Guidelines for Earthquake Resistant Non … for Earthquake Resistant Non-Engineered Construction Revised edition of “Basic concept of Seismic codes” Volume I part 2, 1980 INTERNATIONAL

Guidelines for Earthquake Resistant Non-Engineered Construction

International Association for Earthquake Engineering

National Information Center of Earthquake Engineering

2004

Guidelines for Earthquake Resistant Non-Engineered

Construction

Revised edition of “Basic concept of Seismic codes” Volume I part 2, 1980

INTERNATIONAL ASSOCIATION FOR EARTHQUAKE ENGINEERING

KENCHIKU KAIKAN, 3RD FLOOR, 5-26-20 Shiba Minato-ku, Tokyo 108, Japan

Reprinted by:

NATIONAL INFORMATION CENTER OF

EARTHQUAKE ENGINEERING

Indian Institute of Technology Kanpur Kanpur 208016, India

First published in 1986 by International Association for Earthquake Engineering, KENCHIKU KAIKAN, 3RD FLOOR, 5-26-20 Shiba Minato-ku, Tokyo 108, Japan. Reprinted in October 2001 in India by The Associated Cement Companies Limited (ACC), Cement House, 121, Maharshi Karve Road, Mumbai 400020, with permission from the International Association for Earthquake Engineering, Japan. Reprinted in June 2004 in India by the National Information Center of Earthquake Engineering, IIT Kanpur, Kanpur-208016, India with permission from The International Association for Earthquake Engineering, Japan. Layout and design of reprint by The Indian Concrete Journal (ICJ) published by ACC. Cover design: Jnananjan Panda, NICEE, IIT Kanpur

Preface to the 2004 English Edition

INTERNATIONAL ASSOCIATION FOR EARTHQUAKE ENGINEERING (IAEE)

Secretary General: Hirokazu Iemura, KECHIKU KAIKAN Bldg.4F., 5-26-20, Shiba Minato-ku, Tokyo 108-0014 JAPAN

E-mail: [email protected] Home Page: http://www.iaee.or.jp Fax +81 3 3453-0428

This English edition of the “Guidelines for Earthquake Resistant Non-Engineered Construction” is a new contribution of the National Information Center of Earthquake Engineering of India (NICEE) to contribute to our common objective of improving the seismic safety of non-engineered housing constructions.

The first edition of the book was published by the International Association for Earthquake Engineering (IAEE) in 1986. It consisted in a revised and amplified version of the original document, “Basic Concepts of Seismic Codes, Vol.1, Part II, Non-Engineered Construction”, published also by IAEE in 1980. The revision resulted from the work of an ad-hoc Committee, integrated by Anand S. Arya, Chairman (India), Teddy Boen (Indonesia), Yuji Ishiyama (Japan), A. I. Martemianov (USSR), Roberto Meli (Mexico), Charles Scawthorn (USA), Julio N. Vargas (Peru) and Ye Yaoxian (China). These efforts were guided by the objectives of our Association, related to the promotion of international cooperation among scientists, engineers and other professionals in the field of earthquake engineering through the exchange of knowledge, ideas and the results of research and practical experience.

The book starts with the presentation of the basic concepts that determine the performance of constructions when subjected to high intensity earthquakes, as well as with the sensitivity of that performance to the basic geometrical and mechanical properties of the systems affected. This information is later applied to the formulation of simplified design rules and to the presentation of practical construction procedures, both intended to prevent system collapse and to control the level of damage produced by seismic excitations. Emphasis is placed on basic principles and simple solutions that can be applied to different types of structural systems, representative of those ordinarily used in low-cost housing construction in different regions and countries in the world.

An electronic version of the document was made available at the website (www.nicee.org) of NICEE at the Indian Institute of Technology Kanpur (IITK), shortly after the January 26, 2001 Gujarat earthquake. The response was very encouraging, thus showing the convenience of preparing a new hard edition, intended to disseminate knowledge useful for the reconstruction process. This new endeavor was undertaken by Dr. Sudhir K. Jain, of the Indian Institute of Technology at Kanpur, with the support of the Associated Cement Companies Ltd (ACC). Permission for publication was requested from IAEE, which immediately expressed our enthusiastic approval for this valuable effort. In 2003, NICEE efforts to disseminate the Guidelines continued, now in the form of a translation into Hindi.

The International Association for Earthquake Engineering enthusiastically endorses this new endeavor of NICEE, which will give us a new English edition of the Guidelines. We want again to express our deepest recognition to Dr. Jain for his continued efforts to disseminate knowledge useful for the enhancement of seismic safety around the world.

June 2004 Luis Esteva, President, IAEE

President Luis Esteva Institute de Ingenieria UNAM Ciudad Universitaria, Apartado Postal 70-472, Coyoacan 04510, Mexico 20, D.F., Mexico Executive Vice-President Hiroyuki Aoyama Aoyama Laboratory, 4-2-13, Takadanobaba, Shinjuku-ku, , Tokyo,169-0075 Japan

Vice-President Donald L. Anderson Civil Eng. Dept., University of British Columbia, 2324 Main Mall, Vancouver, B.C.,V6T 1Z4, Canada Past-President Sheldon Cherry Civil Eng. Dept., University of British Columbia, 2324 Main Mall, Vancouver, B.C., V6T 1Z4, Canada

Directors H. Bachmann J.M. Eisenberg L. Garcia P. Gulkan P. Hidalgo D.C. Hopkins Y. Hu S.K. Jain A. Pecker A.V. Rutenberg

L.A. Wyllie, Jr. Consultative Member A.K. Chopra Honorary Members N.N. Ambraseys R.W. Clough L. Esteva R. Flores A. G.Grandori

G.W. Housner K. Kanai T. Kobori S. Okamoto R. Park T. Paulay J. Penzien J. Petrovski G.B. Warburton R. Yarar

Foreward

The Bhuj earthquake of 26 January 2001 in Gujarat (India) has caused wide spread concerns about the seismic safety of built environment in India. The vulnerability of our modern constructions was clearly seen around the country on the television screens. About 130 multistorey buildings collapsed in Ahmedabad (seismic zone III) located far away from the earthquake epicenter (250 km); most of these buildings had been constructed fairly recently. Total losses during the earthquake have been estimated at Rs 25,000 crores. The earthquake has created a very substantial demand for know how on seismic safety. Immediately after the earthquake, the National Information Centre of Earthquake Engineering (www.nicee.org) made an electronic copy of the Guidelines for Earthquake Resistant Non-Engineered Construction available on its web site with permission of the International Association for Earthquake Engineering (IAEE). This was very well received. Subsequently, the ACC Ltd printed about of the Guidelines for dissemination with permission of the IAEE. There was so much demand, that these copies were soon exhausted. In the meanwhile, the guidelines have been translated into Hindi and printed by NICEE in November 2003. NICEE is very pleased to now offer another reprint of the English version of the Guidelines. We are thankful to the International Association for Earthquake Engineering for granting permission for this reprint. Special thanks are due to Professor Luis Esteva, President of IAEE, for his message for this reprint. The excellent layout and design developed by The Indian Concrete Journal (ICJ) of ACC has been used in this reprint; we are thankful to them. Finally, NICEE is thankful to the Government of NCT of Delhi for sponsoring the reprint.

June 2004 Sudhir K Jain

Coordinator, National Information Centre of Earthquake Engineering Indian Institute of Technology Kanpur

[email protected]

CHAPTER TOPIC PAGE NO

1 THE PROBLEM, OBJECTIVE AND SCOPE .. 11.1 THE PROBLEM .. 11.2 SOCIO-ECONOMIC CONSIDERATIONS IN SEISMIC SAFETY OF BUILDINGS 11.3 OBJECT AND SCOPE .. 2

2 STRUCTURAL PERFORMANCE DURING EARTHQUAKES .. 32.1 INTRODUCTION .. 32.2 EARTHQUAKE EFFECTS .. 4

2.2.1 Ground shaking .. 42.2.2 Ground failure .. 42.2.3 Tsunamis .. 42.2.4 Fire .. 4

2.3 GROUND SHAKING EFFECT ON STRUCTURES .. 42.3.1 Inertia forces .. 42.3.2 Seismic load .. 52.3.3 Factors affecting seismic load .. 62.3.4 Nature of seismic stresses .. 62.3.5 Important parameters in seismic design .. 6

2.4 EFFECT OF SITE CONDITIONS ON BUILDING DAMAGE .. 72.5 OTHER FACTORS AFFECTING DAMAGE .. 8

2.5.1 Building configuration .. 82.5.2 Opening size .. 82.5.3 Rigidity distribution .. 82.5.4 Ductility .. 82.5.5 Foundation .. 82.5.6 Construction quality .. 9

2.6 FAILURE MECHANISMS OF EARTHQUAKES .. 92.6.1 Free standing masonry wall .. 92.6.2 Wall enclosure without roof .. 92.6.3 Roof on two walls .. 102.6.4 Roof on wall enclosure .. 112.6.5 Roofs and floors .. 112.6.6 Long building with roof trusses .. 122.6.7 Shear wall with openings .. 13

2.7 EARTHQUAKE DAMAGE CATEGORIES .. 14

3 GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN .. 153.1 INTRODUCTION .. 153.2 CATEGORIES OF BUILDINGS .. 16

CONTENTS

3.2.1 Seismic zones .. 163.2.2 Importance of building .. 163.2.3 Bearing capacity of foundation soil .. 163.2.4 Combination of parameters .. 17

3.3 GENERAL PLANNING AND DESIGN ASPECTS .. 173.3.1. Plan of building .. 173.3.2 Choice of site .. 193.3.3. Structural design .. 203.3.4 Fire resistance .. 20

3.4 STRUCTURAL FRAMING .. 203.5 REQUIREMENTS OF STRUCTURAL SAFETY .. 203.6 CONCEPTS OF DUCTILITY, DEFORMABILITY AND DAMAGEABILITY 21

3.6.1 Ductility .. 223.6.2 Deformability .. 223.6.3 Damageability .. 22

3.7 CONCEPT OF ISOLATION .. 233.8 FOUNDATIONS .. 23

3.8.1 Firm soil .. 233.8.2 Soft soil .. 23

4 BUILDINGS IN FIRED-BRICK AND OTHER MASONRY UNITS .. 254.1 INTRODUCTION .. 254.2 TYPICAL DAMAGE AND FAILURE OF MASONRY BUILDINGS .. 25

4.2.1 Non-structural damage .. 254.2.2 Damage and failure of bearing walls .. 264.2.3 Failure of ground .. 284.2.4 Failure of roofs and floors .. 284.2.5 Causes of damage in masonry buildings .. 28

4.3 TYPICAL STRENGTHS OF MASONRY .. 294.4 GENERAL CONSTRUCTION ASPECTS .. 31

4.4.1 Mortar .. 314.4.2. Wall enclosure .. 314.4.3 Openings in walls .. 324.4.4 Masonry bond .. 34

4.5 HORIZONTAL REINFORCEMENT IN WALLS .. 344.5.1 Horizontal bands or ring beams .. 354.5.2 Section of bands or ring beams .. 364.5.3 Dowels at corners and junctions .. 36

4.6 VERTICAL REINFORCEMENT IN WALLS .. 384.7 FRAMING OF THIN LOAD BEARING WALLS .. 384.8 REINFORCING DETAILS FOR HOLLOW BLOCK MASONRY .. 40

4.8.1 Horizontal band .. 404.8.2 Vertical reinforcement .. 40

5 STONE BUILDINGS .. 435.1 INTRODUCTION .. 435.2 TYPICAL DAMAGE AND FAILURE OF STONE BUILDINGS .. 435.3 TYPICAL STRUCTURAL PROPERTIES .. 44

5.4 GENERAL CONSTRUCTION ASPECTS .. 445.4.1 Overall dimensions .. 445.4.2 Mortar .. 455.4.3 Openings in walls .. 465.4.4 Masonry bond .. 465.4.5 Horizontal reinforcing of walls .. 475.4.6 Vertical reinforcing of walls .. 47

6 WOODEN BUILDINGS .. 496.1 INTRODUCTION .. 496.2 TYPICAL DAMAGE AND FAILURE OF WOODEN BUILDINGS .. 496.3 TYPICAL CHARACTERISTICS OF WOOD .. 506.4 TYPICAL STRUCTURAL PROPERTIES .. 526.5 THE BUILDING PLAN .. 536.6 STUD WALL CONSTRUCTION .. 546.7 BRICK NOGGED TIMBER FRAME .. 586.8 JOINTS IN WOOD FRAMES .. 596.9 FOUNDATIONS .. 59

7 EARTHEN BUILDINGS .. 617.1 INTRODUCTION .. 617.2 TYPICAL DAMAGE AND COLLAPSE OF EARTHEN BUILDINGS.. 617.3 CLASSIFICATION OF WALLS AND MATERIAL PROPERTIES .. 64

7.3.1 Classification of earthen constructions .. 647.3.2 Suitability of soil .. 647.3.3 Strength test of adobe .. 65

7.4 CONSTRUCTIONS OF WALLS .. 657.4.1 Hand-moulded layered construction .. 667.4.2 Adobe or block construction .. 667.4.3 Tapial pise construction .. 677.4.4 Earthen construction with wood or cane structure .. 68

7.5 GENERAL RECOMMENDATIONS FOR SEISMIC AREAS .. 717.5.1 Walls .. 717.5.2 Foundations .. 727.5.3 Roofing .. 74

7.6 SEISMIC STRENGTHENING FEATURES .. 747.6.1 Collar beam or horizontal band .. 747.6.2 Pillasters and buttresses .. 747.6.3 Vertical reinforcement in walls .. 757.6.4 Diagonal bracing .. 75

7.7 PLASTERING AND PAINTING .. 767.8 SUMMARY OF DESIRABLE FEATURES .. 777.9 WORKING STRESSES .. 77

7.9.1 Unit compressive strength .. 777.9.2 Masonry compressive strength .. 777.9.3 Shear strength of masonry .. 787.9.4 Permissible tensile strength of masonry for loads perpendicular

to its plane (fa) .. 78

8 NON ENGINEERED REINFORCED CONCRETE BUILDINGS .. 798.1 INTRODUCTION .. 798.2 TYPICAL DAMAGE AND COLLAPSE OF RC BUILDINGS .. 798.3 CARE IN CONCRETE CONSTRUCTION .. 818.4 TYPICAL MATERIAL PROPERTIES .. 838.5 DETAILING OF BEAMS .. 858.6 DETAILING OF COLUMNS .. 898.7 CONNECTION .. 898.8 ILLUSTRATE SKETCHES .. 89

9 REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGS 919.1 INTRODUCTION .. 919.2 REPAIR, RESTORATION AND STRENGTHENING CONCEPTS .. 92

9.2.1 Repairs .. 929.2.2 Restoration .. 929.2.3 Strengthening of existing buildings .. 93

9.3 REPAIR MATERIALS .. 949.3.1. Shotcrete .. 949.3.2 Epoxy resins .. 949.3.3 Epoxy mortar .. 949.3.4 Gypsum cement mortar .. 959.3.5 Quick-setting cement mortar .. 959.3.6 Mechanical anchors .. 95

9.4 TECHNIQUES TO RESTORE ORIGINAL STRENGTH .. 959.4.1 Small cracks .. 959.4.2 Large cracks and crushed concrete .. 979.4.3 Fractured, excessively yielded and buckled reinforcement .. 989.4.4 Fractured wooden members and joints .. 100

9.5 MODIFICATION OF ROOFS .. 1009.6 SUBSTITUTION OR STRENGTHENING OF SLABS 1029.7 PLANNAR MODIFICATIONS AND STRENGTHENING OF WALLS 102

.. 9.7.1 Inserting new walls .. 1029.7.2 Strengthening existing walls .. 1049.7.3 External binding .. 1099.7.4 Other points .. 110

9.8 STRENGTHENING RC MEMBERS .. 1129.9 STRENGTHENING OF FOUNDATIONS .. 113

APPENDIX .. 115

1

THE PROBLEM, OBJECTIVE AND SCOPE

Chapter 1

THE PROBLEM, OBJECTIVE AND SCOPE

1.1 THE PROBLEMMost of the loss of life in past earthquakeshas occurred due to the collapse ofbuildings, constructed in traditionalmaterials like stone, brick, adobe and wood,which were not particularly engineered tobe earthquake resistant. In view of thecontinued use of such buildings in mostcountries of the world, it is essential tointroduce earthquake resistance features intheir construction.

1.2 SOCIO-ECONOMICCONSIDERATIONS IN SEISMICSAFETY OF BUILDINGSFrom the results of studies on theperformance of buildings during pastearthquakes, it appears that

(i) certain building types should en-tirely be ruled out in seismic zoneshaving probable seismic intensity ofVIII or more on Modified Mercalli orthe MSK Intensity Scales. This wouldinclude earthen houses, random rub-ble masonry as well as brickwork inclay mud mortar, and the like;

(ii) rich mortars involving cement andlime should be used in fired brick andcoursed stone masonry; and

(iii) substantial steel reinforcementshould be introduced in the walls inboth directions of the building.

But there are a number of socio-eco-nomic constraints such as the followingwhich do not permit the adoption of highlevel of safety in the buildings for themasses:

(i) lack of concern about seismic safetydue to infrequent occurrence of earth-quakes;

(ii) lack of awareness that buildingscould be made earthquake resistantat small additional cost only, hencelack of motivation;

(iii) lack of financial resources for addi-tional inputs for meeting earthquakeresistance requirements in buildingconstruction;

(iv) other normal priorities on financialinputs in the daily life of the people;

(v) scarcity of cement, steel as well astimber in the developing countries ingeneral; and

(vi) lack of skill in aseismic design andconstruction techniques and unor-ganised nature of the building sec-tor.

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IAEE MANUAL

Such considerations therefore compelthe continued use of seismically unsuitableconstruction practices.

While theoretically, if appropriate re-sources and building materials are madeavailable, it may be possible to constructbuildings which can withstand the effectsof earthquake without any appreciabledamage, but practically it is not feasible todo so due to very high costs involved. Fromthe safety view point, the safety of humanlives is the primary concern and the func-tioning of the buildings has lower priorityexcept the buildings required for commu-nity activities such as schools, assemblyhalls, places of worship, and cinema halls,etc., and those required for the emergency,such as, buildings for hospital, operationtheatre, telephone and telegraph, fire fight-ing and the like. The safety aims wouldtherefore be met, if a building is designedand constructed in such a way that even inthe event of the probable maximum earth-quake intensity in the region,

(i) an ordinary building should not suf-fer total or partial collapse,

(ii) it should not suffer such irreparabledamage which would require demol-ishing and rebuilding

(iii) it may sustain such damage whichcould be repaired quickly and thebuilding put back to its usual func-tioning; and

(iv) the damage to an important build-ing should even be less so that thefunctioning of the activities duringpost-emergency period may con-tinue unhampered and the commu-nity buildings may be used as tem-porary shelters for the adversely af-fected people.

The present state of research indicatesthat fortunately the above structural safetycan be achieved by adopting appropriatedesign and construction details involvingonly small extra expenditure which shouldbe within the economic means of people inmost countries.

1.3 OBJECT AND SCOPEThe object of this book is to deal with thebasic concepts involved in achieving ap-propriate earthquake resistance of suchbuildings as stated above, which may becollectively called as Non-EngineeredBuildings; to include suitable illustrationsto explain the important points, and topresent such data which could be used toproportion the critical strengthening ele-ments. The term non-engineered buildingmay only be vaguely defined as buildingswhich are spontaneously and informallyconstructed in the traditional manner with-out intervention by qualified architects andengineers in their design but may follow aset of recommendations derived from ob-served behaviour of such buildings duringpast earthquakes and trained engineeringjudgement. Specifically such buildings willinclude load bearing masonry wall build-ings, stud-wall and brick-nogged construc-tions in wood, and composite constructionsusing combinations of load bearing wallsand piers in masonry, reinforced concrete,steel or wood, and the like.

Reinforced masonry, reinforced concreteor steel frame buildings, tall buildings us-ing various types of structural systems, andmajor industrial buildings, etc., are ex-cluded from consideration although someof the principles stated herein will apply tothese constructions with equal force.

•••

1

STRUCTURAL PERFORMANCE DURING EARTHQUAKES

Chapter 2

STRUCTURAL PERFORMANCE DURINGEARTHQUAKES

2.1 INTRODUCTIONEarthquakes are natural hazards underwhich disasters are mainly caused by dam-age to or collapse of buildings and otherman-made structures. Experience hasshown that for new constructions, estab-lishing earthquake resistant regulationsand their implementation is the criticalsafeguard against earthquake-induceddamage. As regards existing structures, itis necessary to evaluate and strengthenthem based on evaluation criteria before anearthquake.

Earthquake damage depends on manyparameters, including intensity, durationand frequency content of ground motion,geologic and soil condition, quality of con-struction, etc. Building design must be suchas to ensure that the building has adequatestrength, high ductility, and will remain asone unit, even while subjected to very largedeformation.

Sociologic factors are also important,such as density of population, time of dayof the earthquake occurrence and commu-

nity preparedness for the possibility of suchan event.

Up to now we can do little to diminishdirect earthquake effects. However we cando much to reduce risks and thereby reducedisasters provided we design and build orstrengthen the buildings so as to minimizethe losses based on the knowledge of theearthquake performance of different build-ing types during an earthquake.

Observation of structural performanceof buildings during an earthquake canclearly identify the strong and weak aspectsof the design, as well as the desirable quali-ties of materials and techniques of construc-tion, and site selection. The study of dam-age therefore provides an important step inthe evolution of strengthening measures fordifferent types of buildings.

This Chapter discusses earthquake per-formance of structures during earthquakeintensity, ground shaking effects on struc-tures, site condition effects on buildingdamage, other factors affecting damage,

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IAEE MANUAL

failure mechanisms of structures, earth-quake damage and damage categories.

Typical patterns of damage for specifictypes of construction are discussed in therespective chapters.

2.2 EARTHQUAKE EFFECTSThere are four basic causes of earthquake-induced damage: ground shaking, groundfailure, tsunamis and fire.

2.2.1 Ground shakingThe principal cause of earthquake-induceddamage is ground shaking. As the earthvibrates, all buildings on the ground sur-face will respond to that vibration in vary-ing degrees. Earthquake inducedaccelerations, velocities and displacementscan damage or destroy a building unless ithas been designed and constructed orstrengthened to be earthquake resistant.Therefore, the effect of ground shaking onbuildings is a principal area of considera-tion in the design of earthquake resistantbuildings. Seismic design loads are ex-tremely difficult to determine due to the ran-dom nature of earthquake motions. How-ever, experiences from past strong earth-quakes have shown that reasonable andprudent practices can keep a building safeduring an earthquake.

2.2.2 Ground failureEarthquake-induced ground failure hasbeen observed in the form of ground rup-ture along the fault zone, landslides, settle-ment and soil liquefaction.

Ground rupture along a fault zone maybe very limited or may extend over hun-dreds of kilometers. Ground displacementalong the fault may be horizontal, vertical

or both, and can be measured in centimetersor even metres. Obviously, a building di-rectly astride such a rupture will be severelydamaged or collapsed.

While landslide can destroy a building,the settlement may only damage the build-ing.

Soil liquefaction can occur in low den-sity saturated sands of relatively uniformsize. The phenomenon of liquefaction isparticularly important for dams, bridges,underground pipelines, and buildingsstanding on such ground.

2.2.3 TsunamisTsunamis or seismic sea waves are gener-ally produced by a sudden movement ofthe ocean floor. As the water waves ap-proach land, their velocity decreases andtheir height increases from 5 to 8 m, or evenmore. Obviously, tsunamis can be devas-tating for buildings built in coastal areas.

2.2.4 FireWhen the fire following an earthquakestarts, it becomes difficult to extinguish it,since a strong earthquake is accompaniedby the loss of water supply and traffic jams.Therefore, the earthquake damage increaseswith the earthquake-induced fire in addi-tion to the damage to buildings directly dueto earthquakes. In the case of the 1923 Kantoearthquake 50% of Tokyo and 70% of thetotal number of houses were burnt and morethan 100,000 people were killed by the fire.

2.3 GROUND SHAKING EFFECTON STRUCTURES2.3.1 Inertia forcesBuildings are fixed to the ground as shownin Fig 2.1(a). As the base of a building moves

3


the superstructure including its contentstends to shake and vibrate from the posi-tion of rest, in a very irregular manner dueto the inertia of the masses.

When the base of the building suddenlymoves to the right, the building moves tothe left relative the base, Fig 2.1(b), as if itwas being pushed to the left by an unseenforce which we call �Inertia Force�. Actu-ally, there is no push at all but, because ofits mass, the building resists any motion.The process is much more complex becausethe ground moves simultaneously in threemutually perpendicular directions duringan earthquake as shown in Fig 2.1 (b), (c),and (d).

2.3.2 Seismic loadThe resultant lateral force or seismic loadis represented by the force F as shown in

Fig 2.1(e). The force F is distinctly differentfrom the dead, live, snow, wind, and im-pact loads. The horizontal ground motionaction is similar to the effect of a horizontalforce acting on the building, hence the term�Seismic Load�. As the base of the build-ing moves in an extremely complicatedmanner, inertia forces are created through-out the mass of the building and its con-tents. It is these reversible forces that causethe building to move and sustain damageor collapse.

Additional vertical load effect is causedon beams and columns due to vertical vi-brations. Being reversible, at certain in-stants of time the effective load is increased,at others it is decreased.

The earthquake loads are dynamic andimpossible to predict precisely in advance,

Fig 2.1 Seismic vibrations of a building and resultant earthquake force

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IAEE MANUAL

since every earthquake exhibits differentcharacteristics. The following equivalentminimum total lateral force is, used for seis-mic design:

F=S.Fs.I.C.W

Where S, Fs, I, C and W are the factorsaffecting seismic load, which will be ex-plained in the following section.

2.3.3 Factors affecting seismic loadThe earthquake zone factor S dependsupon the ground intensity of the earth-quake. The value of S usually is plotted onmaps in terms of seismic intensity isolinesor maximum acceleration isolines. Obvi-ously, the higher the intensity or accelera-tion, the larger will be the seismic force.

The soil-foundation factor Fs dependsupon the ratio of fundamental elastic pe-riod of vibration of a building in the direc-tion under consideration and the charac-teristic site period. Therefore, Fs is a numeri-cal coefficient for site-building resonance.

The occupancy importance or hazardfactor I depends upon the usage of thebuilding. The higher the importance orlarger the hazard caused by the failure ofthe building, the greater the value of thefactor I.

The C is a factor depending on the stiff-ness and damping of the structure. Largerthe stiffness for given mass, shorter the fun-damental period of vibration of the struc-ture and larger the value of C. Damping isthe energy dissipation property of the build-ing; larger the damping, smaller the valueof C.

The W is the total weight of the super-structure of a building including its con-tents. The inertia forces are proportional tothe mass of the building and only that partof the loading action that possesses masswill give rise to seismic force on the build-ing. Therefore, the lighter the material, thesmaller will be the seismic force.

2.3.4 Nature of seismic stressesThe horizontal seismic forces are reversiblein direction. The structural elements suchas walls, beams and columns that werebearing only vertical loads before the earth-quake, have now to carry horizontal bend-ing and shearing effects as well. When thebending tension due to earthquake exceedsthe vertical compression, net tensile stresswill occur. If the building material is weakin tension such as brick or stone masonry,cracking occurs which reduces the effec-tive area for resisting bending moment, asshown in Fig 2.2. It follows that the strengthin tension and shear is important for earth-quake resistance.

2.3.5 Important parameters inseismic designIt follows that the following properties andparameters are most important from thepoint of view of the seismic design.

(i) Building material properties

� Strength in compression, tensionand shear, including dynamic ef-fects

� Unit weight

� Modulus of elasticity

(ii) Dynamic characteristics of the build-ing system, including periods,modes and dampings.

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(iii) Load-deflection characteristics ofbuilding components.

2.4 Effect of site conditions onbuilding damagePast earthquakes show that site conditionsignificantly affects the building damage.Earthquake studies have almost invariablyshown that the intensity of a shock is di-rectly related to the type of soil layers sup-porting the building. Structures built onsolid rock and firm soil frequently fares bet-ter than buildings on soft ground. This wasdramatically demonstrated in the 1985Mexico City earthquake, where the damageon soft soils in Mexico City, at an epicentraldistance of 400 km, was substantiallyhigher than at closer locations.

From studies of the July 28, 1957 earth-quake in Mexico City, it was already knownfor example that the damage on the soft soils

in the center of the city could be 5 to 50 timeshigher than on firmer soils in the surround-ing area. Another example occurred in the1976 Tangshan, China earthquake, inwhich 50% of the buildings on thick soilsites were razed to the ground, while only12% of the buildings on the rock subsoilnear the mountain areas totally collapsed.Rigid masonry buildings resting on rockmay on the contrary show more severe dam-age than when built on soil during a nearearthquake as in Koyna (India) earthquakeof 1967 and North Yemen earthquake of1980.

Lessons learned from recent earthquakeshow that the topography of a building sitecan also have an effect on damage. Build-ings built on sites with open and even to-pography are usually less damaged in anearthquake than buildings on strip-shapedhill ridges, separated high hills, and steep

Fig 2.2 Stress condition in a wall element

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IAEE MANUAL

slopes.

2.5 OTHER FACTORSAFFECTING DAMAGEThe extent of damage to a building dependsmuch on the strength, ductility, and integ-rity of a building and the stiffness of groundbeneath it in a given intensity of the earth-quake motions.

Almost any building can be designed tobe earthquake resistant provided its site issuitable. Buildings suffer during an earth-quake primarily because horizontal forcesare exerted on a structure that often meantto contend only with vertical stresses. Theprincipal factors that influence damage tobuildings and other man-made structuresare listed below:

2.5.1 Building configurationAn important feature is regularity and sym-metry in the overall shape of a building. Abuilding shaped like a box, as rectangularboth in plan and elevation, is inherentlystronger than one that is L-shaped or U-shaped, such as a building with wings. Anirregularly shaped building will twist as itshakes, increasing the damage.

2.5.2 Opening sizeIn general, openings in walls of a buildingtend to weaken the walls, and fewer theopenings less the damage it will suffer dur-ing an earthquake. If it is necessary to havelarge openings through a building, or if anopen first floor is desired, then special pro-visions should be made to ensure structuralintegrity.

2.5.3 Rigidity distributionThe rigidity of a building along the verticaldirection should be distributed uniformly.

Therefore, changes in the structural systemof a building from one floor to the next willincrease the potential for damage, andshould be avoided. Columns or shear wallsshould run continuously from foundationto the roof, without interruptions or changesin material.

2.5.4 DuctilityBy ductility is meant the ability of the build-ing to bend, sway, and deform by largeamounts without collapse. The oppositecondition in a building is called brittlenessarising both from the use of materials thatare inherently brittle and from the wrongdesign of structures using otherwise duc-tile materials. Brittle materials crack underload; some examples are adobe, brick andconcrete blocks. It is not surprising thatmost of the damage during the past earth-quakes was to unreinforced masonry struc-tures constructed of brittle materials, poorlytied together. The addition of steel reinforce-ments can add ductility to brittle materials.Reinforced concrete, for example, can bemade ductile by proper use of reinforcingsteel and closely spaced steel ties.

2.5.5 FoundationBuildings, which are structurally strong towithstand earthquakes sometimes fail dueto inadequate foundation design. Tilting,cracking and failure of superstructuresmay result from soil liquefaction and dif-ferential settlement of footing.

Certain types of foundations are moresusceptible to damage than others. For ex-ample, isolated footings of columns arelikely to be subjected to differential settle-ment particularly where the supportingground consists of different or soft types ofsoil. Mixed types of foundations within the

7


same building may also lead to damage dueto differential settlement.

Very shallow foundations deterioratebecause of weathering, particularly whenexposed to freezing and thawing in the re-gions of cold climate.

2.5.6 Construction qualityIn many instances the failure of buildingsin an earthquake has been attributed topoor quality of construction, substandardmaterials, poor workmanship, e. g., inad-equate skill in bonding, absence of�through stones� or bonding units, andimproper and inadequate construction.

2.6 FAILURE MECHANISMS OFEARTHQUAKES2.6.1 Free standing masonrywallConsider the free standing masonry wallsshown in Fig 2.3. In Fig 2.3(a), the groundmotion is acting transverse to a free stand-ing wall A. The force acting on the mass ofthe wall tends to overturn it. The seismicresistance of the wall is by virtue of itsweight and tensile strength of mortar andit is obviously very small. This wall willcollapse by overturning under the groundmotion.

The free standing wall B fixed on theground in Fig 2.3(b) is subjected to groundmotion in its own plane. In this case, thewall will offer much greater resistance be-cause of its large depth in the plane of bend-ing. Such a wall is termed a shear wall.The damage modes of an unreinforcedshear wall depend on the length-to-widthratio of the wall. A wall with small length-to-depth ratio will generally develop a hori-zontal crack due to bending tension and

then slide due to shearing. A wall withmoderate length-to-width ratio and bound-ing frame diagonally cracks due to shear-ing as shown at Fig 2.3 (c).

A wall with large length-to-width ratio,on the other hand, may develop diagonaltension cracks at both sides and horizontalcracks at the middle as shown at Fig 2.3 (d).

2.6.2 Wall enclosure without roofNow consider the combination of walls Aand B as an enclosure shown in Fig 2.4. Forthe X direction of force as shown, walls Bact as shear walls and, besides taking theirown inertia, they offer resistance againstthe collapse of wall A as well. As a resultwalls A now act as vertical slabs supportedon two vertical sides and the bottom plinth.The walls A are subjected to the inertia forceon their own mass. Near the vertical edges,the wall will carry reversible bending mo-

Fig 2.3 Failure mechanism of free standing walls

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IAEE MANUAL

ments in the horizontal plane for which themasonry has little strength. Consequentlycracking and separation of the walls mayoccur along these edges shown in the fig-ure.

It can be seen that in the action of wallsB as shear walls, the walls A will act asflanges connected to the walls B acting asweb. Thus if the connection between wallsA and B is not lost due to their bonding ac-

tion as plates, the building will tend to actas a box and its resistance to horizontalloads will be much larger than that of wallsB acting separately. Most unreinforced ma-sonry enclosures, however, have very weakvertical joints between walls meeting atright angles due to the construction proce-dure involving toothed joint that is gener-ally not properly filled with mortar. Conse-quently the corners fail and lead to collapseof the walls. It may also be easily imaginedthat the longer the walls in plan, the smallerwill be the support to them from the crosswalls and the lesser will be the box effect.

2.6.3 Roof on two wallsIn Fig 2.5 (a) roof slab is shown to be restingon two parallel walls B and the earthquakeforce is acting in the plane of the walls.Assuming that there is enough adhesionbetween the slab and the walls, the slabwill transfer its inertia force at the top ofwalls B, causing shearing and overturningaction in them. To be able to transfer its in-ertia force to the two end walls, the slabmust have enough strength in bending inthe horizontal plane. This action of slab isknown as diaphragm action. Reinforcedconcrete or reinforced brick slabs have suchstrength inherently and act as rigid dia-phragms. However, other types of roofs orfloors such as timber or reinforced concretejoists with brick tile covering will be veryflexible. The joists will have to be connectedtogether and fixed to the walls suitably sothat they are able to transfer their inertiaforce to the walls. At the same time, the wallsB must have enough strength as shear wallsto withstand the force from the roof and itsown inertia force. Obviously, the structureshown in Fig 2.5, when subjected to groundmotion perpendicular to its plane, will col-

Fig 2.4 Failure mechanism of wall enclosure without roof

Fig 2.5 Roof on two walls

9


lapse very easily because walls B have lit-tle bending resistance in the plane perpen-dicular to it. In long barrack type buildingswithout intermediate walls, the end wallswill be too far to offer much support to thelong walls and the situation will be simi-lar to the one just mentioned above.

2.6.4 Roof on wall enclosureNow consider a complete wall enclosurewith a roof on the top subjected to earth-quake force acting along X-axis as shownin Fig 2.6. If the roof is rigid and acts as ahorizontal diaphragm, its inertia will bedistributed to the four walls in proportionto their stiffness. The inertia of roof will al-most entirely go to walls B since the stiff-ness of the walls B is much greater than thewalls A in X direction. In this case, the plateaction of walls A will be restrained by theroof at the top and horizontal bending ofwall A will be reduced. On the other hand,if the roof is flexible the roof inertia will goto the wall on which it is supported andthe support provided to plate action ofwalls A will also be little or zero. Again theenclosure will act as a box for resisting thelateral loads, this action decreasing in valueas the plan dimensions of the enclosuresincrease.

2.6.5 Roofs and floorsThe earthquake-induced inertia force canbe distributed to the vertical structural ele-ments in proportion to their stiffness, pro-vided the roofs and floors are rigid to act ashorizontal diaphragms. Otherwise, the roofand floor inertia will only go to the verticalelements on which they are supported.Therefore, the stiffness and integrity of roofsand floors are important for earthquake re-sistance.

The roofs and floors, which are rigidand flat and are bonded or tied to the ma-sonry, have a positive effect on the wall,such as the slab or slab and beam construc-tion be directly cast over the walls or jackarch floors or roofs provided with horizon-tal ties and laid over the masonry wallsthrough good quality mortar. Others thatsimply rest on the masonry walls will offerresistance to relative motion only throughfriction, which may or may not be adequatedepending on the earthquake intensity. Inthe case of a floor consisting of timber joistsplaced at center to center spacing of 20 to25 cm with brick tiles placed in directly overthe joists and covered with clayey earth, thebrick tiles have no binding effect on the

Fig 2.6 Roof on wall enclosure

Fig 2.7 Long building with roof trusses

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IAEE MANUAL

joists. Therefore, relative displacement ofthe joists is quite likely to occur during anearthquake, which could easily bring downthe tiles, damaging property and causinginjury to people. Similar behaviour may bevisualized with the floor consisting ofprecast reinforced concrete elements notadequately tied together. In this case, rela-tive displacement of the supporting wallscould bring down the slabs.

2.6.6 Long building with rooftrussesConsider a long building with a single spanand roof trusses as shown in Fig 2.7. Thetrusses rest on the walls A. The walls B aregabled to receive the purlins of the endbays. Assuming that the ground motion isalong the X-axis, the inertia forces will betransmitted from sheeting to purlins totrusses and from trusses to wall A.

Fig 2.8 Deformation of a shear wall with openings.

11


The end purlins will transmit someforce directly to gable ends. Under the seis-mic force the trusses may slide on the wallsunless anchored into them by bolts. Also,the wall A, which does, not get much sup-port from the walls B in this case, may over-turn unless made strong enough in the ver-tical bending as a cantilever or other suit-able arrangement, such as adding horizon-tal bracings between the trusses, is madeto transmit the force horizontally to endwalls B.

When the ground motion is along Y di-rection, walls A will be in a position to actas shear walls and all forces may be trans-

mitted to them. In this case, the purlins actas ties and struts and transfer the inertiaforce of roof to the gable ends.

As a result the gable ends may fail. Whenthe gable triangles are very weak in stabil-ity, they may fail even in small earthquakes.Also, if there is insufficient bracing in theroof trusses, they may overturn even whenthe walls are intact.

2.6.7 Shear wall with openingsShear walls are the main lateral earthquakeresistant elements in many buildings. Forunderstanding their action, let us considera shear wall with three openings shown in

Table 2.1 Categories of damage

Damage category Extent of damage Suggested post- earthquakein general actions

0 No damage No damage No action required

I Slighty non-structural Thin cracks in plaster, falling of Building need not be vacated.damage plaster bits in limited parts. Only architectural repairs

needed.

II Slight Structural Small cracks in walls, failing of Building need not be vacated.Damage plaster in large bits over large areas; Architectural repairs required

damage to non-structural parts like to achieve durability.chimneys, projecting cornices, etc.The load carrying capacity of thestructure is not reduced appreciably.

III Moderate structural Large and deep cracks in walls; Building needs to be vacated, todamage widespread cracking of walls, be reoccupied after restoration

columns, piers and tilting or failing and strengthening.of chimneys. The load carrying Structural restoration andcapacity of the structure is partially seismic strengthening arereduced. necessary after which architec

tural treatment may be carriedout.

IV Severe structural Gaps occur in walls; inner and outer Building has to be vacated.damage walls collapse; failure of ties to Either the building has to be

separate parts of buildings. Approx. demolished or extensive50 % of the main structural restoration and strengtheningelements fail.The building takes work has to be carried outdangerous state. before reoccupation.

V Collapse A large part or whole of the building Clearing the site andcollapses. reconstruction.

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IAEE MANUAL

Fig 2.8. Obviously, the piers between theopenings are more flexible than the portionof wall below (sill masonry) or above(spandrel masonry) the openings. The de-flected form under horizontal seismic forceis also sketched in the figure.

The sections at the level of the top andbottom of opening are found to be the worststressed in tension as well as in compres-sion and those near the mid-height of pierscarry the maximum shears. Under reverseddirection of horizontal loading the sectionscarrying tensile and compressive stresses

are also reversed. Thus it is seen that ten-sion occurs in the jambs of openings and atthe corners of the walls.

2.7 EARTHQUAKE DAMAGECATEGORIESIn this section, an outline of damage cat-egories is simply described in Table 2.1 onthe basis of past earthquake experience.Therein the appropriate post-earthquakeaction for each category of damage is alsosuggested.

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1

GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN

Chapter 3

GENERAL CONCEPTS OF EARTHQUAKERESISTANT DESIGN

3.1 INTRODUCTIONExperience in past earthquakes has dem-onstrated that many common buildingsand typical methods of construction lackbasic resistance to earthquake forces. Inmost cases this resistance can be achievedby following simple, inexpensive princi-ples of good building construction prac-tice. Adherence to these simple rules willnot prevent all damage in moderate or largeearthquakes, but life threatening collapsesshould be prevented, and damage limitedto repairable proportions. These principlesfall into several broad categories:

(i) Planning and layout of the buildinginvolving consideration of the loca-tion of rooms and walls, openingssuch as doors and windows, thenumber of storeys, etc. At this stage,site and foundation aspects shouldalso be considered.

(ii) Lay out and general design of thestructural framing system with spe-cial attention to furnishing lateralresistance, and

(iii) Consideration of highly loaded andcritical sections with provision of

reinforcement as required.

Chapter 2 has provided a good overviewof structural action, mechanism of damageand modes of failure of buildings. Fromthese studies, certain general principleshave emerged:

(i) Structures should not be brittle orcollapse suddenly. Rather, theyshould be tough, able to deflect ordeform a considerable amount.

(ii) Resisting elements, such as bracingor shear walls, must be providedevenly throughout the building, inboth directions side-to-side, as wellas top to bottom.

(iii) All elements, such as walls and theroof, should be tied together so as toact as an integrated unit duringearthquake shaking, transferringforces across connections and pre-venting separation.

(iv) The building must be well connectedto a good foundation and the earth.Wet, soft soils should be avoided, andthe foundation must be well tied to-gether, as well as tied to the wall.

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IAEE MANUAL

Where soft soils cannot be avoided,special strengthening must be pro-vided.

(v) Care must be taken that all materialsused are of good quality, and are pro-tected from rain, sun, insects andother weakening actions, so that theirstrength lasts.

(vi) Unreinforced earth and masonryhave no reliable strength in tension,and are brittle in compression. Gen-erally, they must be suitably rein-forced by steel or wood.

These principles will be discussed andillustrated in this Chapter.

3.2 CATEGORIES OFBUILDINGSFor categorising the buildings with thepurpose of achieving seismic resistance ateconomical cost, three parameters turn outto be significant:

(i) Seismic intensity zone where thebuilding is located,

(ii) How important the building is, and

(iii) How stiff is the foundation soil.

A combination of these parameters willdetermine the extent of appropriate seismicstrengthening of the building.

3.2.1 Seismic zonesIn most countries, the macro level seismiczones are defined on the basis of SeismicIntensity Scales. In this guide, we shall re-fer to seismic zones as defined with refer-ence to MSK Intensity Scale as described inAppendix I for buildings.

Zone A: Risk of Widespread Collapseand Destruction (MSK IX orgreater),

Zone B: Risk of Collapse and HeavyDamage (MSK VIII likely),

Zone C: Risk of Damage (MSK VII likely),

Zone D: Risk of Minor Damage(MSK VI maximum).

The extent of special earthquakestrengthening should be greatest in ZoneA and, for reasons of economy, can be de-creased in Zone C, with relatively little spe-cial strengthening in Zone D. However,since the principles stated in 3.1, are goodprinciples for building in general (not justfor earthquake), they should always be fol-lowed.

3.2.2 Importance of buildingThe importance of the building should be afactor in grading it for strengtheningpurposes,and the following buildings aresuggested as specially important:

IMPORTANT � Hospitals, clinics, com-munication buildings, fire and police sta-tions, water supply facilities, cinemas, thea-tres and meeting halls, schools, dormito-ries, cultural treasures such as museums,monuments and temples, etc.

ORDINARY � Housings, hostels, of-fices, warehouses, factories, etc.

3.2.3 Bearing capacity offoundation soilThree soil types are considered here:

Firm: Those soils which have an allowablebearing capacity of morethan 10 t/m2

3


Soft: Those soils, which have allowablebearing capacity less than or equalto 10 t/m2.

Weak: Those soils, which are liable to largedifferential settlement, or liquefac-tion during an earthquake.

Buildings can be constructed on firmand soft soils but it will be dangerous tobuild them on weak soils. Hence appropri-ate soil investigations should be carried outto establish the allowable bearing capacityand nature of soil. Weak soils must beavoided or compacted to improve them soas to qualify as firm or soft.

3.2.4 Combination ofparametersFor defining the categories of buildings forseismic strengthening purposes, four cat-egories I to IV are defined in Table 3.1. inwhich category I will require maximumstrengthening and category IV the least in-puts. The general planning and designingprinciples are, however, equally applica-ble to them.

3.3. GENERAL PLANNING ANDDESIGN ASPECTS3.3.1. Plan of building

(i) Symmetry: The building as a wholeor its various blocks should be keptsymmetrical about both the axes.Asymmetry leads to torsion duringearthquakes and is dangerous,Fig 3.1. Symmetry is also desirablein the placing and sizing of door andwindow openings, as far as possi-ble.

(ii) Regularity: Simple rectangularshapes, Fig 3.2 (a) behave better inan earthquake than shapes with

many projections Fig 3.2 (b). Tor-sional effects of ground motion arepronounced in long narrow rectan-gular blocks. Therefore, it is desirableto restrict the length of a block tothree times its width. If longerlengths are required two separateblocks with sufficient separation inbetween should be provided,Fig 3.2 (c).

(iii) Separation of Blocks: Separation of alarge building into several blocksmay be required so as to obtain sym-metry and regularity of each block.

Fig 3.1 Torsion of unsymmetrical plans

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IAEE MANUAL

For preventing hammering orpounding damage between blocks aphysical separation of 3 to 4 cmthroughout the height above theplinth level will be adequate as wellas practical for upto 3 storeyedbuildings, Fig 3.2 (c).

The separation section can be treatedjust like expansion joint or it may befilled or covered with a weak mate-rial which would easily crush andcrumble during earthquake shaking.Such separation may be considered

in larger buildings since it may notbe convenient in small buildings.

(iv) Simplicity: Ornamentationinvo1ving large cornices, vertical orhorizontal cantilever projections, fa-cia stones and the like are danger-ous and undesirable from a seismicviewpoint. Simplicity is the best ap-proach.

Where ornamentation is insistedupon, it must be reinforced withsteel, which should be properly em-

Fig 3.2 Plan of building blocks.

5


bedded or tied into the main struc-ture of the building.

Note: If designed, a seismic coeffi-cient about 5 times the coefficientused for designing the main struc-ture should be used for cantileverornamentation.

(v) Enclosed Area: A small building en-closure with properly intercon-nected walls acts like a rigid boxsince the earthquake strength whichlong walls derive from transversewalls increases as their length de-creases.

Therefore structurally it will be ad-visable to have separately enclosedrooms rather than one long room,Fig 3.3. For unframed walls of thick-ness t and wall spacing of a, a ratioof a/t = 40 should be the upper limitbetween the cross walls for mortarsof cement sand 1:6 or richer, and lessfor poor mortars. For larger panelsor thinner walls, framing elementsshould be introduced as shown atFig 3.3(c).

(vi) Separate Buildings for DifferentFunctions: In view of the differencein importance of hospitals, schools,assembly halls, residences, commu-nication and security buildings, etc.,it may be economical to plan sepa-rate blocks for different functions soas to affect economy in strengthen-ing costs.

3.3.2 Choice of siteThe choice of site for a building from theseismic point of view is mainly concernedwith the stability of the ground. The fol-lowing are important:

(i) Stability of Slope: Hillside slopes li-able to slide during an earthquakeshould be avoided and only stableslopes should be chosen to locate thebuilding. Also it will be preferable

Fig 3.3 Enclosed area forming box units

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IAEE MANUAL

3.3.4 Fire resistanceIt is not unusual during earthquakes thatdue to snapping of electrical fittings shortcircuiting takes place, or gas pipes maydevelop leaks and catch fire. Fire could alsobe started due to kerosene lamps andkitchen fires. The fire hazard sometimescould even be more serious than the earth-quake damage. The buildings should there-fore preferably be constructed of fire resist-ant materials.

3.4 STRUCTURAL FRAMINGThere are basically two types structuralframing possible to withstand gravity andseismic load, viz. bearing wall constructionand framed construction. The framed con-struction may again consist of:

(i) Light framing members which musthave diagonal bracing such as woodframes (see Chapter 6) or infill wallsfor lateral load resistance, Fig 3.3 (c),or

(ii) Substantial rigid jointed beams andcolumns capable of resisting the lat-eral loads by themselves.

The latter will be required for large col-umn free spaces such as assembly halls.

The framed constructions can be usedfor a greater number of storeys compared tobearing wall construction. The strength andductility can be better controlled in framedconstruction through design. The strengthof the framed construction is not affectedby the size and number of openings. Suchframes fall in the category of engineeredconstruction, hence outside the scope of thepresent book.

to have several blocks on terracesthan have one large block withfootings at very different elevations.A site subject to the danger of rockfalls has to be avoided.

(ii) Very Loose Sands or Sensitive Clays:These two types of soils are liable tobe destroyed by the earthquake somuch as to lose their original struc-ture and thereby undergocompaction. This would result inlarge unequal settlements and dam-age the building. If the loosecohesionless soils are saturated withwater they are apt to lose their shearresistance altogether during shakingand become liquefied.

Although such soils can be compacted,for small buildings the operation may betoo costly and these soils are better avoided.For large building complexes, such as hous-ing developments, new towns, etc., this fac-tor should be thoroughly investigated andappropriate action taken.

Therefore a site with sufficient bearingcapacity and free from the above defectsshould be chosen and its drainage condi-tion improved so that no water accumu-lates and saturates the ground close to thefooting level.

3.3.3. Structural designDuctility (defined in Section 3.6) is the mostdesirable quality for good earthquake per-formance and can be incorporated to someextent in otherwise brittle masonry con-structions by introduction of steel reinforc-ing bars at critical sections as indicatedlater in Chapters 4 and 5.

7


The strengthening measures necessaryto meet these safety requirements are pre-sented in the following Chapters for vari-ous building types. In view of the lowseismicity of Zone D, no strengtheningmeasures from seismic consideration areconsidered necessary except an emphasison good quality of construction. The fol-lowing recommendations are therefore in-tended for Zones A, B and C. For this pur-pose certain categories of construction in anumber of situations were defined inTable 3.1.

3.6 CONCEPTS OF DUCTILITY,DEFORMABILITY ANDDAMAGEABILITYDesirable properties of earthquake-resist-ant design include ductility, deformabilityand damageability. Ductility anddeformability are interrelated concepts sig-nifying the ability of a structure to sustainlarge deformations without collapse.Damageability refers to the ability of a struc-

3.5 REQUIREMENTS OFSTRUCTURAL SAFETYAs a result of the discussion of structuralaction and mechanism of failure of Chap-ter 2, the following main requirements ofstructural safety of buildings can be arrivedat.

(i) A free standing wall must be de-signed to be safe as a vertical canti-lever.

This requirement will be difficult toachieve in un-reinforced masonry inZone A. Therefore all partitions in-side the buildings must be held onthe sides as well as top. Parapets ofcategory I and II buildings must bereinforced and held to the mainstructural slabs or frames.

(ii) Horizontal reinforcement in walls isrequired for transferring their ownout-of-plane inertia load horizon-tally to the shear walls.

(iii) The walls must be effectively tiedtogether to avoid separation at verti-cal joints due to ground shaking.

(iv) Shear walls must be present alongboth axes of the building.

(v) A shear wall must be capable of re-sisting all horizontal forces due toits own mass and those transmittedto it.

(vi) Roof or floor elements must be tiedtogether and be capable of exhibit-ing diaphragm action.

(vii) Trusses must be anchored to the sup-porting walls and have an arrange-ment for transferring their inertiaforce to the end walls.

Table 3.1 Categories of buildings for strengthening purposes

Category Combination of conditions for the Category

I Important building on soft soil in zone A

II Important building on firm soil in zone AImportant building on soft soil in zone BOrdinary building on soft soil in zone A

III Important building on firm soil in zone BImportant building on soft soil in zone COrdinary building on firm soil in zone AOrdinary building on soft soil in zone B

IV Important building on firm soil in zone COrdinary building on firm soil in zone BOrdinary building on firm soil in zone C

Notes: (i)Seismic zones A, B and C and important buildings are definedin Section 3.2.

(ii) Firm soil refers to those having safe bearing value more than10 t/m2 and soft those less than 10 t/m2.

(iii) Weak soils liable to compaction and liquefaction under earth-quake condition are not covered here.

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IAEE MANUAL

together so that excessive stress concentra-tions are avoided and forces are capable ofbeing transmitted from one component toanother even through large deformations.

Ductility is a term applied to materialand structures, while deformability is ap-plicable only to structures.

Even when ductile materials are presentin sufficient amounts in structural compo-nents such as beams and walls, overallstructural deformability requires that geo-metrical and material instability beavoided. That is, components must haveproper aspect ratios (that is not be too high),must be adequately connected to resistingelements (for example sufficient wall tiesfor a masonry wall, tying it to floors, roofand shear walls), and must be well tied to-gether (for example positive connection atbeam seats, so that deformations do notpermit a beam to simply fall off a post) soas to permit large deformations and dy-namic motions to occur without suddencollapse.

3.6.3 DamageabilityDamageability is also a desirable qualityfor construction, and refers to the ability ofa structure to undergo substantial damages,without partial or total collapse

A key to good damageability is redun-dancy, or provision of several supports forkey structural members, such as ridgebeams, and avoidance of central columnsor walls supporting excessively large por-tions of a building. A key to achieving gooddamageability is to always ask the ques-tion, �if this beam or column, wall connec-tion, foundation, etc. fails, what is the con-sequence?�. If the consequence is total col-

ture to undergo substantial damage, with-out partial or total collapse. This is desir-able because it means that structures canabsorb more damage, and because it per-mits the deformations to be observed andrepairs or evacuation to proceed, prior tocollapse. In this sense, a warning is receivedand lives are saved.

3.6.1 DuctilityFormally, ductility refers to the ratio of thedisplacement just prior to ultimate dis-placement or collapse to the displacementat first damage or yield. Some materials areinherently ductile, such as steel, wroughtiron and wood. Other materials are notductile (this is termed brittle), such as castiron, plain masonry, adobe or concrete, thatis, they break suddenly, without warning.Brittle materials can be made ductile, usu-ally by the addition of modest amounts ofductile materials, Such as wood elementsin adobe construction, or steel reinforcingin masonry and concrete constructions.

For these ductile materials to achieve aductile effect in the overall behaviour of thecomponent, they must be proportioned andplaced so that they come in tension and aresubjected to yielding. Thus, a necessary re-quirement for good earthquake-resistantdesign is to have sufficient ductile materi-als at points of tensile stresses.

3.6.2 DeformabilityDeformability is a less formal term refer-ring to the ability of a structure to displaceor deform substantial amounts withoutcollapsing. Besides inherently relying onductility of materials and components,deformability requires that structures bewell-proportioned, regular and well tied

9


lapse of the structure, additional supportsor alternative structural layouts should beexamined, or an additional factor of safetybe furnished for such critical members orconnections.

3.7 CONCEPT OF ISOLATIONThe foregoing discussion of earthquake-resistant design has emphasized the tradi-tional approach of resisting the forces anearthquake imposes on a structure. An al-ternative approach which is presentlyemerging is to avoid these forces, by isola-tion of the structure from the ground mo-tions which actually impose the forces onthe structure.

This is termed base-isolation. For sim-ple buildings, base- friction isolation maybe achieved by reducing the coefficient offriction between the structure and its foun-dation, or by placing a flexible connectionbetween the structure and its foundation.

For reduction of the coefficient of fric-tion between the structure and its founda-tion, one suggested technique is to placetwo layers of good quality plastic betweenthe structure and its foundation, so that theplastic layers may slide over each other.

Flexible connections between the struc-ture and its foundation are also difficult toachieve on a permanent basis. One tech-nique that has been used for generationshas been to build a house on short postsresting on large stones, so that under earth-quake motions, the posts are effectively pin-connected at the top and bottom and thestructure can rock to and fro somewhat.This has the advantage of substantially re-ducing the lateral forces, effectively isolat-ing the structure from the high amplitude

high frequency motions. Unfortunately, tra-ditional applications of this technique usu-ally do not account for occasional largedisplacements of this pin-connectedmechanism, due to rare very large earth-quakes or unusually large low-frequencycontent in the ground motion, so that whenlateral displacements reach a certain point,collapse results. A solution to this problemwould be provision of a plinth slightly be-low the level of the top of the posts, so thatwhen the posts rock too far, the structure isonly dropped a centimeter or so.

3.8 FOUNDATIONSFor the purpose of making a building trulyearthquake resistant, it will be necessary tochoose an appropriate foundation type forit. Since loads from typical low heightbuildings will be light, providing the re-quired bearing area will not usually be aproblem. The depth of footing in the soilshould go below the zone of deep freezingin cold countries and below the level ofshrinkage cracks in clayey soils. For choos-ing the type of footing from the earthquakeangle, the soils may be grouped as Firm andSoft (see Section 3.2.3) avoiding the weaksoil unless compacted and brought to Softor Firm condition.

3.8.1 Firm soilIn firm soil conditions, any type of footing(individual or strip type) can be used. Itshould of course have a firm base of lime orcement concrete with requisite width overwhich the construction of the footing maystart. It will be desirable to connect the in-dividual reinforced concrete columnfootings in Zone A by means of RC beamsjust below plinth level intersecting at rightangles.

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IAEE MANUAL

3.8.2 Soft soilIn soft soil, it will be desirable to use a plinthband in all walls and where necessary toconnect the individual column footings bymeans of plinth beams as suggested above.It may be mentioned that continuous rein-forced concrete footings are considered tobe most effective from earthquake consid-erations as well as to avoid differential set-tlements under normal vertical loads. De-tails of plinth band and continuous RC

footings are presented in Chapters 4 and 9respectively.

These should ordinarily be providedcontinuously under all the walls. Continu-ous footing should be reinforced both inthe top and bottom faces, width of the foot-ing should be wide enough to make thecontact pressures uniform, and the depthof footing should be below the lowest levelof weathering.

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1

BUILDINGS IN FIRED-BRICK AND OTHER MASONRY UNITS

Chapter 4

BUILDINGS IN FIRED-BRICK ANDOTHER MASONRY UNITS

4.1 INTRODUCTIONThe buildings in fired bricks, solid concreteblocks and hollow concrete or mortarblocks are dealt with in this chapter. Thegeneral principles and most details ofearthquake resistant design and construc-tion of brick-buildings are applicable tothose using other rectangular masonryunits such as solid blocks of mortar, con-crete, or stabilized soil, or hollow blocks ofmortar, or concrete having adequatecompressive strength. Some constructiondetails only differ for hollow blocks, whichare also indicated as necessary.

4.2 TYPICAL DAM AGE ANDFAILURE OF MASONRYBUILDINGSThe creation of tensile and shearingstresses in walls of masonry buildings isthe primary cause of different types of dam-age suffered by such buildings. The typi-cal damages and modes of failure are brieflydescribed below:

4.2.1 Non-structural damageThe non-structural damage is that due towhich the strength and stability of thebuilding is not affected. Such damage oc-curs very frequently even under moderateintensifies of earthquakes:

� Cracking and overturning of ma-sonry parapets, roof chimney, largecantilever cornices and balconies.

� Falling of plaster from walls and ceil-ing particularly where it was loose.

� Cracking and overturning of parti-tion walls, filler walls and claddingwalls from inside of frames. (Thoughnot usually accounted for in calcu-lations, this type of damage reducedthe lateral strength of the building).

� Cracking and failing of ceilings.

� Cracking of glass panes.

� Failing of loosely placed objects, over-turning of cupboards, etc.

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4.2.2 Damage and failure ofbearing walls

(i) Failure due to racking shear is char-acterized by diagonal cracks whichcould be due to diagonal compres-sion or diagonal tension. Such fail-ure may be either through the pat-tern of joints or diagonally throughmasonry units. These cracks usuallyinitiate at the corner of openings andsometimes at centre of wall segment.This kind of failure can cause par-tial or complete collapse of the struc-ture, Fig 4.1.

(ii) A wall can fail as a bending memberloaded by seismic inertia forces onthe mass of the wall itself in a direc-tion, transverse to the plane of the

wall. Tension cracks occur verticallyat the centre, ends or corners of thewalls. Longer the wall and longer theopenings, more prominent is thedamage, Fig 4.1. Since earthquake ef-fects occur along both axes of a build-ing simultaneously, bending andshearing effects occur often togetherand the two modes of failures areoften combined. Failure in the piersoccur due to combined action offlexure and shear.

(iii) Unreinforced gable end masonrywalls are very unstable and the strut-ting action of purlins imposes addi-tional force to cause their failure.Horizontal bending tension cracksare caused in the gables.

Fig 4.1 Cracking in bearing wall building due to bending and shear

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(iv) The deep beam between two open-ings one above the other is a weakpoint of the wall under lateralinplane forces. Cracking in this zoneoccurs before diagonal cracking ofpiers, Fig 4.2. In order to prevent itand to enable the full distribution ofshear among all piers, either a rigidslab or RC band must exist betweenthem.

(v) Walls can be damaged due to the seis-mic force of the roof, which cancause the formation of tension cracksand separation of supporting walls,Fig 4.3. This mode of failure is thecharacteristic of massive flat roofs (orfloors) supported by joists, which inturn are supported by bearing walls,but without proper connection withthem. Also if the connection withfoundation is not adequate, wallscrack there and slide. This may causefailure of plumbing pipes too.

(vi) Failure due to torsion and warping:The damage in unsymmetrical build-ing occurs due to torsion and warp-ing in an earthquake, Fig 3.1. Thismode of failure causes excessivecracking due to shear in all walls.Larger damage occurs near the cor-ner of the building.

(vii) Arches across openings in walls areoften badly cracked since the archestend to lose their end thrust underin-plane shaking of walls.

(viii) Under severe prolonged intenseground motions, the following hap-pens:

- the cracks become wider and themasonary units become loose

Fig 4.2 Cracking of spandrel wall between opening

Fig 4.3 Fall of roof because of inadequate connection between roof andwall

- partial collapse and gaps inwalls occur due to falling ofloose masonry units, particu-larly at location of piers.

- falling of spandrel masonry dueto collapse of piers

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- falling of gable masonry due toout of plane cantilever action

- walls get separated at cornersand intermediate T-junctionsand fall outwards.

- roof collapse, either partial or full

- certain types of roofs may slideoff the top of walls and the roofbeams fall down

- masonry arches across wallopenings as well as those usedfor roof collapse completely.

4.2.3 Failure of ground(i) Inadequate depth of foundation:

Shallow foundations deteriorate asa result of weathering and conse-quently become weak for earthquakeresistance.

(ii) Differential settlement of founda-tion: During severe ground shaking,liquefaction of loose water-saturatedsands and differential cornpactionof weak loose soils occur which leadto excessive cracking and tilting ofbuildings which may even collapsecompletely.

(iii) Sliding of slopes: Earthquakes causesliding failures in man-made as wellas natural hill slopes and any build-ing resting on such a slope have adanger of complete disastrous dis-integration.

4.2.4 Failure of roofs and floors(i) Dislodging of roofing material: Im-

properly tied roofing material is dis-lodged due to inertia forces actingon the roof. This mode of failure is

typical of sloping roofs, particularlywhen slates, clay, tiles etc. are usedas roofing material.

Brittle material like asbestos cementmay be broken if the trusses andsheeting purlins are not properlybraced together.

(ii) Weak roof to support connection isthe cause of separation of roof trussfrom supports,although completeroof collapse mostly occurs due tocollapse of supporting structure. Therupture of bottom chord of roof trussmay cause a complete collapse oftruss as well as that of walls, Fig 4.4.

(iii) Heavy roofs as used in rural areaswith large thickness of earth overround timbers cause large inertiaforces on top of walls and may leadto complete collapse in severe earth-quake shocks.

(iv) Lean-to roofs easily cause instabil-ity in the lower supporting walls orpiers and collapse easily due to lackof ties.

4.2.5 Causes of damage inmasonry buildingsThe following are the main weaknesses inthe materials and unreinforced masonryconstructions and other reasons for the ex-tensive damage of such buildings:

� Heavy weight and very stiff build-ings, attracting large seismic inertiaforces.

� Very low tensile strength, particu-larly with poor mortars.

� Low shear strength, particularlywith poor mortars.

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� Brittle behaviour in tension as wellas compression.

� Weak connection between wall andwall.

� Stress concentration at corners ofwindows and doors.

� Overall unsymmetry in plan and el-evation of building.

� Unsymmetry due to imbalance in thesizes and positions of openings inthe walls.

� Defects in construction such as useof substandard materials, unfilledjoints between bricks, not-plumbwalls, improper bonding betweenwalls at right angles, etc.

4.2 TYPICAL STRENGTHS OFMASONRYThe crushing strength of masonry used inthe position of walls depends on many fac-tors such as the following:

(i) Crushing strength of the masonryunit.

(ii) Mix of the mortar used and age atwhich tested. The mortar used fordifferent wall constructions varies inquality as well as strength. It is gen-erally described on the basis of themain binding material such as ce-ment or lime mortar, cement limecomposite mortar, lime-pozzolana orhydraulic lime mortar. Clay mudmortar is also used in many coun-tries particular in rural areas.

(iii) Slenderness ratio of the wall, that is,smaller of the ratio of effective heightand effective length of the wall to itsthickness. Larger is the slendernessratio, smaller the strength.

(iv) Eccentricity of the vertical toad onthe wall- Larger the eccentricity,smaller the strength.

(v) Percentage of openings in the wall� larger the openings, smaller thestrength. The tensile and shearingstrengths of masonry mainly dependupon the bond or adhesion at thecontact surface between the masonry

Fig 4.4 Failure due to rupture of bottom chord of roof truss

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unit and the mortar and, in general,their values are only a small percent-age of the crushing strength. Richeris a mortar in cement or lime con-tent, higher is the percentage of ten-sile and shearing strength in relationto the crushing strength. Test carriedout on brick-couplets using handmade bricks in cement mortar givethe typical values as shown inTable 4.1.

Brick couplet tests under combined ten-sion-shear and compression-shear stresses

show that the shearing strength decreaseswhen acting with tension and increaseswhen acting with compression. Fig 4.5shows the combined strengths.

The tensile strength of masonry is notgenerally relied upon for design purposesunder normal loads and the area subjectedto tension is assumed cracked. Under seis-mic conditions, it is recommended that thepermissible tensile and shear stresses onthe area of horizontal mortar bed joint inmasonry may be adopted as given inTable 4.2.

The modulus of elasticity of masonryvery much depends upon the density andstiffness of masonry unit, besides the mor-tar mix. For brickwork the values are of theorder 2000 MPa for cement-sand mortar in1:6 proportion. The mass density of ma-sonry mainly depends on the type of ma-sonry unit. For example brickwork willhave a mass density of about 1900 kg/m3

and dressed stone masonry 2400 kg/m3.

The slenderness ratio of the wall is takenas the lesser of h/t and l/t where h = effectiveheight of the wall and L = its effectivelength. The allowable stresses in Table 4.2must be modified for eccentricity of verticalloading due to its position and seismicmoment and the slenderness ratio multi-plying factors given in Table 4.3. The effec-tive height h may be taken as a factor timesthe actual height of wall between floors, thefactor being 0.75 when floors are rigid dia-phragms and 1.00 for flexible roofs; it willbe 2.0 for parapets.

The effective length L will be a fractionof actual length between lateral supports,the factor being 0.8 for wall continuous

Table 4.1 Typical strengths of masonry

Mortar mix Tensile Shearing Compressive strength in MPacement sand strength, strength, corresponding to crushing

MPa MPa strength of masonry unit

3.5 7.0 10.5 14.0

1 12 0.04 0.22 1.5 2.4 3.3 3.9

1 6 0.25 0.39 2.1 3.3 5.1 6.0

1 3 0.71 1.04 2.4 4.2 6.3 7.5

Fig 4.5 Combined stress couplet test results

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with cross walls or buttresses at both ends,1.0 for continuous at one end and sup-ported on the other and 1.5 for continuousat one and free at the other.

4.4 GENERAL CONSTRUCTIONASPECTS4.4.1 MortarSince tensile and shear strength are impor-tant for seismic resistance of masonry walls,use of mud or very lean mortars will beunsuitable. A mortar mix cement: sandequal to 1:6 by volume or equivalent instrength should be the minimum. Appro-priate mixes for various categories of con-struction are recommended in Table 4.4. Useof a rich mortar in narrow piers betweenopenings will be desirable even if a leanmix is used for walls in general.

4.4.2. Wall enclosureIn load bearing wall construction, the wallthickness �t� should not be kept less than190 mm, wall height not more than 20 t andwall length between cross-walls not morethan 40 t. If longer rooms are required, ei-ther the wall thickness is to be increased, orbuttresses of full height should be providedat 20 t or less apart. The minimum dimen-sions of the buttress shall be as thicknessand top width equal to t and bottom widthequal to one sixth the wall height.

4.4.3 Openings in wallsStudies carried out on the effect of open-ings on the strength of walls indicate thatthey should be small in size and centrallylocated. The following are the guidelineson the size and position of openings:

Table 4.3 Stress factor for slenderness ratio and eccentricity of loading

Slenderness Stress factor, K, for eccentricity ratio, e/t Remarksratio 0 0.04 0.10 0.20 0.30 0.33 0.50

6 1.000 1.000 1.000 0.996 0.984 0.980 0.970 Linear interpolation

8 0.920 0.920 0.920 0.910 0.880 0.870 0.850 may be used.

10 0.840 0.835 0.830 0.810 0.770 0.760 0.730

12 0.760 0.750 0.740 0.706 0.664 0.650 0.600

14 0.670 0.660 0.640 0.604 0.556 0.540 0.480 Values for e/t = 0.5 are

16 0.580 0.565 0.545 0.500 0.440 0.420 0.350 for interpolation only

18 0.500 0.480 0.450 0.396 0.324 0.300 0.230

21 0.470 0.448 0.420 0.354 0.276 0.250 0.170

24 0.440 0.415 0.380 0.310 0.220 0.190 0.110

Table 4.2 Typical permissible stresses

Mortar mix or equivalent Permissible stresses Compression for strength of unit, MPa

cement lime sand tension shear 3.5 7.0 10.5 14.0MPa MPa

1 - 6 0.05 0.08 0.35 0.55 0.85 1.00

1 1 6 0.13 0.20 0.35 0.70 1.00 1.10

1 - 3 0.13 0.20 0.35 0.70 1.05 1.25

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(i) Openings to be located away fromthe inside corner by a clear distanceequal to at least 1/4 of the height ofopenings but not less than 60 cm.

(ii) The total length of openings not toexceed 50 percent of the length of the

wall between consecutive cross wallsin single-storey construction, 42 per-cent in two-storey construction and33 percent in three storey buildings.

(iii) The horizontal distance (pier width)between two openings to be not lessthan half the height of the shorteropening, Fig 4.6, but not less than60 cm.

(iv) The vertical distance from an open-ing to an opening directly above itnot to be less than 60 cm nor lessthan 1/2 of the width of the smalleropening, Fig 4.6.

(v) When the openings do not complywith requirements (i) to (iv), they

Fig 4.6 Recommendation regarding openings in bearing walls

Table 4.4 Recommended mortar mixes

Category of Proportion of cement-lime-sandconstruction*

I Cement-sand 1:4 or cement-lime-sand 1:1:6 or richer

II Cement-lime-sand 1:2:9 or richer

III Cement-sand 1:6 or richer

IV Cement-sand 1:6 or lime-cinder** 1:3 or richer

Notes:* Category of construction is defined in Table 3.1.

** In this case some other pozzolonic material like trass (Indonesia)and surkhi (burnt brick fine powder in India) may be used in placeof cinder.

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should either be boxed in reinforcedconcrete alround or reinforcing barsprovided at the jambs through theMasonry, Fig 4.7.

4.4.4 Masonry bondFor achieving full strength of masonry theusual bonds specified for masonry shouldbe followed so that the vertical joints are

Fig 4.7 Strengthening of masonry around openings

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broken properly from course to course. Thefollowing deserves special mention.

Vertical joint betweenperpendicular wallsFor convenience of construction, buildersprefer to make a toothed joint which is

Fig 4.8 A typical detail of masonry

many times left hollow and weak. To ob-tain full bond it is necessary to make a slop-ing (stepped) joint by making the cornersfirst to a height of 600 mm and then build-ing the wall in between them. Otherwise,the toothed joint should be made in boththe walls alternately in lifts of about 45 cm,Fig 4.8.

4.5 HORIZONTALREINFORCEMENT IN WALLSHorizontal reinforcing of walls is requiredfor imparting to them horizontal bendingstrength against plate action for out ofplane inertia load and for tying the perpen-dicular wall together. In the partition walls,horizontal reinforcement helps preventingshrinkage and temperature cracks. The fol-lowing reinforcing arrangements are nec-essary.

4.5.1 Horizontal bands or ringbeamsThe most important horizontal reinforcing

Table 4.5 Recommendation for steel in RC band

Longitudinal steel in R.C. bandsSpan, m category I category II category III category IV

no of diameter of no of diameter of no of diameter of no of diameter ofbars bars, mm bars bars, mm Bars Bars, mm Bars Bars, mm

5 2 12 2 10 2 10 2 10

6 2 16 2 12 2 10 2 10

7 2 16 2 16 2 12 2 10

8 4 12 2 16 2 16 2 12

9 4 16 4 12 2 16 2 12

Notes: (i) Width of the RC band is assumed to be the same as the thickness of wall. Wall thickness shall be 20 cm minimum. Acover of 25 mm from face of wall will be maintained. For thicker walls, the quantity of steel need not be increased.For thinner walls, see 4.7.

(ii) The vertical thickness of RC band may be kept minimum 75 mm where two longitudinal bars are specified and 150mm where four longitudinal bars are specified.

(iii) Concrete mix to be 1:2:4 by volume or having 15 MPa cube crushing strength at 28 days.

(iv) The longitudinal bars shall be held in position by steel links or stirrups 6 mm diameter spaced at 150 mm apart(see Fig 4.10 (a))

(v) Bar diameters are for mild-steel. For high strength must deformed bars, equivalent diameter may be used.

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Fig 4.9 Gable band and roof band in barrack type buildings

is through reinforced concrete bands pro-vided continuously through all load bear-ing longitudinal and transverse walls atplinth, lintel, and roof-eave levels, also attop of gables according to requirements asstated hereunder:

(i) Plinth band: This should be pro-vided in those cases where the soilis soft or uneven in their propertiesas it usually happens in hill tracts.It will also serve as damp proofcourse. This band is not too critical.

(ii) Lintel band: This is the most impor-tant band and will incorporate in it-self all door and window lintels the

reinforcement of which should be ex-tra to the lintel band steel. It must beprovided in all storeys in buildingsas per Table 4.5.

(iii) Roof band: This band will be re-quired at eave level of trussed roofs,Fig 4.9 and also below or in level withsuch floors, which consist of joistsand covering elements so as to prop-erly integrate them at ends and fixinto the walls.

(iv) Gable band: Masonry gable endsmust have the triangular portion ofmasonry enclosed in a band, the hori-zontal part will be continuous withthe eave level band on longitudinal

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walls, Fig 4.9.

4.5.2 Section of bands or ringbeamsThe reinforcement and dimensions of these

Fig 4.10 Reinforcement in RC band

bands may be kept as follows for wall spansupto 9 m between the cross walls or but-tresses. For longer spans, the size of bandmust be calculated.

A band consists of two (or four) longitu-dinal steel bars with links or stirrups em-bedded in 75 mm (or 50 mm), thick con-crete, Fig 4.10. The thickness of band maybe made equal to or a multiple of masonryunit and its width should equal the thick-ness of wall. The steel bars are located closeto the wall faces with 25 mm cover and fullcontinuity is provided at corners and junc-tions. The minimum size of band andamount of reinforcing will depend uponthe unsupported length of wall betweencross walls and the effective seismic coeffi-cient based on seismic zone, importance ofbuildings, type of soil and storey of thebuilding.

Appropriate steel and concrete sizes arerecommended for various buildings inTable 4.5. Such bands are to be located atcritical levels of the building, namely plinth,lintel, roof and gables according to require-ments (see 4.5.1).

4.5.3 Dowels at corners andjunctionsAs a supplement to the bands described in(a) above, steel dowel bars may be used atcorners and T-junctions to integrate the boxaction of walls. Dowels, Fig 4.11, are placedin every fourth course or at about 50 cmintervals and taken into the walls to suffi-cient length so as to provide the full bondstrength. Wooden dowels can also be usedinstead of steel. However, the dowels donot serve to reinforce the walls in horizon-tal bending except near the junctions.

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Fig 4.11 (a) Corner-strengthening by dowel reinforcement placed in one joint (b) Corner-strengthening by dowelreinforcement placed in two consecutive joints. (c) T-junction - strengthening by dowel reinforcements(d) Strengthening by wire fabric at junction and corner

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4.6 VERTICALREINFORCEMENT IN WALLSThe need for vertical reinforcing of shearwalls at critical sections was establishedin Para 2.6.7. The critical sections were thejambs of openings and the corners of walls.The amount of vertical reinforcing steel willdepend upon several factors like thenumber of storeys, storey heights, the effec-tive seismic coefficient based on seismiczone, importance of building and soil foun-dation type. Values based on rough esti-mates for building are given in Table 4.6 forready use. The steel bars are to be installedat the critical sections, that is the corners ofwalls and jambs of doors right, from thefoundation concrete and covered with ce-ment concrete in cavities made around themduring masonry construction. This concretemix should be kept 1:2:4 by volume or richer.Typical arrangements of placing the verti-cal steel in brick work are shown inFig 4.12.

The jamb steel was shown in Fig 4.7.The jamb steel of window openings will beeasiest to provide in box form around it.The vertical steel of opening may bestopped by embedding it into the lintel bandbut the vertical steel at corners and junc-tions of walls must be taken into the floorand roof slabs or roof band

The total arrangement of providing re-inforcing steel in masonry wall construc-tion is schematically shown in Fig 4.13.

4.7 FRAMING OF THIN LOADBEARING WALLSIf load-bearing walls are made thinner than200 mm, say 150 mm inclusive of plaster-ing on both sides, reinforced concrete fram-ing columns and collar beams are neces-sary which are constructed to have fullbond with the walls. Columns are to be lo-cated at all corners and junctions of wallsand at not more than 1.5 m apart but solocated as to frame up the doors and win-dows. The horizontal bands or ring beamsare located at all floors, roof as well as lin-tel levels of the openings. The sequence ofconstruction between walls and columnsis: first to build the wall upto 4 to 6 coursesheight leaving toothed gaps (tooth projec-tion being about 40 mm only) for the col-umns and second to pour 1:2:4 concrete tofill the columns against the walls usingwood -forms only or two sides. Needless tosay that column steel should be accuratelyheld in position all along. The band con-crete should be cast on the wall masonrydirectly so as to develop full bond with it.

Such construction may be limited to onlytwo storeys maximum in view of its verti-cal load carrying capacity. The horizontallength of walls between cross walls may be

Table 4.6 Recommendation for vertical steel at critical sections

No of Storeys Diameter of mild steel single bar in mm ateach critical section for category (1)

category I category II categoryIII category IV

One 16 12 12 Nil

Two Top 16 12 12 NilBottom 20 16 16 Nil

Three Top 16 12 12 NilMiddle 20 16 12 NilBottom 20 16 16 Nil

Four Top (2) (2) 12 12Third 12 12

Second 16 12Bottom 16 12

Notes: (i)Category of construction is defined in Table 3.1. Equivalentarea of twisted grip bars or a number of mild steel bars could beused but the diameter should not be less than 12 mm.

(ii) Four storeyed load bearing wall construction may not be usedfor categories I and II buildings.

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Fig 4.12 Vertical reinforcement in walls

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restricted to 7 m and the storey height to3 m.

4.8 REINFORCING DETAILSFOR HOLLOW BLOCKMASONRYThe following details may be followed inplacing the horizontal and vertical steel in

hollow block masonry using cement-sandor cement concrete blocks.

4.8.1 Horizontal bandU-shaped blocks may best be used for con-struction the horizontal bands at variouslevels of the storeys as per seismic require-ments, as shown in, Fig 4.14.

The amount of horizontal reinforcementmay be taken 25 percent more than thatgiven in Table 4.5 and provided by usingfour bars and 6mm dia stirrups. Other con-tinuity details shall be followed as shownin Fig 4.10.

4.8.2 Vertical reinforcementThe vertical bars as specified in Table 4.6may conveniently be located inside thecavities of the hollow blocks, one bar in onecavity. Where more than one bar is planned,

Fig 4.14 U-blocks for horizontal bands

Fig 4.15 Vertical reinforcement in cavities

Fig 4.13 Overall arrangement of reinforcing masonry buildings

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these can be located in two or three con-secutive cavities as shown inFig 4.15. The cavities containing bars areto be filled by using micro-concrete 1:2:3 orcement- coarse sand mortar 1:3 and prop-erly rodded for compaction.

Practical difficulty is faced in thread-ing the bars through the hollow blockssince the bars have to be set in footings andhave to be kept standing vertically whilelifting the blocks whole storey heights,threading the bar into the cavity and low-ering it down to the bedding level. To avoidlifting of blocks too high, the bars are madeshorter and overlapped with upper por-tions of bars. This is wastefull of steel aswell as the bond strength in small cavitiesremains doubtful. For solving this problem,two alternatives may be used as shown inFig.4.16 (a) use of three sided or U-block (b)bent interlocked bars.

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Chapter 2

STRUCTURAL PERFORMANCE DURINGEARTHQUAKES

2.1 INTRODUCTIONEarthquakes are natural hazards underwhich disasters are mainly caused by dam-age to or collapse of buildings and otherman-made structures. Experience hasshown that for new constructions, estab-lishing earthquake resistant regulationsand their implementation is the criticalsafeguard against earthquake-induceddamage. As regards existing structures, itis necessary to evaluate and strengthenthem based on evaluation criteria before anearthquake.

Earthquake damage depends on manyparameters, including intensity, durationand frequency content of ground motion,geologic and soil condition, quality of con-struction, etc. Building design must be suchas to ensure that the building has adequatestrength, high ductility, and will remain asone unit, even while subjected to very largedeformation.

Sociologic factors are also important,such as density of population, time of dayof the earthquake occurrence and commu-

nity preparedness for the possibility of suchan event.

Up to now we can do little to diminishdirect earthquake effects. However we cando much to reduce risks and thereby reducedisasters provided we design and build orstrengthen the buildings so as to minimizethe losses based on the knowledge of theearthquake performance of different build-ing types during an earthquake.

Observation of structural performanceof buildings during an earthquake canclearly identify the strong and weak aspectsof the design, as well as the desirable quali-ties of materials and techniques of construc-tion, and site selection. The study of dam-age therefore provides an important step inthe evolution of strengthening measures fordifferent types of buildings.

This Chapter discusses earthquake per-formance of structures during earthquakeintensity, ground shaking effects on struc-tures, site condition effects on buildingdamage, other factors affecting damage,

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failure mechanisms of structures, earth-quake damage and damage categories.

Typical patterns of damage for specifictypes of construction are discussed in therespective chapters.

2.2 EARTHQUAKE EFFECTSThere are four basic causes of earthquake-induced damage: ground shaking, groundfailure, tsunamis and fire.

2.2.1 Ground shakingThe principal cause of earthquake-induceddamage is ground shaking. As the earthvibrates, all buildings on the ground sur-face will respond to that vibration in vary-ing degrees. Earthquake inducedaccelerations, velocities and displacementscan damage or destroy a building unless ithas been designed and constructed orstrengthened to be earthquake resistant.Therefore, the effect of ground shaking onbuildings is a principal area of considera-tion in the design of earthquake resistantbuildings. Seismic design loads are ex-tremely difficult to determine due to the ran-dom nature of earthquake motions. How-ever, experiences from past strong earth-quakes have shown that reasonable andprudent practices can keep a building safeduring an earthquake.

2.2.2 Ground failureEarthquake-induced ground failure hasbeen observed in the form of ground rup-ture along the fault zone, landslides, settle-ment and soil liquefaction.

Ground rupture along a fault zone maybe very limited or may extend over hun-dreds of kilometers. Ground displacementalong the fault may be horizontal, vertical

or both, and can be measured in centimetersor even metres. Obviously, a building di-rectly astride such a rupture will be severelydamaged or collapsed.

While landslide can destroy a building,the settlement may only damage the build-ing.

Soil liquefaction can occur in low den-sity saturated sands of relatively uniformsize. The phenomenon of liquefaction isparticularly important for dams, bridges,underground pipelines, and buildingsstanding on such ground.

2.2.3 TsunamisTsunamis or seismic sea waves are gener-ally produced by a sudden movement ofthe ocean floor. As the water waves ap-proach land, their velocity decreases andtheir height increases from 5 to 8 m, or evenmore. Obviously, tsunamis can be devas-tating for buildings built in coastal areas.

2.2.4 FireWhen the fire following an earthquakestarts, it becomes difficult to extinguish it,since a strong earthquake is accompaniedby the loss of water supply and traffic jams.Therefore, the earthquake damage increaseswith the earthquake-induced fire in addi-tion to the damage to buildings directly dueto earthquakes. In the case of the 1923 Kantoearthquake 50% of Tokyo and 70% of thetotal number of houses were burnt and morethan 100,000 people were killed by the fire.

2.3 GROUND SHAKING EFFECTON STRUCTURES2.3.1 Inertia forcesBuildings are fixed to the ground as shownin Fig 2.1(a). As the base of a building moves

3


the superstructure including its contentstends to shake and vibrate from the posi-tion of rest, in a very irregular manner dueto the inertia of the masses.

When the base of the building suddenlymoves to the right, the building moves tothe left relative the base, Fig 2.1(b), as if itwas being pushed to the left by an unseenforce which we call �Inertia Force�. Actu-ally, there is no push at all but, because ofits mass, the building resists any motion.The process is much more complex becausethe ground moves simultaneously in threemutually perpendicular directions duringan earthquake as shown in Fig 2.1 (b), (c),and (d).

2.3.2 Seismic loadThe resultant lateral force or seismic loadis represented by the force F as shown in

Fig 2.1(e). The force F is distinctly differentfrom the dead, live, snow, wind, and im-pact loads. The horizontal ground motionaction is similar to the effect of a horizontalforce acting on the building, hence the term�Seismic Load�. As the base of the build-ing moves in an extremely complicatedmanner, inertia forces are created through-out the mass of the building and its con-tents. It is these reversible forces that causethe building to move and sustain damageor collapse.

Additional vertical load effect is causedon beams and columns due to vertical vi-brations. Being reversible, at certain in-stants of time the effective load is increased,at others it is decreased.

The earthquake loads are dynamic andimpossible to predict precisely in advance,

Fig 2.1 Seismic vibrations of a building and resultant earthquake force

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IAEE MANUAL

since every earthquake exhibits differentcharacteristics. The following equivalentminimum total lateral force is, used for seis-mic design:

F=S.Fs.I.C.W

Where S, Fs, I, C and W are the factorsaffecting seismic load, which will be ex-plained in the following section.

2.3.3 Factors affecting seismic loadThe earthquake zone factor S dependsupon the ground intensity of the earth-quake. The value of S usually is plotted onmaps in terms of seismic intensity isolinesor maximum acceleration isolines. Obvi-ously, the higher the intensity or accelera-tion, the larger will be the seismic force.

The soil-foundation factor Fs dependsupon the ratio of fundamental elastic pe-riod of vibration of a building in the direc-tion under consideration and the charac-teristic site period. Therefore, Fs is a numeri-cal coefficient for site-building resonance.

The occupancy importance or hazardfactor I depends upon the usage of thebuilding. The higher the importance orlarger the hazard caused by the failure ofthe building, the greater the value of thefactor I.

The C is a factor depending on the stiff-ness and damping of the structure. Largerthe stiffness for given mass, shorter the fun-damental period of vibration of the struc-ture and larger the value of C. Damping isthe energy dissipation property of the build-ing; larger the damping, smaller the valueof C.

The W is the total weight of the super-structure of a building including its con-tents. The inertia forces are proportional tothe mass of the building and only that partof the loading action that possesses masswill give rise to seismic force on the build-ing. Therefore, the lighter the material, thesmaller will be the seismic force.

2.3.4 Nature of seismic stressesThe horizontal seismic forces are reversiblein direction. The structural elements suchas walls, beams and columns that werebearing only vertical loads before the earth-quake, have now to carry horizontal bend-ing and shearing effects as well. When thebending tension due to earthquake exceedsthe vertical compression, net tensile stresswill occur. If the building material is weakin tension such as brick or stone masonry,cracking occurs which reduces the effec-tive area for resisting bending moment, asshown in Fig 2.2. It follows that the strengthin tension and shear is important for earth-quake resistance.

2.3.5 Important parameters inseismic designIt follows that the following properties andparameters are most important from thepoint of view of the seismic design.

(i) Building material properties

� Strength in compression, tensionand shear, including dynamic ef-fects

� Unit weight

� Modulus of elasticity

(ii) Dynamic characteristics of the build-ing system, including periods,modes and dampings.

5


(iii) Load-deflection characteristics ofbuilding components.

2.4 Effect of site conditions onbuilding damagePast earthquakes show that site conditionsignificantly affects the building damage.Earthquake studies have almost invariablyshown that the intensity of a shock is di-rectly related to the type of soil layers sup-porting the building. Structures built onsolid rock and firm soil frequently fares bet-ter than buildings on soft ground. This wasdramatically demonstrated in the 1985Mexico City earthquake, where the damageon soft soils in Mexico City, at an epicentraldistance of 400 km, was substantiallyhigher than at closer locations.

From studies of the July 28, 1957 earth-quake in Mexico City, it was already knownfor example that the damage on the soft soils

in the center of the city could be 5 to 50 timeshigher than on firmer soils in the surround-ing area. Another example occurred in the1976 Tangshan, China earthquake, inwhich 50% of the buildings on thick soilsites were razed to the ground, while only12% of the buildings on the rock subsoilnear the mountain areas totally collapsed.Rigid masonry buildings resting on rockmay on the contrary show more severe dam-age than when built on soil during a nearearthquake as in Koyna (India) earthquakeof 1967 and North Yemen earthquake of1980.

Lessons learned from recent earthquakeshow that the topography of a building sitecan also have an effect on damage. Build-ings built on sites with open and even to-pography are usually less damaged in anearthquake than buildings on strip-shapedhill ridges, separated high hills, and steep

Fig 2.2 Stress condition in a wall element

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IAEE MANUAL

slopes.

2.5 OTHER FACTORSAFFECTING DAMAGEThe extent of damage to a building dependsmuch on the strength, ductility, and integ-rity of a building and the stiffness of groundbeneath it in a given intensity of the earth-quake motions.

Almost any building can be designed tobe earthquake resistant provided its site issuitable. Buildings suffer during an earth-quake primarily because horizontal forcesare exerted on a structure that often meantto contend only with vertical stresses. Theprincipal factors that influence damage tobuildings and other man-made structuresare listed below:

2.5.1 Building configurationAn important feature is regularity and sym-metry in the overall shape of a building. Abuilding shaped like a box, as rectangularboth in plan and elevation, is inherentlystronger than one that is L-shaped or U-shaped, such as a building with wings. Anirregularly shaped building will twist as itshakes, increasing the damage.

2.5.2 Opening sizeIn general, openings in walls of a buildingtend to weaken the walls, and fewer theopenings less the damage it will suffer dur-ing an earthquake. If it is necessary to havelarge openings through a building, or if anopen first floor is desired, then special pro-visions should be made to ensure structuralintegrity.

2.5.3 Rigidity distributionThe rigidity of a building along the verticaldirection should be distributed uniformly.

Therefore, changes in the structural systemof a building from one floor to the next willincrease the potential for damage, andshould be avoided. Columns or shear wallsshould run continuously from foundationto the roof, without interruptions or changesin material.

2.5.4 DuctilityBy ductility is meant the ability of the build-ing to bend, sway, and deform by largeamounts without collapse. The oppositecondition in a building is called brittlenessarising both from the use of materials thatare inherently brittle and from the wrongdesign of structures using otherwise duc-tile materials. Brittle materials crack underload; some examples are adobe, brick andconcrete blocks. It is not surprising thatmost of the damage during the past earth-quakes was to unreinforced masonry struc-tures constructed of brittle materials, poorlytied together. The addition of steel reinforce-ments can add ductility to brittle materials.Reinforced concrete, for example, can bemade ductile by proper use of reinforcingsteel and closely spaced steel ties.

2.5.5 FoundationBuildings, which are structurally strong towithstand earthquakes sometimes fail dueto inadequate foundation design. Tilting,cracking and failure of superstructuresmay result from soil liquefaction and dif-ferential settlement of footing.

Certain types of foundations are moresusceptible to damage than others. For ex-ample, isolated footings of columns arelikely to be subjected to differential settle-ment particularly where the supportingground consists of different or soft types ofsoil. Mixed types of foundations within the

7


same building may also lead to damage dueto differential settlement.

Very shallow foundations deterioratebecause of weathering, particularly whenexposed to freezing and thawing in the re-gions of cold climate.

2.5.6 Construction qualityIn many instances the failure of buildingsin an earthquake has been attributed topoor quality of construction, substandardmaterials, poor workmanship, e. g., inad-equate skill in bonding, absence of�through stones� or bonding units, andimproper and inadequate construction.

2.6 FAILURE MECHANISMS OFEARTHQUAKES2.6.1 Free standing masonrywallConsider the free standing masonry wallsshown in Fig 2.3. In Fig 2.3(a), the groundmotion is acting transverse to a free stand-ing wall A. The force acting on the mass ofthe wall tends to overturn it. The seismicresistance of the wall is by virtue of itsweight and tensile strength of mortar andit is obviously very small. This wall willcollapse by overturning under the groundmotion.

The free standing wall B fixed on theground in Fig 2.3(b) is subjected to groundmotion in its own plane. In this case, thewall will offer much greater resistance be-cause of its large depth in the plane of bend-ing. Such a wall is termed a shear wall.The damage modes of an unreinforcedshear wall depend on the length-to-widthratio of the wall. A wall with small length-to-depth ratio will generally develop a hori-zontal crack due to bending tension and

then slide due to shearing. A wall withmoderate length-to-width ratio and bound-ing frame diagonally cracks due to shear-ing as shown at Fig 2.3 (c).

A wall with large length-to-width ratio,on the other hand, may develop diagonaltension cracks at both sides and horizontalcracks at the middle as shown at Fig 2.3 (d).

2.6.2 Wall enclosure without roofNow consider the combination of walls Aand B as an enclosure shown in Fig 2.4. Forthe X direction of force as shown, walls Bact as shear walls and, besides taking theirown inertia, they offer resistance againstthe collapse of wall A as well. As a resultwalls A now act as vertical slabs supportedon two vertical sides and the bottom plinth.The walls A are subjected to the inertia forceon their own mass. Near the vertical edges,the wall will carry reversible bending mo-

Fig 2.3 Failure mechanism of free standing walls

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ments in the horizontal plane for which themasonry has little strength. Consequentlycracking and separation of the walls mayoccur along these edges shown in the fig-ure.

It can be seen that in the action of wallsB as shear walls, the walls A will act asflanges connected to the walls B acting asweb. Thus if the connection between wallsA and B is not lost due to their bonding ac-

tion as plates, the building will tend to actas a box and its resistance to horizontalloads will be much larger than that of wallsB acting separately. Most unreinforced ma-sonry enclosures, however, have very weakvertical joints between walls meeting atright angles due to the construction proce-dure involving toothed joint that is gener-ally not properly filled with mortar. Conse-quently the corners fail and lead to collapseof the walls. It may also be easily imaginedthat the longer the walls in plan, the smallerwill be the support to them from the crosswalls and the lesser will be the box effect.

2.6.3 Roof on two wallsIn Fig 2.5 (a) roof slab is shown to be restingon two parallel walls B and the earthquakeforce is acting in the plane of the walls.Assuming that there is enough adhesionbetween the slab and the walls, the slabwill transfer its inertia force at the top ofwalls B, causing shearing and overturningaction in them. To be able to transfer its in-ertia force to the two end walls, the slabmust have enough strength in bending inthe horizontal plane. This action of slab isknown as diaphragm action. Reinforcedconcrete or reinforced brick slabs have suchstrength inherently and act as rigid dia-phragms. However, other types of roofs orfloors such as timber or reinforced concretejoists with brick tile covering will be veryflexible. The joists will have to be connectedtogether and fixed to the walls suitably sothat they are able to transfer their inertiaforce to the walls. At the same time, the wallsB must have enough strength as shear wallsto withstand the force from the roof and itsown inertia force. Obviously, the structureshown in Fig 2.5, when subjected to groundmotion perpendicular to its plane, will col-

Fig 2.4 Failure mechanism of wall enclosure without roof

Fig 2.5 Roof on two walls

9


lapse very easily because walls B have lit-tle bending resistance in the plane perpen-dicular to it. In long barrack type buildingswithout intermediate walls, the end wallswill be too far to offer much support to thelong walls and the situation will be simi-lar to the one just mentioned above.

2.6.4 Roof on wall enclosureNow consider a complete wall enclosurewith a roof on the top subjected to earth-quake force acting along X-axis as shownin Fig 2.6. If the roof is rigid and acts as ahorizontal diaphragm, its inertia will bedistributed to the four walls in proportionto their stiffness. The inertia of roof will al-most entirely go to walls B since the stiff-ness of the walls B is much greater than thewalls A in X direction. In this case, the plateaction of walls A will be restrained by theroof at the top and horizontal bending ofwall A will be reduced. On the other hand,if the roof is flexible the roof inertia will goto the wall on which it is supported andthe support provided to plate action ofwalls A will also be little or zero. Again theenclosure will act as a box for resisting thelateral loads, this action decreasing in valueas the plan dimensions of the enclosuresincrease.

2.6.5 Roofs and floorsThe earthquake-induced inertia force canbe distributed to the vertical structural ele-ments in proportion to their stiffness, pro-vided the roofs and floors are rigid to act ashorizontal diaphragms. Otherwise, the roofand floor inertia will only go to the verticalelements on which they are supported.Therefore, the stiffness and integrity of roofsand floors are important for earthquake re-sistance.

The roofs and floors, which are rigidand flat and are bonded or tied to the ma-sonry, have a positive effect on the wall,such as the slab or slab and beam construc-tion be directly cast over the walls or jackarch floors or roofs provided with horizon-tal ties and laid over the masonry wallsthrough good quality mortar. Others thatsimply rest on the masonry walls will offerresistance to relative motion only throughfriction, which may or may not be adequatedepending on the earthquake intensity. Inthe case of a floor consisting of timber joistsplaced at center to center spacing of 20 to25 cm with brick tiles placed in directly overthe joists and covered with clayey earth, thebrick tiles have no binding effect on the

Fig 2.6 Roof on wall enclosure

Fig 2.7 Long building with roof trusses

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joists. Therefore, relative displacement ofthe joists is quite likely to occur during anearthquake, which could easily bring downthe tiles, damaging property and causinginjury to people. Similar behaviour may bevisualized with the floor consisting ofprecast reinforced concrete elements notadequately tied together. In this case, rela-tive displacement of the supporting wallscould bring down the slabs.

2.6.6 Long building with rooftrussesConsider a long building with a single spanand roof trusses as shown in Fig 2.7. Thetrusses rest on the walls A. The walls B aregabled to receive the purlins of the endbays. Assuming that the ground motion isalong the X-axis, the inertia forces will betransmitted from sheeting to purlins totrusses and from trusses to wall A.

Fig 2.8 Deformation of a shear wall with openings.

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The end purlins will transmit someforce directly to gable ends. Under the seis-mic force the trusses may slide on the wallsunless anchored into them by bolts. Also,the wall A, which does, not get much sup-port from the walls B in this case, may over-turn unless made strong enough in the ver-tical bending as a cantilever or other suit-able arrangement, such as adding horizon-tal bracings between the trusses, is madeto transmit the force horizontally to endwalls B.

When the ground motion is along Y di-rection, walls A will be in a position to actas shear walls and all forces may be trans-

mitted to them. In this case, the purlins actas ties and struts and transfer the inertiaforce of roof to the gable ends.

As a result the gable ends may fail. Whenthe gable triangles are very weak in stabil-ity, they may fail even in small earthquakes.Also, if there is insufficient bracing in theroof trusses, they may overturn even whenthe walls are intact.

2.6.7 Shear wall with openingsShear walls are the main lateral earthquakeresistant elements in many buildings. Forunderstanding their action, let us considera shear wall with three openings shown in

Table 2.1 Categories of damage

Damage category Extent of damage Suggested post- earthquakein general actions

0 No damage No damage No action required

I Slighty non-structural Thin cracks in plaster, falling of Building need not be vacated.damage plaster bits in limited parts. Only architectural repairs

needed.

II Slight Structural Small cracks in walls, failing of Building need not be vacated.Damage plaster in large bits over large areas; Architectural repairs required

damage to non-structural parts like to achieve durability.chimneys, projecting cornices, etc.The load carrying capacity of thestructure is not reduced appreciably.

III Moderate structural Large and deep cracks in walls; Building needs to be vacated, todamage widespread cracking of walls, be reoccupied after restoration

columns, piers and tilting or failing and strengthening.of chimneys. The load carrying Structural restoration andcapacity of the structure is partially seismic strengthening arereduced. necessary after which architec

tural treatment may be carriedout.

IV Severe structural Gaps occur in walls; inner and outer Building has to be vacated.damage walls collapse; failure of ties to Either the building has to be

separate parts of buildings. Approx. demolished or extensive50 % of the main structural restoration and strengtheningelements fail.The building takes work has to be carried outdangerous state. before reoccupation.

V Collapse A large part or whole of the building Clearing the site andcollapses. reconstruction.

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Fig 2.8. Obviously, the piers between theopenings are more flexible than the portionof wall below (sill masonry) or above(spandrel masonry) the openings. The de-flected form under horizontal seismic forceis also sketched in the figure.

The sections at the level of the top andbottom of opening are found to be the worststressed in tension as well as in compres-sion and those near the mid-height of pierscarry the maximum shears. Under reverseddirection of horizontal loading the sectionscarrying tensile and compressive stresses

are also reversed. Thus it is seen that ten-sion occurs in the jambs of openings and atthe corners of the walls.

2.7 EARTHQUAKE DAMAGECATEGORIESIn this section, an outline of damage cat-egories is simply described in Table 2.1 onthe basis of past earthquake experience.Therein the appropriate post-earthquakeaction for each category of damage is alsosuggested.

��

1


Chapter 3

GENERAL CONCEPTS OF EARTHQUAKERESISTANT DESIGN

3.1 INTRODUCTIONExperience in past earthquakes has dem-onstrated that many common buildingsand typical methods of construction lackbasic resistance to earthquake forces. Inmost cases this resistance can be achievedby following simple, inexpensive princi-ples of good building construction prac-tice. Adherence to these simple rules willnot prevent all damage in moderate or largeearthquakes, but life threatening collapsesshould be prevented, and damage limitedto repairable proportions. These principlesfall into several broad categories:

(i) Planning and layout of the buildinginvolving consideration of the loca-tion of rooms and walls, openingssuch as doors and windows, thenumber of storeys, etc. At this stage,site and foundation aspects shouldalso be considered.

(ii) Lay out and general design of thestructural framing system with spe-cial attention to furnishing lateralresistance, and

(iii) Consideration of highly loaded andcritical sections with provision of

reinforcement as required.

Chapter 2 has provided a good overviewof structural action, mechanism of damageand modes of failure of buildings. Fromthese studies, certain general principleshave emerged:

(i) Structures should not be brittle orcollapse suddenly. Rather, theyshould be tough, able to deflect ordeform a considerable amount.

(ii) Resisting elements, such as bracingor shear walls, must be providedevenly throughout the building, inboth directions side-to-side, as wellas top to bottom.

(iii) All elements, such as walls and theroof, should be tied together so as toact as an integrated unit duringearthquake shaking, transferringforces across connections and pre-venting separation.

(iv) The building must be well connectedto a good foundation and the earth.Wet, soft soils should be avoided, andthe foundation must be well tied to-gether, as well as tied to the wall.

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Where soft soils cannot be avoided,special strengthening must be pro-vided.

(v) Care must be taken that all materialsused are of good quality, and are pro-tected from rain, sun, insects andother weakening actions, so that theirstrength lasts.

(vi) Unreinforced earth and masonryhave no reliable strength in tension,and are brittle in compression. Gen-erally, they must be suitably rein-forced by steel or wood.

These principles will be discussed andillustrated in this Chapter.

3.2 CATEGORIES OFBUILDINGSFor categorising the buildings with thepurpose of achieving seismic resistance ateconomical cost, three parameters turn outto be significant:

(i) Seismic intensity zone where thebuilding is located,

(ii) How important the building is, and

(iii) How stiff is the foundation soil.

A combination of these parameters willdetermine the extent of appropriate seismicstrengthening of the building.

3.2.1 Seismic zonesIn most countries, the macro level seismiczones are defined on the basis of SeismicIntensity Scales. In this guide, we shall re-fer to seismic zones as defined with refer-ence to MSK Intensity Scale as described inAppendix I for buildings.

Zone A: Risk of Widespread Collapseand Destruction (MSK IX orgreater),

Zone B: Risk of Collapse and HeavyDamage (MSK VIII likely),

Zone C: Risk of Damage (MSK VII likely),

Zone D: Risk of Minor Damage(MSK VI maximum).

The extent of special earthquakestrengthening should be greatest in ZoneA and, for reasons of economy, can be de-creased in Zone C, with relatively little spe-cial strengthening in Zone D. However,since the principles stated in 3.1, are goodprinciples for building in general (not justfor earthquake), they should always be fol-lowed.

3.2.2 Importance of buildingThe importance of the building should be afactor in grading it for strengtheningpurposes,and the following buildings aresuggested as specially important:

IMPORTANT � Hospitals, clinics, com-munication buildings, fire and police sta-tions, water supply facilities, cinemas, thea-tres and meeting halls, schools, dormito-ries, cultural treasures such as museums,monuments and temples, etc.

ORDINARY � Housings, hostels, of-fices, warehouses, factories, etc.

3.2.3 Bearing capacity offoundation soilThree soil types are considered here:

Firm: Those soils which have an allowablebearing capacity of morethan 10 t/m2

3


Soft: Those soils, which have allowablebearing capacity less than or equalto 10 t/m2.

Weak: Those soils, which are liable to largedifferential settlement, or liquefac-tion during an earthquake.

Buildings can be constructed on firmand soft soils but it will be dangerous tobuild them on weak soils. Hence appropri-ate soil investigations should be carried outto establish the allowable bearing capacityand nature of soil. Weak soils must beavoided or compacted to improve them soas to qualify as firm or soft.

3.2.4 Combination ofparametersFor defining the categories of buildings forseismic strengthening purposes, four cat-egories I to IV are defined in Table 3.1. inwhich category I will require maximumstrengthening and category IV the least in-puts. The general planning and designingprinciples are, however, equally applica-ble to them.

3.3. GENERAL PLANNING ANDDESIGN ASPECTS3.3.1. Plan of building

(i) Symmetry: The building as a wholeor its various blocks should be keptsymmetrical about both the axes.Asymmetry leads to torsion duringearthquakes and is dangerous,Fig 3.1. Symmetry is also desirablein the placing and sizing of door andwindow openings, as far as possi-ble.

(ii) Regularity: Simple rectangularshapes, Fig 3.2 (a) behave better inan earthquake than shapes with

many projections Fig 3.2 (b). Tor-sional effects of ground motion arepronounced in long narrow rectan-gular blocks. Therefore, it is desirableto restrict the length of a block tothree times its width. If longerlengths are required two separateblocks with sufficient separation inbetween should be provided,Fig 3.2 (c).

(iii) Separation of Blocks: Separation of alarge building into several blocksmay be required so as to obtain sym-metry and regularity of each block.

Fig 3.1 Torsion of unsymmetrical plans

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IAEE MANUAL

For preventing hammering orpounding damage between blocks aphysical separation of 3 to 4 cmthroughout the height above theplinth level will be adequate as wellas practical for upto 3 storeyedbuildings, Fig 3.2 (c).

The separation section can be treatedjust like expansion joint or it may befilled or covered with a weak mate-rial which would easily crush andcrumble during earthquake shaking.Such separation may be considered

in larger buildings since it may notbe convenient in small buildings.

(iv) Simplicity: Ornamentationinvo1ving large cornices, vertical orhorizontal cantilever projections, fa-cia stones and the like are danger-ous and undesirable from a seismicviewpoint. Simplicity is the best ap-proach.

Where ornamentation is insistedupon, it must be reinforced withsteel, which should be properly em-

Fig 3.2 Plan of building blocks.

5


bedded or tied into the main struc-ture of the building.

Note: If designed, a seismic coeffi-cient about 5 times the coefficientused for designing the main struc-ture should be used for cantileverornamentation.

(v) Enclosed Area: A small building en-closure with properly intercon-nected walls acts like a rigid boxsince the earthquake strength whichlong walls derive from transversewalls increases as their length de-creases.

Therefore structurally it will be ad-visable to have separately enclosedrooms rather than one long room,Fig 3.3. For unframed walls of thick-ness t and wall spacing of a, a ratioof a/t = 40 should be the upper limitbetween the cross walls for mortarsof cement sand 1:6 or richer, and lessfor poor mortars. For larger panelsor thinner walls, framing elementsshould be introduced as shown atFig 3.3(c).

(vi) Separate Buildings for DifferentFunctions: In view of the differencein importance of hospitals, schools,assembly halls, residences, commu-nication and security buildings, etc.,it may be economical to plan sepa-rate blocks for different functions soas to affect economy in strengthen-ing costs.

3.3.2 Choice of siteThe choice of site for a building from theseismic point of view is mainly concernedwith the stability of the ground. The fol-lowing are important:

(i) Stability of Slope: Hillside slopes li-able to slide during an earthquakeshould be avoided and only stableslopes should be chosen to locate thebuilding. Also it will be preferable

Fig 3.3 Enclosed area forming box units

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IAEE MANUAL

3.3.4 Fire resistanceIt is not unusual during earthquakes thatdue to snapping of electrical fittings shortcircuiting takes place, or gas pipes maydevelop leaks and catch fire. Fire could alsobe started due to kerosene lamps andkitchen fires. The fire hazard sometimescould even be more serious than the earth-quake damage. The buildings should there-fore preferably be constructed of fire resist-ant materials.

3.4 STRUCTURAL FRAMINGThere are basically two types structuralframing possible to withstand gravity andseismic load, viz. bearing wall constructionand framed construction. The framed con-struction may again consist of:

(i) Light framing members which musthave diagonal bracing such as woodframes (see Chapter 6) or infill wallsfor lateral load resistance, Fig 3.3 (c),or

(ii) Substantial rigid jointed beams andcolumns capable of resisting the lat-eral loads by themselves.

The latter will be required for large col-umn free spaces such as assembly halls.

The framed constructions can be usedfor a greater number of storeys compared tobearing wall construction. The strength andductility can be better controlled in framedconstruction through design. The strengthof the framed construction is not affectedby the size and number of openings. Suchframes fall in the category of engineeredconstruction, hence outside the scope of thepresent book.

to have several blocks on terracesthan have one large block withfootings at very different elevations.A site subject to the danger of rockfalls has to be avoided.

(ii) Very Loose Sands or Sensitive Clays:These two types of soils are liable tobe destroyed by the earthquake somuch as to lose their original struc-ture and thereby undergocompaction. This would result inlarge unequal settlements and dam-age the building. If the loosecohesionless soils are saturated withwater they are apt to lose their shearresistance altogether during shakingand become liquefied.

Although such soils can be compacted,for small buildings the operation may betoo costly and these soils are better avoided.For large building complexes, such as hous-ing developments, new towns, etc., this fac-tor should be thoroughly investigated andappropriate action taken.

Therefore a site with sufficient bearingcapacity and free from the above defectsshould be chosen and its drainage condi-tion improved so that no water accumu-lates and saturates the ground close to thefooting level.

3.3.3. Structural designDuctility (defined in Section 3.6) is the mostdesirable quality for good earthquake per-formance and can be incorporated to someextent in otherwise brittle masonry con-structions by introduction of steel reinforc-ing bars at critical sections as indicatedlater in Chapters 4 and 5.

7


The strengthening measures necessaryto meet these safety requirements are pre-sented in the following Chapters for vari-ous building types. In view of the lowseismicity of Zone D, no strengtheningmeasures from seismic consideration areconsidered necessary except an emphasison good quality of construction. The fol-lowing recommendations are therefore in-tended for Zones A, B and C. For this pur-pose certain categories of construction in anumber of situations were defined inTable 3.1.

3.6 CONCEPTS OF DUCTILITY,DEFORMABILITY ANDDAMAGEABILITYDesirable properties of earthquake-resist-ant design include ductility, deformabilityand damageability. Ductility anddeformability are interrelated concepts sig-nifying the ability of a structure to sustainlarge deformations without collapse.Damageability refers to the ability of a struc-

3.5 REQUIREMENTS OFSTRUCTURAL SAFETYAs a result of the discussion of structuralaction and mechanism of failure of Chap-ter 2, the following main requirements ofstructural safety of buildings can be arrivedat.

(i) A free standing wall must be de-signed to be safe as a vertical canti-lever.

This requirement will be difficult toachieve in un-reinforced masonry inZone A. Therefore all partitions in-side the buildings must be held onthe sides as well as top. Parapets ofcategory I and II buildings must bereinforced and held to the mainstructural slabs or frames.

(ii) Horizontal reinforcement in walls isrequired for transferring their ownout-of-plane inertia load horizon-tally to the shear walls.

(iii) The walls must be effectively tiedtogether to avoid separation at verti-cal joints due to ground shaking.

(iv) Shear walls must be present alongboth axes of the building.

(v) A shear wall must be capable of re-sisting all horizontal forces due toits own mass and those transmittedto it.

(vi) Roof or floor elements must be tiedtogether and be capable of exhibit-ing diaphragm action.

(vii) Trusses must be anchored to the sup-porting walls and have an arrange-ment for transferring their inertiaforce to the end walls.

Table 3.1 Categories of buildings for strengthening purposes

Category Combination of conditions for the Category

I Important building on soft soil in zone A

II Important building on firm soil in zone AImportant building on soft soil in zone BOrdinary building on soft soil in zone A

III Important building on firm soil in zone BImportant building on soft soil in zone COrdinary building on firm soil in zone AOrdinary building on soft soil in zone B

IV Important building on firm soil in zone COrdinary building on firm soil in zone BOrdinary building on firm soil in zone C

Notes: (i)Seismic zones A, B and C and important buildings are definedin Section 3.2.

(ii) Firm soil refers to those having safe bearing value more than10 t/m2 and soft those less than 10 t/m2.

(iii) Weak soils liable to compaction and liquefaction under earth-quake condition are not covered here.

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IAEE MANUAL

together so that excessive stress concentra-tions are avoided and forces are capable ofbeing transmitted from one component toanother even through large deformations.

Ductility is a term applied to materialand structures, while deformability is ap-plicable only to structures.

Even when ductile materials are presentin sufficient amounts in structural compo-nents such as beams and walls, overallstructural deformability requires that geo-metrical and material instability beavoided. That is, components must haveproper aspect ratios (that is not be too high),must be adequately connected to resistingelements (for example sufficient wall tiesfor a masonry wall, tying it to floors, roofand shear walls), and must be well tied to-gether (for example positive connection atbeam seats, so that deformations do notpermit a beam to simply fall off a post) soas to permit large deformations and dy-namic motions to occur without suddencollapse.

3.6.3 DamageabilityDamageability is also a desirable qualityfor construction, and refers to the ability ofa structure to undergo substantial damages,without partial or total collapse

A key to good damageability is redun-dancy, or provision of several supports forkey structural members, such as ridgebeams, and avoidance of central columnsor walls supporting excessively large por-tions of a building. A key to achieving gooddamageability is to always ask the ques-tion, �if this beam or column, wall connec-tion, foundation, etc. fails, what is the con-sequence?�. If the consequence is total col-

ture to undergo substantial damage, with-out partial or total collapse. This is desir-able because it means that structures canabsorb more damage, and because it per-mits the deformations to be observed andrepairs or evacuation to proceed, prior tocollapse. In this sense, a warning is receivedand lives are saved.

3.6.1 DuctilityFormally, ductility refers to the ratio of thedisplacement just prior to ultimate dis-placement or collapse to the displacementat first damage or yield. Some materials areinherently ductile, such as steel, wroughtiron and wood. Other materials are notductile (this is termed brittle), such as castiron, plain masonry, adobe or concrete, thatis, they break suddenly, without warning.Brittle materials can be made ductile, usu-ally by the addition of modest amounts ofductile materials, Such as wood elementsin adobe construction, or steel reinforcingin masonry and concrete constructions.

For these ductile materials to achieve aductile effect in the overall behaviour of thecomponent, they must be proportioned andplaced so that they come in tension and aresubjected to yielding. Thus, a necessary re-quirement for good earthquake-resistantdesign is to have sufficient ductile materi-als at points of tensile stresses.

3.6.2 DeformabilityDeformability is a less formal term refer-ring to the ability of a structure to displaceor deform substantial amounts withoutcollapsing. Besides inherently relying onductility of materials and components,deformability requires that structures bewell-proportioned, regular and well tied

9


lapse of the structure, additional supportsor alternative structural layouts should beexamined, or an additional factor of safetybe furnished for such critical members orconnections.

3.7 CONCEPT OF ISOLATIONThe foregoing discussion of earthquake-resistant design has emphasized the tradi-tional approach of resisting the forces anearthquake imposes on a structure. An al-ternative approach which is presentlyemerging is to avoid these forces, by isola-tion of the structure from the ground mo-tions which actually impose the forces onthe structure.

This is termed base-isolation. For sim-ple buildings, base- friction isolation maybe achieved by reducing the coefficient offriction between the structure and its foun-dation, or by placing a flexible connectionbetween the structure and its foundation.

For reduction of the coefficient of fric-tion between the structure and its founda-tion, one suggested technique is to placetwo layers of good quality plastic betweenthe structure and its foundation, so that theplastic layers may slide over each other.

Flexible connections between the struc-ture and its foundation are also difficult toachieve on a permanent basis. One tech-nique that has been used for generationshas been to build a house on short postsresting on large stones, so that under earth-quake motions, the posts are effectively pin-connected at the top and bottom and thestructure can rock to and fro somewhat.This has the advantage of substantially re-ducing the lateral forces, effectively isolat-ing the structure from the high amplitude

high frequency motions. Unfortunately, tra-ditional applications of this technique usu-ally do not account for occasional largedisplacements of this pin-connectedmechanism, due to rare very large earth-quakes or unusually large low-frequencycontent in the ground motion, so that whenlateral displacements reach a certain point,collapse results. A solution to this problemwould be provision of a plinth slightly be-low the level of the top of the posts, so thatwhen the posts rock too far, the structure isonly dropped a centimeter or so.

3.8 FOUNDATIONSFor the purpose of making a building trulyearthquake resistant, it will be necessary tochoose an appropriate foundation type forit. Since loads from typical low heightbuildings will be light, providing the re-quired bearing area will not usually be aproblem. The depth of footing in the soilshould go below the zone of deep freezingin cold countries and below the level ofshrinkage cracks in clayey soils. For choos-ing the type of footing from the earthquakeangle, the soils may be grouped as Firm andSoft (see Section 3.2.3) avoiding the weaksoil unless compacted and brought to Softor Firm condition.

3.8.1 Firm soilIn firm soil conditions, any type of footing(individual or strip type) can be used. Itshould of course have a firm base of lime orcement concrete with requisite width overwhich the construction of the footing maystart. It will be desirable to connect the in-dividual reinforced concrete columnfootings in Zone A by means of RC beamsjust below plinth level intersecting at rightangles.

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IAEE MANUAL

3.8.2 Soft soilIn soft soil, it will be desirable to use a plinthband in all walls and where necessary toconnect the individual column footings bymeans of plinth beams as suggested above.It may be mentioned that continuous rein-forced concrete footings are considered tobe most effective from earthquake consid-erations as well as to avoid differential set-tlements under normal vertical loads. De-tails of plinth band and continuous RC

footings are presented in Chapters 4 and 9respectively.

These should ordinarily be providedcontinuously under all the walls. Continu-ous footing should be reinforced both inthe top and bottom faces, width of the foot-ing should be wide enough to make thecontact pressures uniform, and the depthof footing should be below the lowest levelof weathering.

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1


Chapter 4

BUILDINGS IN FIRED-BRICK ANDOTHER MASONRY UNITS

4.1 INTRODUCTIONThe buildings in fired bricks, solid concreteblocks and hollow concrete or mortarblocks are dealt with in this chapter. Thegeneral principles and most details ofearthquake resistant design and construc-tion of brick-buildings are applicable tothose using other rectangular masonryunits such as solid blocks of mortar, con-crete, or stabilized soil, or hollow blocks ofmortar, or concrete having adequatecompressive strength. Some constructiondetails only differ for hollow blocks, whichare also indicated as necessary.

4.2 TYPICAL DAM AGE ANDFAILURE OF MASONRYBUILDINGSThe creation of tensile and shearingstresses in walls of masonry buildings isthe primary cause of different types of dam-age suffered by such buildings. The typi-cal damages and modes of failure are brieflydescribed below:

4.2.1 Non-structural damageThe non-structural damage is that due towhich the strength and stability of thebuilding is not affected. Such damage oc-curs very frequently even under moderateintensifies of earthquakes:

� Cracking and overturning of ma-sonry parapets, roof chimney, largecantilever cornices and balconies.

� Falling of plaster from walls and ceil-ing particularly where it was loose.

� Cracking and overturning of parti-tion walls, filler walls and claddingwalls from inside of frames. (Thoughnot usually accounted for in calcu-lations, this type of damage reducedthe lateral strength of the building).

� Cracking and failing of ceilings.

� Cracking of glass panes.

� Failing of loosely placed objects, over-turning of cupboards, etc.

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IAEE MANUAL

4.2.2 Damage and failure ofbearing walls

(i) Failure due to racking shear is char-acterized by diagonal cracks whichcould be due to diagonal compres-sion or diagonal tension. Such fail-ure may be either through the pat-tern of joints or diagonally throughmasonry units. These cracks usuallyinitiate at the corner of openings andsometimes at centre of wall segment.This kind of failure can cause par-tial or complete collapse of the struc-ture, Fig 4.1.

(ii) A wall can fail as a bending memberloaded by seismic inertia forces onthe mass of the wall itself in a direc-tion, transverse to the plane of the

wall. Tension cracks occur verticallyat the centre, ends or corners of thewalls. Longer the wall and longer theopenings, more prominent is thedamage, Fig 4.1. Since earthquake ef-fects occur along both axes of a build-ing simultaneously, bending andshearing effects occur often togetherand the two modes of failures areoften combined. Failure in the piersoccur due to combined action offlexure and shear.

(iii) Unreinforced gable end masonrywalls are very unstable and the strut-ting action of purlins imposes addi-tional force to cause their failure.Horizontal bending tension cracksare caused in the gables.

Fig 4.1 Cracking in bearing wall building due to bending and shear

3


(iv) The deep beam between two open-ings one above the other is a weakpoint of the wall under lateralinplane forces. Cracking in this zoneoccurs before diagonal cracking ofpiers, Fig 4.2. In order to prevent itand to enable the full distribution ofshear among all piers, either a rigidslab or RC band must exist betweenthem.

(v) Walls can be damaged due to the seis-mic force of the roof, which cancause the formation of tension cracksand separation of supporting walls,Fig 4.3. This mode of failure is thecharacteristic of massive flat roofs (orfloors) supported by joists, which inturn are supported by bearing walls,but without proper connection withthem. Also if the connection withfoundation is not adequate, wallscrack there and slide. This may causefailure of plumbing pipes too.

(vi) Failure due to torsion and warping:The damage in unsymmetrical build-ing occurs due to torsion and warp-ing in an earthquake, Fig 3.1. Thismode of failure causes excessivecracking due to shear in all walls.Larger damage occurs near the cor-ner of the building.

(vii) Arches across openings in walls areoften badly cracked since the archestend to lose their end thrust underin-plane shaking of walls.

(viii) Under severe prolonged intenseground motions, the following hap-pens:

- the cracks become wider and themasonary units become loose

Fig 4.2 Cracking of spandrel wall between opening

Fig 4.3 Fall of roof because of inadequate connection between roof andwall

- partial collapse and gaps inwalls occur due to falling ofloose masonry units, particu-larly at location of piers.

- falling of spandrel masonry dueto collapse of piers

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IAEE MANUAL

- falling of gable masonry due toout of plane cantilever action

- walls get separated at cornersand intermediate T-junctionsand fall outwards.

- roof collapse, either partial or full

- certain types of roofs may slideoff the top of walls and the roofbeams fall down

- masonry arches across wallopenings as well as those usedfor roof collapse completely.

4.2.3 Failure of ground(i) Inadequate depth of foundation:

Shallow foundations deteriorate asa result of weathering and conse-quently become weak for earthquakeresistance.

(ii) Differential settlement of founda-tion: During severe ground shaking,liquefaction of loose water-saturatedsands and differential cornpactionof weak loose soils occur which leadto excessive cracking and tilting ofbuildings which may even collapsecompletely.

(iii) Sliding of slopes: Earthquakes causesliding failures in man-made as wellas natural hill slopes and any build-ing resting on such a slope have adanger of complete disastrous dis-integration.

4.2.4 Failure of roofs and floors(i) Dislodging of roofing material: Im-

properly tied roofing material is dis-lodged due to inertia forces actingon the roof. This mode of failure is

typical of sloping roofs, particularlywhen slates, clay, tiles etc. are usedas roofing material.

Brittle material like asbestos cementmay be broken if the trusses andsheeting purlins are not properlybraced together.

(ii) Weak roof to support connection isthe cause of separation of roof trussfrom supports,although completeroof collapse mostly occurs due tocollapse of supporting structure. Therupture of bottom chord of roof trussmay cause a complete collapse oftruss as well as that of walls, Fig 4.4.

(iii) Heavy roofs as used in rural areaswith large thickness of earth overround timbers cause large inertiaforces on top of walls and may leadto complete collapse in severe earth-quake shocks.

(iv) Lean-to roofs easily cause instabil-ity in the lower supporting walls orpiers and collapse easily due to lackof ties.

4.2.5 Causes of damage inmasonry buildingsThe following are the main weaknesses inthe materials and unreinforced masonryconstructions and other reasons for the ex-tensive damage of such buildings:

� Heavy weight and very stiff build-ings, attracting large seismic inertiaforces.

� Very low tensile strength, particu-larly with poor mortars.

� Low shear strength, particularlywith poor mortars.

5


� Brittle behaviour in tension as wellas compression.

� Weak connection between wall andwall.

� Stress concentration at corners ofwindows and doors.

� Overall unsymmetry in plan and el-evation of building.

� Unsymmetry due to imbalance in thesizes and positions of openings inthe walls.

� Defects in construction such as useof substandard materials, unfilledjoints between bricks, not-plumbwalls, improper bonding betweenwalls at right angles, etc.

4.2 TYPICAL STRENGTHS OFMASONRYThe crushing strength of masonry used inthe position of walls depends on many fac-tors such as the following:

(i) Crushing strength of the masonryunit.

(ii) Mix of the mortar used and age atwhich tested. The mortar used fordifferent wall constructions varies inquality as well as strength. It is gen-erally described on the basis of themain binding material such as ce-ment or lime mortar, cement limecomposite mortar, lime-pozzolana orhydraulic lime mortar. Clay mudmortar is also used in many coun-tries particular in rural areas.

(iii) Slenderness ratio of the wall, that is,smaller of the ratio of effective heightand effective length of the wall to itsthickness. Larger is the slendernessratio, smaller the strength.

(iv) Eccentricity of the vertical toad onthe wall- Larger the eccentricity,smaller the strength.

(v) Percentage of openings in the wall� larger the openings, smaller thestrength. The tensile and shearingstrengths of masonry mainly dependupon the bond or adhesion at thecontact surface between the masonry


6

IAEE MANUAL

unit and the mortar and, in general,their values are only a small percent-age of the crushing strength. Richeris a mortar in cement or lime con-tent, higher is the percentage of ten-sile and shearing strength in relationto the crushing strength. Test carriedout on brick-couplets using handmade bricks in cement mortar givethe typical values as shown inTable 4.1.

Brick couplet tests under combined ten-sion-shear and compression-shear stresses

show that the shearing strength decreaseswhen acting with tension and increaseswhen acting with compression. Fig 4.5shows the combined strengths.

The tensile strength of masonry is notgenerally relied upon for design purposesunder normal loads and the area subjectedto tension is assumed cracked. Under seis-mic conditions, it is recommended that thepermissible tensile and shear stresses onthe area of horizontal mortar bed joint inmasonry may be adopted as given inTable 4.2.

The modulus of elasticity of masonryvery much depends upon the density andstiffness of masonry unit, besides the mor-tar mix. For brickwork the values are of theorder 2000 MPa for cement-sand mortar in1:6 proportion. The mass density of ma-sonry mainly depends on the type of ma-sonry unit. For example brickwork willhave a mass density of about 1900 kg/m3

and dressed stone masonry 2400 kg/m3.

The slenderness ratio of the wall is takenas the lesser of h/t and l/t where h = effectiveheight of the wall and L = its effectivelength. The allowable stresses in Table 4.2must be modified for eccentricity of verticalloading due to its position and seismicmoment and the slenderness ratio multi-plying factors given in Table 4.3. The effec-tive height h may be taken as a factor timesthe actual height of wall between floors, thefactor being 0.75 when floors are rigid dia-phragms and 1.00 for flexible roofs; it willbe 2.0 for parapets.

The effective length L will be a fractionof actual length between lateral supports,the factor being 0.8 for wall continuous

Table 4.1 Typical strengths of masonry

Mortar mix Tensile Shearing Compressive strength in MPacement sand strength, strength, corresponding to crushing

MPa MPa strength of masonry unit

3.5 7.0 10.5 14.0

1 12 0.04 0.22 1.5 2.4 3.3 3.9

1 6 0.25 0.39 2.1 3.3 5.1 6.0

1 3 0.71 1.04 2.4 4.2 6.3 7.5

Fig 4.5 Combined stress couplet test results

7


with cross walls or buttresses at both ends,1.0 for continuous at one end and sup-ported on the other and 1.5 for continuousat one and free at the other.

4.4 GENERAL CONSTRUCTIONASPECTS4.4.1 MortarSince tensile and shear strength are impor-tant for seismic resistance of masonry walls,use of mud or very lean mortars will beunsuitable. A mortar mix cement: sandequal to 1:6 by volume or equivalent instrength should be the minimum. Appro-priate mixes for various categories of con-struction are recommended in Table 4.4. Useof a rich mortar in narrow piers betweenopenings will be desirable even if a leanmix is used for walls in general.

4.4.2. Wall enclosureIn load bearing wall construction, the wallthickness �t� should not be kept less than190 mm, wall height not more than 20 t andwall length between cross-walls not morethan 40 t. If longer rooms are required, ei-ther the wall thickness is to be increased, orbuttresses of full height should be providedat 20 t or less apart. The minimum dimen-sions of the buttress shall be as thicknessand top width equal to t and bottom widthequal to one sixth the wall height.

4.4.3 Openings in wallsStudies carried out on the effect of open-ings on the strength of walls indicate thatthey should be small in size and centrallylocated. The following are the guidelineson the size and position of openings:

Table 4.3 Stress factor for slenderness ratio and eccentricity of loading

Slenderness Stress factor, K, for eccentricity ratio, e/t Remarksratio 0 0.04 0.10 0.20 0.30 0.33 0.50

6 1.000 1.000 1.000 0.996 0.984 0.980 0.970 Linear interpolation

8 0.920 0.920 0.920 0.910 0.880 0.870 0.850 may be used.

10 0.840 0.835 0.830 0.810 0.770 0.760 0.730

12 0.760 0.750 0.740 0.706 0.664 0.650 0.600

14 0.670 0.660 0.640 0.604 0.556 0.540 0.480 Values for e/t = 0.5 are

16 0.580 0.565 0.545 0.500 0.440 0.420 0.350 for interpolation only

18 0.500 0.480 0.450 0.396 0.324 0.300 0.230

21 0.470 0.448 0.420 0.354 0.276 0.250 0.170

24 0.440 0.415 0.380 0.310 0.220 0.190 0.110

Table 4.2 Typical permissible stresses

Mortar mix or equivalent Permissible stresses Compression for strength of unit, MPa

cement lime sand tension shear 3.5 7.0 10.5 14.0MPa MPa

1 - 6 0.05 0.08 0.35 0.55 0.85 1.00

1 1 6 0.13 0.20 0.35 0.70 1.00 1.10

1 - 3 0.13 0.20 0.35 0.70 1.05 1.25

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IAEE MANUAL

(i) Openings to be located away fromthe inside corner by a clear distanceequal to at least 1/4 of the height ofopenings but not less than 60 cm.

(ii) The total length of openings not toexceed 50 percent of the length of the

wall between consecutive cross wallsin single-storey construction, 42 per-cent in two-storey construction and33 percent in three storey buildings.

(iii) The horizontal distance (pier width)between two openings to be not lessthan half the height of the shorteropening, Fig 4.6, but not less than60 cm.

(iv) The vertical distance from an open-ing to an opening directly above itnot to be less than 60 cm nor lessthan 1/2 of the width of the smalleropening, Fig 4.6.

(v) When the openings do not complywith requirements (i) to (iv), they

Fig 4.6 Recommendation regarding openings in bearing walls

Table 4.4 Recommended mortar mixes

Category of Proportion of cement-lime-sandconstruction*

I Cement-sand 1:4 or cement-lime-sand 1:1:6 or richer

II Cement-lime-sand 1:2:9 or richer

III Cement-sand 1:6 or richer

IV Cement-sand 1:6 or lime-cinder** 1:3 or richer

Notes:* Category of construction is defined in Table 3.1.

** In this case some other pozzolonic material like trass (Indonesia)and surkhi (burnt brick fine powder in India) may be used in placeof cinder.

9


should either be boxed in reinforcedconcrete alround or reinforcing barsprovided at the jambs through theMasonry, Fig 4.7.

4.4.4 Masonry bondFor achieving full strength of masonry theusual bonds specified for masonry shouldbe followed so that the vertical joints are

Fig 4.7 Strengthening of masonry around openings

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IAEE MANUAL

broken properly from course to course. Thefollowing deserves special mention.

Vertical joint betweenperpendicular wallsFor convenience of construction, buildersprefer to make a toothed joint which is

Fig 4.8 A typical detail of masonry

many times left hollow and weak. To ob-tain full bond it is necessary to make a slop-ing (stepped) joint by making the cornersfirst to a height of 600 mm and then build-ing the wall in between them. Otherwise,the toothed joint should be made in boththe walls alternately in lifts of about 45 cm,Fig 4.8.

4.5 HORIZONTALREINFORCEMENT IN WALLSHorizontal reinforcing of walls is requiredfor imparting to them horizontal bendingstrength against plate action for out ofplane inertia load and for tying the perpen-dicular wall together. In the partition walls,horizontal reinforcement helps preventingshrinkage and temperature cracks. The fol-lowing reinforcing arrangements are nec-essary.

4.5.1 Horizontal bands or ringbeamsThe most important horizontal reinforcing

Table 4.5 Recommendation for steel in RC band

Longitudinal steel in R.C. bandsSpan, m category I category II category III category IV

no of diameter of no of diameter of no of diameter of no of diameter ofbars bars, mm bars bars, mm Bars Bars, mm Bars Bars, mm

5 2 12 2 10 2 10 2 10

6 2 16 2 12 2 10 2 10

7 2 16 2 16 2 12 2 10

8 4 12 2 16 2 16 2 12

9 4 16 4 12 2 16 2 12

Notes: (i) Width of the RC band is assumed to be the same as the thickness of wall. Wall thickness shall be 20 cm minimum. Acover of 25 mm from face of wall will be maintained. For thicker walls, the quantity of steel need not be increased.For thinner walls, see 4.7.

(ii) The vertical thickness of RC band may be kept minimum 75 mm where two longitudinal bars are specified and 150mm where four longitudinal bars are specified.

(iii) Concrete mix to be 1:2:4 by volume or having 15 MPa cube crushing strength at 28 days.

(iv) The longitudinal bars shall be held in position by steel links or stirrups 6 mm diameter spaced at 150 mm apart(see Fig 4.10 (a))

(v) Bar diameters are for mild-steel. For high strength must deformed bars, equivalent diameter may be used.

11


Fig 4.9 Gable band and roof band in barrack type buildings

is through reinforced concrete bands pro-vided continuously through all load bear-ing longitudinal and transverse walls atplinth, lintel, and roof-eave levels, also attop of gables according to requirements asstated hereunder:

(i) Plinth band: This should be pro-vided in those cases where the soilis soft or uneven in their propertiesas it usually happens in hill tracts.It will also serve as damp proofcourse. This band is not too critical.

(ii) Lintel band: This is the most impor-tant band and will incorporate in it-self all door and window lintels the

reinforcement of which should be ex-tra to the lintel band steel. It must beprovided in all storeys in buildingsas per Table 4.5.

(iii) Roof band: This band will be re-quired at eave level of trussed roofs,Fig 4.9 and also below or in level withsuch floors, which consist of joistsand covering elements so as to prop-erly integrate them at ends and fixinto the walls.

(iv) Gable band: Masonry gable endsmust have the triangular portion ofmasonry enclosed in a band, the hori-zontal part will be continuous withthe eave level band on longitudinal

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IAEE MANUAL

walls, Fig 4.9.

4.5.2 Section of bands or ringbeamsThe reinforcement and dimensions of these

Fig 4.10 Reinforcement in RC band

bands may be kept as follows for wall spansupto 9 m between the cross walls or but-tresses. For longer spans, the size of bandmust be calculated.

A band consists of two (or four) longitu-dinal steel bars with links or stirrups em-bedded in 75 mm (or 50 mm), thick con-crete, Fig 4.10. The thickness of band maybe made equal to or a multiple of masonryunit and its width should equal the thick-ness of wall. The steel bars are located closeto the wall faces with 25 mm cover and fullcontinuity is provided at corners and junc-tions. The minimum size of band andamount of reinforcing will depend uponthe unsupported length of wall betweencross walls and the effective seismic coeffi-cient based on seismic zone, importance ofbuildings, type of soil and storey of thebuilding.

Appropriate steel and concrete sizes arerecommended for various buildings inTable 4.5. Such bands are to be located atcritical levels of the building, namely plinth,lintel, roof and gables according to require-ments (see 4.5.1).

4.5.3 Dowels at corners andjunctionsAs a supplement to the bands described in(a) above, steel dowel bars may be used atcorners and T-junctions to integrate the boxaction of walls. Dowels, Fig 4.11, are placedin every fourth course or at about 50 cmintervals and taken into the walls to suffi-cient length so as to provide the full bondstrength. Wooden dowels can also be usedinstead of steel. However, the dowels donot serve to reinforce the walls in horizon-tal bending except near the junctions.

13


Fig 4.11 (a) Corner-strengthening by dowel reinforcement placed in one joint (b) Corner-strengthening by dowelreinforcement placed in two consecutive joints. (c) T-junction - strengthening by dowel reinforcements(d) Strengthening by wire fabric at junction and corner

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IAEE MANUAL

4.6 VERTICALREINFORCEMENT IN WALLSThe need for vertical reinforcing of shearwalls at critical sections was establishedin Para 2.6.7. The critical sections were thejambs of openings and the corners of walls.The amount of vertical reinforcing steel willdepend upon several factors like thenumber of storeys, storey heights, the effec-tive seismic coefficient based on seismiczone, importance of building and soil foun-dation type. Values based on rough esti-mates for building are given in Table 4.6 forready use. The steel bars are to be installedat the critical sections, that is the corners ofwalls and jambs of doors right, from thefoundation concrete and covered with ce-ment concrete in cavities made around themduring masonry construction. This concretemix should be kept 1:2:4 by volume or richer.Typical arrangements of placing the verti-cal steel in brick work are shown inFig 4.12.

The jamb steel was shown in Fig 4.7.The jamb steel of window openings will beeasiest to provide in box form around it.The vertical steel of opening may bestopped by embedding it into the lintel bandbut the vertical steel at corners and junc-tions of walls must be taken into the floorand roof slabs or roof band

The total arrangement of providing re-inforcing steel in masonry wall construc-tion is schematically shown in Fig 4.13.

4.7 FRAMING OF THIN LOADBEARING WALLSIf load-bearing walls are made thinner than200 mm, say 150 mm inclusive of plaster-ing on both sides, reinforced concrete fram-ing columns and collar beams are neces-sary which are constructed to have fullbond with the walls. Columns are to be lo-cated at all corners and junctions of wallsand at not more than 1.5 m apart but solocated as to frame up the doors and win-dows. The horizontal bands or ring beamsare located at all floors, roof as well as lin-tel levels of the openings. The sequence ofconstruction between walls and columnsis: first to build the wall upto 4 to 6 coursesheight leaving toothed gaps (tooth projec-tion being about 40 mm only) for the col-umns and second to pour 1:2:4 concrete tofill the columns against the walls usingwood -forms only or two sides. Needless tosay that column steel should be accuratelyheld in position all along. The band con-crete should be cast on the wall masonrydirectly so as to develop full bond with it.

Such construction may be limited to onlytwo storeys maximum in view of its verti-cal load carrying capacity. The horizontallength of walls between cross walls may be

Table 4.6 Recommendation for vertical steel at critical sections

No of Storeys Diameter of mild steel single bar in mm ateach critical section for category (1)

category I category II categoryIII category IV

One 16 12 12 Nil

Two Top 16 12 12 NilBottom 20 16 16 Nil

Three Top 16 12 12 NilMiddle 20 16 12 NilBottom 20 16 16 Nil

Four Top (2) (2) 12 12Third 12 12

Second 16 12Bottom 16 12

Notes: (i)Category of construction is defined in Table 3.1. Equivalentarea of twisted grip bars or a number of mild steel bars could beused but the diameter should not be less than 12 mm.

(ii) Four storeyed load bearing wall construction may not be usedfor categories I and II buildings.

15


Fig 4.12 Vertical reinforcement in walls

16

IAEE MANUAL

restricted to 7 m and the storey height to3 m.

4.8 REINFORCING DETAILSFOR HOLLOW BLOCKMASONRYThe following details may be followed inplacing the horizontal and vertical steel in

hollow block masonry using cement-sandor cement concrete blocks.

4.8.1 Horizontal bandU-shaped blocks may best be used for con-struction the horizontal bands at variouslevels of the storeys as per seismic require-ments, as shown in, Fig 4.14.

The amount of horizontal reinforcementmay be taken 25 percent more than thatgiven in Table 4.5 and provided by usingfour bars and 6mm dia stirrups. Other con-tinuity details shall be followed as shownin Fig 4.10.

4.8.2 Vertical reinforcementThe vertical bars as specified in Table 4.6may conveniently be located inside thecavities of the hollow blocks, one bar in onecavity. Where more than one bar is planned,

Fig 4.14 U-blocks for horizontal bands


Fig 4.13 Overall arrangement of reinforcing masonry buildings

17


these can be located in two or three con-secutive cavities as shown inFig 4.15. The cavities containing bars areto be filled by using micro-concrete 1:2:3 orcement- coarse sand mortar 1:3 and prop-erly rodded for compaction.

Practical difficulty is faced in thread-ing the bars through the hollow blockssince the bars have to be set in footings andhave to be kept standing vertically whilelifting the blocks whole storey heights,threading the bar into the cavity and low-ering it down to the bedding level. To avoidlifting of blocks too high, the bars are madeshorter and overlapped with upper por-tions of bars. This is wastefull of steel aswell as the bond strength in small cavitiesremains doubtful. For solving this problem,two alternatives may be used as shown inFig.4.16 (a) use of three sided or U-block (b)bent interlocked bars.

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1

STONE BUILDINGS

Chapter 5

STONE BUILDINGS

5.1 INTRODUCTIONStone buildings using fully dressedrectangularized stone units, or cast solidblocks consisting of large stone pieces incement concrete mix 1:3.6 may be built ac-cording to the details given in Chapter 4.Those also generally apply to the random-rubble and half -dressed stone buildingsexcept such details as are dealt with in thisChapter.

5.2 TYPICAL DAMAGE ANDFAILURE OF STONEBuildings Random rubble and half -dressed stone buildings, Fig 5.l, have suf-fered extensive damage and complete col-lapse during past earthquakes having in-tensifies of MSK VII and more.

The following are the main ways inwhich such buildings are seen to be dam-aged :

� Separation of walls at corners andT-junctions takes place even moreeasily than in brick buildings due topoorer connection between thewalls.

� Delamination and bulging of walls,that is, vertical separation of inter-nal wythe and external wythethrough the middle of wall thickness,Fig 5.2. This occurs due mainly to theabsence of �through� or bond stonesand weak mortar filling between thewythes. In half-dressed stone ma-sonry, the surface stones are pyrami-dal in shape having more or less anedge contact one over the other, thusthe stones have an unstable equilib-

Fig 5.1 Schematic cross-section through a traditional stone house

2

IAEE MANUAL

rium and easily disturbed undershaking condition.

� Crumbling and collapsing of bulgedwythes after delamination underheavy weight of roofs/ floors, lead-ing to collapse of roof along withwalls or causing large gaps in walls.

� Outward overturning of stone wallsafter separation at corners due to in-ertia of roofs and floors and theirown inertia when the roofs were in-capable of acting as horizontal dia-phragms. This particularly hap-pened when the roof consisted ofround poles, reed matting and claycovering.

Frequently, such stone houses, underMSK VII or higher intensifies, are completelyshattered and razed to the ground, thewalls reduced to only heaps of rubble. Peo-ple get buried and more often killed. Thussuch buildings, without the seismic im-provements as suggested here below, canbe considered as dangerous particularly inseismic zones defined by Zones A and B inChapter 3.

5.3 TYPICAL STRUCTURALPROPERTIESTest data on the strength characteristics ofrandom rubble and half-dressed stone ma-sonry is not available. It is, however, quali-tatively known that the compressivestrength even while using clay mud asmortar will be enough to support three sto-reys but the tensile strength could only benear about zero. Sliding shear strength willonly be due to frictional resistance.

5.4 GENERAL CONSTRUCTIONASPECTS5.4.1 Overall dimensions

� The height of the construction maybe restricted to one storey of categoryI and II buildings and two storeys ofcategories III and IV buildings.Where light sheeted roof is used, anattic floor may also be used.

� The height of a storey may be kept aslow as 2.5 m but not more than 3.5m.

� The wall thickness should be usedas small as feasible, say 300 to 450mm.

� The unsupported length of a wallbetween cross walls may be limitedto 7 m.Fig 5.3 Recommended openings in bearing walls in rubble masonry

Fig 5.2 Wall delaminated with buckled wythes

3

STONE BUILDINGS

� For longer walls, buttresses may beused at intermediate points not far-ther apart than 3 m. The size of but-tress may be kept as: thickness = topwidth = t and base width = h/6

where, t = thickness of wall and h =actual wall height.

5.4.2 Mortar� Clay mud mortar should be avoided

as far as possible.

Fig 5.4 “Through” stones or “Bond” elements.

Fig 5.5 Lintel level wooden band on all load bearing walls

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IAEE MANUAL

� Mortars as specified in Table 4.4 maybe used for stone walls.

5.4.3 Openings in walls� Openings should be as small and as

centrally located as practicable.

� The recommended opening limita-tions are shown in Fig 5.3.

� Ventilator, where used, may be made450 x 450 mm or smaller.

5.4.4 Masonry bond� Random rubble masonry construc-

tion should be brought to courses atnot more than 600 mm lift.

� �Through� stones of full lengthequal to wall thickness should beused in every 600 mm lift at not morethan 1.2 m apart horizontally. If fulllength stones are not available,stones in pairs, each of about 3/4 ofthe wall thickness may be used inplace of one full length stone so as toprovide an overlap between them,Fig 5.4.

� In place of �through� stones, bond-ing elements of steel bars 8 to 10 mmφ in S-shape or as a hooked link maybe used with a cover of 25 mm fromeach face of the wall, Fig 5.4.

� Alternatively, wood bars of 38 mm x38 mm cross-section or equivalentmay be used for the �through�stones. Wood should be well pre-served through seasoning and

Fig 5.6 Details of wood reinforcing at corners and T-junctions

Fig 5.7 Vertical steel in random rubble masonry

5

STONE BUILDINGS

chemical treatment so as to be dura-ble against weathering action andinsect attack, Fig 5.4.

� Use of long stones should also bemade at corners and junction of wallsto break the vertical joint and pro-vide bonding between perpendicu-lar walls.

5.4.5 Horizontal reinforcing ofwallsAll the horizontal reinforcing recom-mended for brick buildings in Section 4.5.1,4.5.2 and 4.5.3 may be use for random rub-ble constructions as well.

As an alternative to steel reinforcingbars, wooden planks of rectangular section,effectively spliced longitudinally and heldby lateral members in lattice form may beused where timber is available and alsomore economical. Recommended sectionsare shown in Fig 5.5 and Fig 5.6

5.4.6 Vertical reinforcing of wallsThe amount of vertical steel in masonrywalls required to be provided at the cor-ners and T-junctions of walls and at jambsof openings is shown in Table 5.1.

Buildings of Category IV need not havethe vertical steel at all. For providing verti-cal bar in stone masonry a casing pipe isrecommended around which the masonryis built to heights of 600 mm, Fig 5.7. Thepipe is kept loose by rotating it during ma-sonry construction. Then the casing pipe israised and the cavity below is filled with1:2:4 concrete mix and rodded to compactit. The concrete will not only provide thebond between the bar and the masonry butwill also protect the bar from corrosion.

The jamb steel may be taken from thefooting upto the lintel band and anchoredinto it. The corner steel must be taken fromthe footing upto the roof slab or roof bandand anchored into it.

��

Table 5.1 Recommended vertical steel at critical sections

No. of storeys Diameter of mild steel single bar in mm at eachcritical section category*

category I category II category III

One 20 16 14

Two ** ** 16

Notes: * Category of construction is defined in Table 3.1. Equivalent areaof twisted grip bars or a number of mild steel bars could be usedalternatively, but the diameter should not be less than 12 mm

** Two-storeyed buildings with load bearing stone masonry ofrandom rubble or half-dressed stone type are not recommendedin categories I and II.

1

WOODEN BUILDINGS

Chapter 6

WOODEN BUILDINGS

6.1 INTRODUCTIONWood has higher strength per unit weightand is, therefore, very suitable for earth-quake resistant construction. But heavycladding walls could impose high lateralload on the frame. Although seismicallysuitable, use of timber is declining in build-ing construction even where it used to bethe prevalent material on account of van-ishing forests due to population pressure.The situation in many countries of theworld has in-fact become rather alarmingon account of the ecological imbalance.Hence use of timber must be restricted inbuilding construction for seismic strength-ening of other weaker constructions suchas adobe and masonry. Timber buildingsmay only be used in those areas and coun-tries where it is still abundantly availableor in unavoidable situations only.

6.2 TYPICAL DAMAGE ANDFAILURE OF WOODENBUILDINGSThe typical features of earthquake damageto wooden buildings are as follows:

(i) Roof tiles easily slide down duringearthquakes, if they are not properly

fastened to the roof. Falling roof tilesmay hurt people, Fig 6.1.

(ii) The failure of the joints connectingcolumns and girders frequently oc-curs, accompanying the falling offinishings. As the inclination of thebuilding increases, its restoring forceagainst distortion decreases due tothe structural deterioration and roofweight, and finally becomes negativewhich results in the complete col-lapse of the building, Figs 6.2 and 6.3.

(iii) In the case of two storey buildings,the first storey usually suffers severerdamage than the second storey. It isoften seen that the first storey falls

Fig 6.1 Falling of roof tiles

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IAEE MANUAL

down while the second storey is un-damaged, Fig 6.4.

(iv) Damage is considerably influencedby the ground condition on whichthe building stands. In general, thesofter the subsoil, the severer thedamage to the building.

The damage due to differential set-tlements of foundations is also ob-served for buildings on soft ground.

Furthermore, the damage due to theliquefaction of subsoil occurs to

buildings on saturated soft sand.

(v) Sliding of the building as a whole issometimes seen when there are noanchor bolts connecting the sill to thefoundation, Fig 6.5.

The damage to superstructure is alsoobserved when the foundation can-not resist the lateral force caused byearthquake motion.

(vii) Other types of damage in woodenbuildings are failure of wooden ga-ble frames, Figs 6.6(a) and (b), and fail-ure due to rupture of bottom chordsof roof truss, Figs 6.7(a) and (b).

(viii) The most crucial destruction ofwooden buildings has been due tofire resulting from electrical short-circuiting or kitchen fires during theearthquake shaking and spreadinginto conflagration thereafter. Precau-tions against fire are most importantin case of wooden buildings.

6.3 TYPICALCHARACTERISTICS OF WOODThough wood has higher strength per unitweight than most other construction mate-rials, it has the following peculiarities thatare not seen in other materials.

(i) It is a non-homogeneous and aniso-tropic material showing differentcharacteristics not only in differentdirections but also in tension andcompression.

(ii) Shrinkage of wood on drying is rela-tively large. Particularly the jointsslack easily by the contraction in thedirection perpendicular to fibres.Therefore dry wood shall be used,

Fig 6.2 Rupture of columns at the connection of knee brace and column

Fig 6.3 Rupture of columns due to large notching at the connection ofgirder and column

3

WOODEN BUILDINGS

and the moisture content should beless than 20%.

(iii) The elastic modulus is small. Con-sequently, members are apt to showlarge deformation.

(iv) A notable creep phenomenon is seendue to permanent vertical loads.This is important especially in snowyarea.

(v) Sinking occurs by compressive forcein the direction perpendicular tofibers. This has a great influence to

the amount of deformation of hori-zontal members and chord members

Fig 6.5 Slide due to insufficient connection between sill and footing

Fig 6.4 Damage of a building having no diagonal bracing

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IAEE MANUAL

of built-up members.

(vi) The defects and notches of wood in-fluence greatly the strength and stiff-ness. Consequently it is necessary toselect and to arrange structural mem-bers considering their structuralproperties.

(vii) Wood is easily decayed by repeatedchanges of moisture. Therefore sea-soned wood should be used in con-struction.

(viii) Preservative treatment is necessaryto avoid rotting and insect attack ontimber so as to derive long life.

(ix) Wood is a combustible material.Therefore precautions must be takento minimize the danger of fire.

(x) Long lengths more than 3.5 m andlarge size timbers are difficult to ob-tain, hence call for splicing throughconnectors or gluing.

In view of its lightness, very easy work-ability like cutting and nailing and safetransportability, timber makes an excellentmaterial for post-earthquake relief and re-habilitation construction.

6.4 TYPICAL STRUCTURALPROPERTIESThere are large varieties of timbers in usein various countries. It will therefore not bepracticable to present their strength prop-erties here. But it will be pertinent to men-tion that these depend on a number of fac-tors as follows:

(i) Wood specy

(ii) Direction of loading relative to grainof wood

(iii) Defects like knots, checks, cracks,splits, shakes and wanes

(iv) Moisture content or seasoning

(v) Sapwood, pith, wood from deadtrees and dried wood conditions

(vi) Location of use, viz inside protected,outside, alternate wetting and dry-ing.

The permissible stresses must be deter-mined taking all these factors into account.Table 6.1 gives typical basic stresses for tim-bers placed in three groups A, B and C clas-sified on the basis of their stiffness. It willbe reasonable to increase the normal per-missible stress by a factor of 1.33 to 1.5 whenearthquake stresses are superimposed.

Fig 6.6 Failure modes of gable frame

5

WOODEN BUILDINGS

6.5 THE BUILDING PLANThe plan of the building should be sur-rounded and divided by bearing wall lines.The maximum spacing of the bearing walllines is 8 m. The maximum width of open-ings in the bearing wall lines is 4 m andthe opening is at least 50 cm away from thecorner. Adjacent openings should be atleast 50 cm apart, Fig 6.8.

All bearing wall lines of the lower sto-rey should be supported by continuousfoundations, through sills or the columnsshould rest on pedestals, for details see sec-tion 6.9. All bearing wall lines of the upperstorey should be supported by the bearingwall lines of the lower storey. The bearingwalls may have stud wall type or brick-nogged type construction as detailed in sec-tion 6.6 and 6.7 respectively. The height of


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IAEE MANUAL

the building will be limited to two storeysor two storeys plus attic.

6.6 STUD WALLCONSTRUCTIONThe stud-wall construction consists of tim-ber studs and corner posts framed into sills,top plates and wall plates. Horizontalstruts and diagonal braces are used tostiffen the frame against lateral loads dueto earthquake and wind. The wall coveringmay consist of matting made from bamboo,reeds, and timber boarding or the like. Typi-cal details of stud walls are shown in Fig6.9.

If the sheathing boards are properlynailed to the timber frame, the diagonalbracing may be omitted. The diagonal brac-ing may be framed into the verticals, ornailed to the surface. Other details are givenbelow:

SillThe dimension of sill is kept 40 × 90,90 × 90 (mm units) or larger. The sill is con-nected to the foundation by anchor boltswhose minimum diameter is 12 mm andlength 35 cm. The anchor bolts are installedat both sides of joints of sills and at themaximum spacing is 2 m.

StudsThe minimum dimension of studs is 40 mm× 90 mm. The maximum spacings of thesestuds are shown in Table 6.2. If 90 mm ×90 mm studs are used the spacing may bedoubled. Storey height should not be morethan 2.70 m.

Top platesThe top of studs is connected to top plateswhose dimension is not less than the di-mension of the stud.

Table 6.1 Basic permissible stresses for timber group*Types of stress Location Permissible stress, MPa

Group A Group B Group C

(i) Bending and tension along inside 18 12 8grain outside 15 10 7

wet 12 8 6

(ii) Shear in beams all 1.2 0.9 0.6shear along grains all 1.7 1.3 0.9

(iii) Compression parallel to inside 12 7 6grain outside 11 6 6

wet 9 6 5

(iv) Compression perpendicular inside 6 2.2 2.2to grain outside 5 1.8 1.7

wet 4 1.5 1.4

* Based on Indian Standard IS:883.Note: Group A, B and C are classified according to Young’s modulus of

elasticity as follows:group A more than 12,600 MPa.group B more than 9,800 to 12,600 MPa.group C 5,600 to 9,800 MPa.

Fig 6.8 Plan divided by bearing wall lines

7

WOODEN BUILDINGS

Bearing wallsWall framing consisting of sills, studs andtop plates should have diagonal braces, orsheathing boards so that the framings actsas bearing walls. In case no sheathingboards are attached, all studs should beconnected to the adjacent studs by horizon-tal blockings at least every 1.5 m in height,Fig 6.9.

The minimum dimension of braces is 20mm × 60 mm. The brace is fastened at bothends and at middle portion by more thantwo nails whose minimum length is 50 mm

to the framing members. The sheathingboard is connected to the framing membersby nails whose minimum length is50 mm and maximum spacing is 150 mm at

Figure 6.9 Stud-wall construction

Table 6.2 Maximum spacing of 40 mm × 90 mm finished size studsin stud wall constructionGroup of Single storeyed or first floor Ground floor of doubletimber of double storeyed buildings storeyed buildings

exterior wall, interior wall, exterior wall, interior wall,mm mm mm mm

A 1000 1000 500 500

B and C 1000 800 500 500

Notes: Group of timber defined in Table 6. 1

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IAEE MANUAL

Figure 6.10 Brick nogged timber frame

Table 6.3 Minimum finished size of diagonal bracesCategory* Group** Single storeyed or first floor Ground floor of double

timber of double storeyed buildings storeyed buildingsexterior wall, interior wall, exterior wall, interior wall,mm × mm mm × mm mm × mm mm × mm

I and II A 20 × 60 20 × 60 20 × 90 20 × 90B and C 20 × 60 20 × 60 20 × 90 20 × 90

III and IV A, B and C 20 × 60 20 × 60 20 × 60 20 × 60

Notes: *Categories of construction defined in Table 3.1.**Group of timber defined in Table 6.1.

9

WOODEN BUILDINGS

Fig 6.11 Foundation and foundation reinforcement in concrete

10

IAEE MANUAL

the fringe of the board and 300 mm at otherparts.

6.7 BRICK NOGGED TIMBERFRAMEThe brick nogged timber frame consists ofintermediate verticals, columns, sills, wallplates, horizontal nogging membersframed into each other. Diagonal bracesmay also be framed with the verticals ornailed or bolted on the faces. The space be-

tween framing members is filled with tightfitting brick or dressed stone masonry instretcher bond.

Typical details of brick nogged timberframe construction are shown in Fig 6.10.The vertical framing members in bricknogged bearing walls should have mini-mum finished sizes as specified in Table 6.4.The sizes of diagonal bracing membershould be the same as in Table 6.3. The hori-

Figure 6.12 Wooden column footings

11

WOODEN BUILDINGS

Table 6.5: Minimum finished sizes ofhorizontal nogging members

Spacing of Size, mm × mmverticals, m

1.5 70 x 100

1.0 50 x 100

0.5 25 x 100

zontal framing members in brick construc-tion should be spaced not more than onemeter apart. Their minimum finished sizesare recommended in Table 6.5.

6.8 JOINTS IN WOOD FRAMESThe joints of structural members should befirmly connected by nails or bolts. The useof metal straps is strongly recommendedat structurally important joints such asthose of studs/columns with sill or wallplates and with horizontal nogging mem-bers.

6.9 FOUNDATIONSThe superstructure should be supported byconcrete or masonry footings as shown in

Fig 6.11. Openings for ventilation need beprovided in continuous foundations,Figs 6.11(a) and (b). Some reinforcement asshown is also preferable in very soft soilareas and in areas where liquefaction is ex-pected. On firm soil, isolated footings orboulders can also be used under the woodcolumns as shown in Fig 6.12.

•••

Table 6.4 Minimum finished sizes of verticals in brick nogged timber frame constructionSpacing (m) Group of Single storeyed or first floor Ground floor of double

timber of double storeyed buildings storeyed buildingsexterior wall, interior wall, exterior wall, interior wall,mm × mm mm × mm mm × mm mm × mm

1.0 m A 50 × 100 50 × 100 50 × 100 70 × 100B and C 50 × 100 50 × 100 70 × 100 90 × 100

1.5 m A 50 × 100 70 × 100 70 × 100 80 × 100B and C 70 × 100 80 × 100 80 × 100 100 × 100

Notes: Groups of timber are defined in Table 6. 1

1

EARTHEN BUILDINGS

Chapter 7

EARTHEN BUILDINGS

7.1 INTRODUCTIONEarthen construction has been, is and willcontinue to be a reality. Recent statisticsshow that the percentage of earthen hous-ing in the next 15 years will be higher than50%.

Even though this material has clear ad-vantages of costs, aesthetics, acoustics andheat insulation and low energy consump-tion, it also has some disadvantages suchas being weak under earthquake forces andwater action. However, the technology de-veloped to date has allowed a reduction inits disadvantages, stressing its most valu-able advantages.

Earthen constructions are, in general,spontaneous and a great difficulty is thedissemination of knowledge about its ad-equate use.

The recommendations presented hereinare applicable to earthen constructions ingeneral, but they are especially oriented topopular housing, aiming to enhance thequality of the spontaneous, informal or

massive constructions which are the onescausing the greatest loss of life and dam-age during seismic events.

Therefore, it does not include solutionsinvolving the use of stabilizers (cement,lime, asphalt, etc.) to improve the strengthor durability. Also for making the strength-ening very economical, minimum use of theexpensive materials (concrete, steel, wood,etc.) has been indicated to enhance the dy-namic behaviour of the structure.

7.2 TYPICAL DAMAGE ANDCOLLAPSE OF EARTHENBUILDINGSEarthquake experience shows that earthenbuildings may be cracked at MSK IntensityVI, wide cracks and even partial collapsemay occur at MSK VII and collapses arewidespread under MSK VIII. Damage is al-ways much more severe in two storeyedbuildings than in one storeyed ones. Sometypical damages are sketched in Fig 7.1.However, single storeyed houses with flatroofs constructed in good clay have beenfound to be undamaged in Intensity VIII

2

IAEE MANUAL

Fig 7.1 Typical damages and collapse of earthen buildings (continued on next page)

(a) Corner failure and out of plane collapse of walls

(b) Gables

(c) Two storey house damage / collapse

(d) Split level roof

3

EARTHEN BUILDINGS

(e) L shaped buildings

Fig 7.1 Typical damages and collapse of earthen buildings (continued from previous page)

(f) High walled houses

(g) Awning

4

IAEE MANUAL

zone whereas at the same location two sto-reyed houses were completely ruined. Themain courses of failure of earthen build-ings in earthquakes are graphically sum-marised in Fig 7.2.

7.3 CLASSIFICATION OFWALLS AND MATERIALPROPERTIESIn earthen construction, the walls are thebasic elements hence it can be classifiedaccording to the wall type as follows:

7.3.1 Classification of earthenconstructionsa. Hand-formed by layers

a.1 Simple forming

a.2 Earth balls, thrown andmoulded as wall

b. Adobe or blocksb.1 Cut from hardened soil

b.2 Formed in mould

Fig 7.2 Graphic summary of causes of failure

b.3 Moulded and compacted

c. Tapial or pise (rammed earth)c.1 Compacted by hand blows

c.2 Mechanized or vibratingcompaction

d. Wood or cane structure, with wood orcane mesh enclosures plastered with mud

d.1 Continuous

d.2 Pre-fabricated panels

Whereas systems (a), (b) and (c) dependfor stability on the strength of earthenwalls, the system d behaves like a woodframe and its construction will be dealtwith separately.

7.3.2 Suitability of soilThe quality of materials, particularly claycontent of the soil may vary somewhat forthe type of construction. But in general thefollowing qualitative tests are sufficient for

5

EARTHEN BUILDINGS

determining the suitability of a soil forearthen construction:

a. Dry strength testFive or Six small balls of soil of approxi-mately 2 cm in diameter are made. Oncethey are dry (after 48 hours), each ball iscrushed between the forefinger and thethumb. If they are strong enough that noneof them breaks, the soil has enough clay tobe used in the adobe construction, providedthat some control over the mortar micro-fissures caused by the drying process isexercised, Fig 7.3(a)

If some of the balls break, the soil is notconsidered to be adequate, because it doesnot have enough clay and should be dis-carded.

b. Fissuring control testAt least eight sandwich units are manu-factured with mortars made with mixturesin different proportions of soil and coarsesand. It is recommended that the propor-tion of soil to coarse sand vary between 1:0and 1:3 in volume. The sandwich havingthe least content of coarse sand which,when opened after 48 hours, does not showvisible fissures in the mortar, will indicatethe most adequate proportion of soil/sandfor adobe constructions, giving the higheststrength.

7.3.3 Strength test of adobeThe strength of adobe can be qualitativelyascertained a follows: After 4 weeks of sundrying the adobe be should be strongenough to support in bending the weightof a man, Fig 7.3 (b). If it breaks, more clayand fibrous material is to be added. Quan-titatively, the compressure strength may bedetermined by testing 10 cm cubes of clay

after completely drying them. A minimumvalue of 1.2 N/mm2 will be desirable.

7.4 CONSTRUCTIONS OFWALLSIn general, the strength of walls is a func-tion of clay content, and its activation byhumidity (promoted by wetting orcompaction procedures) and the control offissuring.

The positive effect of a traditional prac-tice, namely �sleeping� the mud (storing itat least for one day but better for more days)before the fabrication of adobe bricks ormortar was confirmed. It seems that thisprocedure allows for a better dispersionand thus for a more uniform action of theclay particles.

Fig 7.3 Field testing of strength of soil and adobe

6

IAEE MANUAL

If the soils are clayey, stronger construc-tions could be built, provided an adequatetechnology is used to control the typical fis-sures caused by drying from high moisturecontent. The most economical and simpleform to control such fissures is by addingcoarse sand to diminish the clay contrac-tion or by adding dry straw to the mud tocontrol the micro-fissures.

In general, there are no �recommended�mixing ratios for the soil to be used inearthen constructions. The different per-centages of clay, lime, fine sand and coarsesand will be defined by the most abun-dantly available nearby soil, its clay con-tent (see dry strength test), the type of con-structions required according to the classi-fication and the amount of coarse sandneeded to control or avoid the visible fis-sures and attain a monolithic behaviour.

It may be concluded that:

(i) Soils with low clay content shouldnot be used (see dry strength test).

(ii) Coarse sand needs be added to avoidfissures and straw to control them.

7.4.1 Hand-moulded layeredconstructionThese are the most primitive and weakesttype of constructions because of the lowpercentage of moisture employed to makethe hand-moulding and the poor level ofcompaction attained. For these reasons, allthe clay is not activated, either by moisture,or by compaction.

Even though a small amount of mois-ture is used (depending on the soil), somehorizontal and also vertical fissures nor-

mally appear. These should be controlledby adding straw as much as possible to at-tain a reasonable workabity of the mixture.If this is not possible, coarse sand could beused as additive, in the smallest experimen-tal proportion able to achieve the disap-pearance of visible fissures (try with in-creasing proportions and wait a few daysto check the results). An excess of coarsesand will inevitably reduce the wallstrength.

Generally, it will be necessary to mois-ten the area of the lower layer which willbe in contact with the mud, in order to avoidsudden drying of the contact zone, whichproduces the fissures.

7.4.2 Adobe or blockconstructionIn the case of cut as well as moulded blocks,the strongest units correspond to plastic orclayey soils. However, the block strengthplays a secondary role in the masonrystrength, since the joints between blocksbecome critical. The blocks used should bewell dried in order to avoid futureretractions. Blocks are made in differentsizes in various countries.

It may be stated that the dimensions ofthe blocks, nor the way these are placed,have a serious effect on their strength.

Traditional practices obtain an adequateblock without important fissures, either bymixing sandy and clayey soils or by look-ing after the block so it dries without re-strictions, thus eliminating the fissures. Thesoil to be used should be verified with thedry strength test, Fig 7.3, to ensure a mini-mum strength.

7

EARTHEN BUILDINGS

The mud used to fill the space betweenblocks (the so called �mortar�) requiresspecial attention.

To guarantee the bond between blocksand mortar, the micro-fissures of the lattershould be avoided. The conditions of themortar drying are very severe because ofthe fact that the mortar gets in contact withblocks which readily absorb moisture andalso they restrict the drying contraction.This produces the above-mentioned micro-fissures, which consequently weaken themasonry.

The joint mud should normally be thesame as used to manufacture the block. If itis found to fissure, some straw (nearly 1:1by volume) should be added to the mortaruntil an acceptable degree of workabilityis attained. Some coarse sand could alsobe added, the adequate proportion beinggiven by the fissuring control test, 7.3.2 (b).

For clayey soils, the adobe blocks shouldbe moistened for a few minutes before plac-ing them. Also it will be useful to moistenthe previous layer of blocks before placingthe joint mortar. For sandy soils, it will beenough only to moisten the preceding layerof blocks.

The usual good principles of bonds inmasonry should be adopted for construc-tion of adobe walls, that is,

(i) All courses should be laid level.

(ii) The vertical joints should be brokenbetween two consecutive courses byoverlap of adobe and must be care-fully filled with mortar.

(iii) The right angle joints between wallsshould be made in such manner thatthe walls are properly joined to-gether and a through vertical joint isavoided.

7.4.3 Tapial or pise constructionTapial or pise constructions are rammedearth constructions in which moist soil ispoured in wooden forms of the walls andcompacted to achieve the desired density.

Whilst adobe constructions acquiretheir strength by activation of the claythrough moisture contained in the soil,tapial constructions owe it to compaction,using small percentages of moisture in thesoil.

High strength is obtained by humidityand compaction when clay is present.There are however, practical limitations torestrict the moisture, such as the feasibilityto pound and compact the soil, the exces-sive deformation occurring when the formsare removed and the fissuring problem.

The use of low moisture content (suchas the optimum in the Proctor test or lower)and the control of the amount of clay byadding coarse sand to the soils are requiredto control the shrinkage fissures on drying.If the amount of coarse sand is excessive,the strength diminishes dangerously. It isrecommended to make wall tests with in-creasing percentage of sand, until fissur-ing is reasonably under control.

The compaction or number of blowsapplied to the wall is a function of theweight and shape of the tool used for thispurpose. Higher strengths are obtained

8

IAEE MANUAL

under higher compaction, but there is apoint at which this is not true any more. Anormal compaction is recommended andthis will be the one under which no mudremains stuck to the form when this is re-moved.

Fifty strokes per 1000 cm2 of wall area,applied with a mallet of about 8 to 10 kgweight is the recommended practice. Therequired height for the blocks varies be-tween 50 and 80 cm, but it is very impor-tant that the compacted layers in the blocksdo not exceed 10 cm each.

The best way to ensure the monolithicstructure of the tapial walls is to sufficientquantity of water at the sub joints at every10 cm. Likewise, between the tapial layers,every 50 to 80 cm, it is necessary to pourplenty of water on the layer before compact-ing further material. The placing of straw

between the tapial layers is not necessary.

The use of excessive amounts of strawin the mud mixture, more than 1:1/4 in vol-ume is selfdefeating, because it causes astrength reduction.

7.4.4 Earthen construction withwood or cane structureThe scheme of earthen construction usingstructural framework of wood or cane isshown in Fig 7.4. It consist of vertical postsand horizontal blocking members of woodor cane or bamboo, the panels being filledwith cane or bamboo, or some kind of reedmatting plastered over both sides with mud.The construction could be done in the ru-dimentary way, building element by ele-ment or by using prefabricated panels.

The behaviour of this type of construc-tion could be very good, as long as the fol-

Fig 7.4 Earthen constructions with wood or cane structures

9

EARTHEN BUILDINGS

lowing fundamental rules are observed:

� Good connections between the woodor cane elements, so as to ensure anintegral behaviour of the structure.The connections are normally fixedwith nails. Their number and dimen-sions should be enough but not ex-

cessive as to split the elements. Theconnections can be also tied withwires, ropes, leather straps, etc.

- Preservation of the wood or cane ele-ments by charring the surface orpainting by coal tar, especially in thepart embedded in the foundation,

Figure 7.5 :: Adequate Configuration

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IAEE MANUAL

which should preferably be of con-crete, stone or bricks laid with ce-ment, lime or gypsum mortar.

- Additionally, it is recommended thatthe panel filling material shouldconsist of wood or cane mesh, overwhich a layer of mud and straw (1:1in volume) is placed on each face in

the form of plaster. Very often, themeshes are knit in themselves andaround the structure.

- In houses built as a continuous sys-tem as well as in those made withpre-fabricated panels, an upper ringbeam should be placed, the purposeof it being two fold:

(i) Ensure the integral behaviour ofall walls, and

(ii) Distribute evenly the roofingload.

Only after fixing this upper ring beamand the roof (after completing the nailing),the mud filling must be placed. This willavoid fissuring caused by the strokes of thenailing operation.

In the case of pre-fabricated panels, theframes could have very small and economi-cal sections 25 × 50 or 25 × 75 mm. The

Fig 7.7 Pillasters at corners

Fig 7.6 Wall dimensions

11

EARTHEN BUILDINGS

connection between panels is madethrough nails, but the wood or cane knitmesh over which the mud filling is placed,can be fixed without the use of nails.

7.5 GENERALRECOMMENDATIONS FORSEISMIC AREAS7.5.1 Walls

(a) The height of the adobe buildingshould be restricted to one story plusattic only in seismic zones A and Band to two storeys in zone C.

(b) The length of a wall, between twoconsecutive walls at right angles toit, should not be greater than 10 timesthe wall thickness �t� nor greater than64t2/h where h is the height of wall.

(c) When a longer wall is required, thewalls should be strengthened by in-termediate vertical buttresses,Fig 7.6 (a).

(d) The height of wall should not begreater than 8 times its thickness.

(e) The width of an opening should notbe greater than 1.20 m.

(f) The distance between an outsidecorner and the opening should benot less than 1.20 m.

(g) The sum of the widths of openingsin a wall should not exceed one-third

of the total wall length in seismiczone A, 40 percent in zones B and C.

(h) The bearing length (embedment) oflintels on each side of an openingshould not be less than 50 cm. Anadequate configuration is shown inFig 7.5 for an adobe and tapial house.

(i) Hand-formed walls could preferablybe made tapering upwards keepingthe minimum thickness 30 cm at topand increasing it with a batter of 1:12at bottom, Fig 7.6 (b).

(j) Providing outside pillasters at allcorners and junctions of walls willincrease the seismic stability of thebuildings a great deal, Fig 7.7.

Fig 7.8 Use of longitudinal wood under roof rafters

Fig 7.9 Reinforcing lintel under floor beam

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IAEE MANUAL

7.5.2 Foundations(a) The brittleness and reduced strength

of adobe constructions restricts thepossible locations of these buildingsto areas associated with firm sub-soils.

Sandy loose soils, poorly compactedclays and fill materials should gen-erally be discarded due to their set-tlements during seismic vibrations.

Also, soils with very high phreaticlevel (water table) should be avoided.These recommendations are particu-larly important for seismic zones Aand B.

(b) Width of strip footings of the wallsmay be kept as follows:

One storey on firm soil � Equal towall thickness

1.5 or 2 storeys on firm soil � 1.5times the wall thickness

One storey on soft soil � 1.5 timesthe wail thickness

1.5 or 2 storeys on soft soil � 2times the wall thickness

The depth of foundation belowground level should at least be 400mm.

(c) The footing should preferably bebuilt using stone, fired brick using

Fig 7.10 Collar band in walls at lintel level

Fig 7.11 Roof band on pillastered walls

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EARTHEN BUILDINGS

cement or lime mortar. Alternativelyit may be made in lean cement con-crete with Plums (cement : sand :gravel : stones as 1:4:6:10) or with-out plums as 1:5:10. Lime could beused in place of cement in the ratiolime:sand:gravel as 1:4:8.

(d) Plinth masonry: The wall above foun-dation up to should preferably beconstructed using stone or burntbricks laid in cement or lime mortar.

Clay mud mortar may be used onlyas a last resort. The height of plinthshould be above the flood water lineor a minimum of 300mm aboveground level. It will be preferable touse a waterproofing layer in the formof waterproof mud (para 7.7 (c)) orheavy block polyethylene sheet at theplinth level before starting the con-struction of superstructure wall. Ifadobe itself is used, the outside face

Fig 7.12 Reinforcement in walls of earthen constructions in zone

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IAEE MANUAL

of plinth should be protected againstdamage by water by suitable facia orplaster. A water drain should bemade slightly away from the wall tosave it from seepage.

7.5.3 RoofingRoofs have two main parts: the structureand the cover. The roofing structure mustbe light, well connected and adequately tiedto the walls.

(a) The roof covering should preferablybe of light material, like sheeting ofany type.

(b) If thatch is used for roof covering, itshould better be made waterproofand fire resistant by applying watermud plaster, para 7.7(c).

(c) The roof beams, rafters or trussesshould preferably be rested on lon-gitudinal wooden elements for dis-tributing the load on adobe, Fig 7.8.If wood is not used preferably twotop courses of burnt bricks may belaid instead of adobe for resting theroof structures.

(d) The slopes and the over-hangingwill depend on local climatic condi-

tions. In zones subjected to rain andsnow, walls protection must be en-sured by projecting the roof by about0.5 m beyond the walls, Fig 7.8.

(e) The roof beams or rafters should belocated to avoid their position abovedoor or window lintels. Otherwise,the lintel should be reinforced by anadditional lumber, Fig 7.9.

7.6 SEISMIC STRENGTHENINGFEATURES7.6.1 Collar beam or horizontalbandTwo horizontal continuous reinforcing andbinding beam or bands should be placed,one coinciding with lintels of doors andwindow openings and the other just belowthe roof in all walls, in all seismic zones forconstructions of types (a), (b), (c), describedin section 7.3.1. Proper connection of tiesplaced at right angles at the corners andjunctions of walls should be insured.Where the height of wall is no more than2.5 m, the lintel band can be avoided butthe lintels should be connected to the roofband as shown, in Fig 7.11. The band couldbe in the following forms:

(a) Unfinished rough cut lumber in sin-gle pieces provided with diagonalmembers for bracing at corners,Fig 7.10(a).

(b) Unfinished rough cut or sawn (50 ×100 mm in section) lumbers twopieces in parallel with halved jointsat corners and junctions of wallsplaced in parallel, Fig 7 10(b).

7.6.2 Pillasters and buttressesWhere pillasters or buttresses are used, asrecommended in 7.5.1(j) at T-junctions, the

Fig 7.13 Diagonal bracing of wood-structures of earthen construction

15

EARTHEN BUILDINGS

collar beam should cover the buttresses aswell as shown in Fig 7.11. Use of diagonalstruts at corners will further stiffen the col-lar beam.

7.6.3 Vertical reinforcement inwallsa. In mesh form of bamboo or caneIn seismic zone A, mesh form of reinforc-ing will be preferable. Here the whole wallsare reinforced by a mesh of canes or bam-boos as shown in Fig 7.12 along with thecollar beams, which may in this case bemade from canes, or bamboos themselves.The vertical canes must be tied to the hori-zontal canes as well as the collar beam atlintel level and the roof beam at eave level.

b. With collar beams or bandsFor seismic zones A and B, in addition tothe collar beams recommended in 7.6.1 and7.6.2 provision of vertical reinforcement in

earthen walls of earthen constructionstypes (a), (b), (c) will be necessary whereasit can be avoided in zone C.

The most effective vertical reinforcementwill be in the form of wood posts, bambooor cane located at corners and junctions ofwalls. It should be started at the founda-tion level and continued through and tiedto the lintel and roof bands by binding wire,fishing line or rope etc.

7.6.4 Diagonal bracingIn case of earthen constructions type d (Sec-tion 7.3.1), it will be necessary for achiev-ing adequate seismic resistance in zones Aand B to provide diagonal bracing mem-bers in the planes of walls as well as hori-zontally at the top level of walls as shownin Fig 7.13. This can be done by using canesor bamboos nailed to the framing members

Fig 7.14 Good features of earthquake resistant construction

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IAEE MANUAL

at the ends and intermediate points of in-tersection.

7.7 PLASTERING ANDPAINTINGThe purpose of plastering and painting isto give protection and durability to thewalls, in addition to obvious aesthetic rea-sons.

(a) Plastering based on natural addi-tives could be formed in two layers.The first one of about 12 to 15 mm, isa mixture of mud and straw (1:1 involume), plus a natural additive likecow dung used to increase the mois-ture resistance of the mud, thus pre-venting the occurrence of fissuresduring the drying process. The natu-ral additive helps to withstand theshrinkage tensions of the restricteddrying process. The second and last

layer is made with fine mud whichwhen dried, should be rubbed withsmall, hard, rounded pebbles.

(b) A technology has been developedwhich consists of plastering wallswith a mud stucco stabilized withcactus as described hereunder:

The main recommendations for plaster-ing adobe walls with this type of stucco areto

(i) Prepare the cactus stabilizer bysoaking cactus chopped piecesuntil the soft (inside) part dis-solves completely leaving theskin only as residue. The ob-tained product is characterizedby gluey consistency, green colorand strong smell of decomposedorganic matter.

(ii) Remove dust from the wall sur-face.

(iii) Apply the stucco in two layers,a first layer of 12 mm thicknessand a second very thin layer (ap-proximately 3 mm). The firstlayer contains straw, and coarsesand in amounts that allow anadequate workability. The sec-ond layer contains straw insmall pieces (approximately50mm) and should not containcoarse sand. The second layercovers the cracks of the first layerand provides a surface adequateto be polished. Both layers aremixed with cactus stabilizer(water is not used).

(iv) Rub the stucco surface with acoarse stone (granitic). Thereaf-Fig 7.15 Axial compression test

17

EARTHEN BUILDINGS

ter, moist the surface with the sta-bilizer and polish it with asmooth stone (basaltic stone).

(v) Paint the finished surface withthe cactus stabi1izer.

(c) To obtain a truly waterproof mudplaster, bitumen may be used in thefollowing way where this materialis found feasible to use: cut-back isprepared by mixing bitumen 80/100grade, kerosene oil and paraffin waxin the ratio 100:20:1. For 1.8 kg cut-back, 1.5 kg bitumen is melted with15 grams of wax and this mixture ispoured in a container having 300millilitre kerosene oil with constantstirring till all ingredients are mixed.This mixture can now be mixed with0.03 m3 of mud mortar to make it bothwater repellent as well as fire pro-tection of thatch.

The exterior of walls may then be suit-ably painted using a water-insoluble paintor wash with water solutions of lime or ce-ment or gypsum and plant extracts.

7.8 SUMMARY OF DESIRABLEFEATURESThe desirable features for earthquake re-sistance of earthen houses are briefly illus-trated in Fig 7.14.

7.9 WORKING STRESSES7.9.1 Unit compressive strengthThe compressive strength of the unit is anindex of its quality and not of the masonry.

It will be determined by testing cubes ofapproximately 100 mm. The compressivestrength fo is the value exceeded by 80% ofthe number of specimens tested.

The minimum number of specimens issix (6) and they should be completely dryat the time of testing. The minimum valueof fo is 1.2 N/mm2.

7.9.2 Masonry compressivestrengthThe masonry compressive strength may bedetermined by:

(a) Prism test with materials and tech-nology to use in the field.

The prisms will be composed by thenumber of full adobes necessary toobtain a height/thickness ratio ofthree.

The minimum number of adobes willbe four and the joint thickness lessthan 20 mm, Fig 7.15.

Special care should be observed tokeep the specimens vertical. They

Fig 7.16 Diagonal compression test

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IAEE MANUAL

should be tested after 30 days of con-struction to obtain a mortar com-pletely dry. The minimum number ofprisms will be three.

The permissible compressive stressfm in wall will be:

fm = 0.4R f � m

where:

R = Reduction factor due to wallslenderness.

R can be obtained by analogy toan elastic column but not greaterthan 0.75.

f � m = Ultimate compressive stress ofprism. Two of every three prismsshould have greater values thanthe compressive strength.

Alternatively the following expres-sion can be used:

fm = 0.2 f � m

(b) If no prism test is conducted, the per-missible compressive stress will be

fm = 0.2 N/mm2

The permissible crushing stress willbe: 1.25 fm

7.9.3 Shear strength of masonryThe shear strength of adobe ma-sonry can be determined by:

(a) Diagonal compression test with ma-terials and technology to be used inthe field, Fig 7.16.

A minimum of three specimensshould be tested. The permissi-ble strength of wall (vm) will beobtained from :

vm = 0.4 f � t

Where: f � t = ultimate strengthof specimen tested. Two of everythree specimens will save valuesthat exceed f � t .

b. When no tests are conducted, the fol-lowing value for the shear strengthmay be used

vm = 0.025 N/mm2

7.9.4 Permissible tensilestrength of masonry for loadsperpendicular to its plane ( fa)

fa = 0.04 N/mm2

��

1

REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGS

Chapter 9

REPAIR, RESTORATION ANDSTRENGTHENING OF BUILDINGS

9.1 INTRODUCTIONThe need to improve the ability of an exist-ing building to withstand seismic forcesarises usually from the evidence of dam-age and poor behaviour during a recentearthquake. It can arise also from calcula-tions or by comparisons with similar build-ings that have been damaged in otherplaces. While in the first case the ownercan be rather easily convinced to take meas-ures to improve the strength of his build-ing, in the second case dwellers that havemuch more stringent day-to-day needs areusually reluctant to invest money in theimprovement of seismic safety. The prob-lems of repairs, restoration and seismicstrengthening of buildings are brieflystated below:

(i) Before the occurrence of the probableearthquake, the required strengthen-ing of seismically weak buildings isto be determined by a survey andanalysis of the structures.

(ii) Just after a damaging earthquake,temporary supports and emergency

repairs are to be carried so that pre-cariously standing buildings maynot collapse during aftershocks andthe less damaged ones could bequickly brought back into use.

(iii) The real repair and strengtheningproblems are faced at the stage afterthe earthquake when things start set-tling down. At this stage distinctionhas to be made in the type of actionrequired, that is, repairs, restorationand strengthening, since the cost,time and skill required in the threemay be quite different.

The decision as to whether a givenbuilding needs to be strengthened and towhat degree, must be based on calculationsthat show if the levels of safety demandedby present codes and recommendations aremet. Difficulties in establishing actualstrength arise from the considerable uncer-tainties related with material properties andwith the amount of strength deteriorationdue to age or to damage suffered from pre-vious earthquakes. Thus, decisions are fre-

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IAEE MANUAL

quently based on gross conservative as-sumptions about actual strength.

The method of repair and strengthen-ing would naturally depend very largelyon the structural scheme and materials usedfor the construction of the building in thefirst instance, the technology that is feasi-ble to adopt quickly and on the amount offunds that can be assigned to the task, usu-ally very limited. Some methods like�splints and bandages�, �wire mesh withgunite�, �epoxy injection,� etc., have al-ready been tried and applied in a few coun-tries for repairing as well as strengtheningearthquake damaged buildings. These aswell as other possible methods will be dis-cussed in this chapter.

9.2 REPAIR, RESTORATIONAND STRENGTHENINGCONCEPTSThe underlying concepts in the three op-erations are stated below:

9.2.1 RepairsThe main purpose of repairs is to bring backthe architectural shape of the building sothat all services start working and the func-tioning of building is resumed quickly. Re-pair does not pretend to improve the struc-tural strength of the building and can bevery deceptive for meeting the strength re-quirements of the next earthquake. The ac-tions will include the following:

(i) Patching up of defects such as cracksand fall of plaster.

(ii) Repairing doors, windows, replace-ment of glass panes.

(iii) Checking and repairing electric wir-ing.

(iv) Checking and repairing gas pipes,water pipes and plumbing services.

(v) Re-building non-structural walls,smoke chimneys, boundary walls,etc.

(vi) Re-plastering of walls as required.

(vii) Rearranging disturbed roofing tiles.

(viii) Relaying cracked flooring at groundlevel.

(ix) Redecoration � whitewashing,painting, etc.

The architectural repairs as stated abovedo not restore the original structuralstrength of cracked walls or columns andmay sometimes be very illusive, since theredecorates building will hide all the weak-nesses and the building will suffer evenmore severe damage if shaken again by anequal shock since the original energy ab-sorbing capacity will not be available.

9.2.2 RestorationIt is the restitution of the strength the build-ing had before the damage occurred. Thistype of action must be undertaken whenthere is evidence that the structural dam-age can be attributed to exceptional phe-nomena that are not likely to happen againand that the original strength provides anadequate level of safety.

The main purpose of restoration is tocarry out structural repairs to load bearingelements. It may involve cutting portionsof the elements and rebuilding them or sim-ply adding more structural material so thatthe original strength is more or less restored.The process may involve inserting tempo-rary supports, underpinning, etc. Some ofthe approaches are stated below:

3


(i) Removal of portions of cracked ma-sonry walls and piers and rebuild-ing them in richer mortar. Use of non-shrinking mortar will be preferable.

(ii) Addition of reinforcing mesh onboth -faces of the cracked wall, hold-ing it to the wall through spikes orbolts and then covering it suitably.Several alternatives have been used.

(iii) Injecting epoxy like material, whichis strong in tension, into the cracksin walls, columns, beams, etc.

Where structural repairs are considerednecessary, these should be carried out priorto or simultaneously with the architecturalrepairs so that total planning of work couldbe done in a coordinated manner and wast-age is avoided.

9.2.3 Strengthening of existingbuildingsThe seismic behaviour of old existing build-ings is affected by their original structuralinadequacies, material degradation due totime, and alterations carried out during useover the years such as making new open-ings, addition of new parts inducing dis-symmetry in plan and elevation, etc.

The possibility of substituting themwith new earthquake resistant buildingsis generally neglected due to historical, ar-tistic, social and economical reasons. Thecomplete replacement of the buildings in agiven area will also lead to destroying anumber of social and human links. There-fore seismic strengthening of existing dam-aged or undamaged buildings can be a defi-nite requirement in same areas.

Strengthening is an improvement overthe original strength when the evaluation

of the building indicates that the strengthavailable before the damage was insuffi-cient and restoration alone will not be ad-equate in future quakes.

The extent of the modifications must bedetermined by the general principles anddesign methods stated in earlier chapters,and should not be limited to increasing thestrength of members that have been dam-aged, but should consider the overall be-haviour of the structure. Commonly,strengthening procedures should aim atone or more of the following objectives:

(i) Increasing the lateral strength in oneor both directions, by reinforcementor by increasing wall areas or thenumber of walls and columns.

(ii) Giving unity to the structure by pro-viding a proper connection betweenits resisting elements, in such a waythat inertia forces generated by thevibration of the building can betransmitted to the members that havethe ability to resist them. Typical im-portant aspects are the connectionsbetween roofs or floors and walls,between intersecting walls and be-tween walls and foundations.

(iii) Eliminating features that are sourcesof weakness or that produce concen-trations of stresses in some members.Asymmetrical plan distribution ofresisting members, abrupt changesof stiffness from one floor to the other,concentration of large masses, largeopenings in walls without a properperipheral reinforcement are exam-ples of defect of this kind.

(iv) Avoiding the possibility of brittlemodes of failure by proper reinforce-

4

IAEE MANUAL

ment and connection of resistingmembers. Since its cost may go to ashigh as 50 to 60% of the cost of re-building, the justification of suchstrengthening must be fully consid-ered.

The extent of modification must befound using the principles of strengthen-ing discussed in Chapters 2, 3 and 4 andin accordance with the local factors appli-cable to each building.

9.3 REPAIR MATERIALSThe most common materials for damagerepair works of various types are cementand steel. In many situations non-shrink-ing cement or an admixture like aluminiumpowder in the ordinary portland cementwill be admissible. Steel may be requiredin many forms, like bolts, rods, angles,channels, expanded metal and weldedwire fabric. Wood and bamboo are the mostcommon material for providing temporarysupports and scaffolding etc., and will berequired in the form of rounds, sleepers,planks, etc.

Besides the above, special materials andtechniques are available for best results inthe repair and strengthening operations.They are described below:

9.3.1. ShotcreteShotcrete is a method of applying a combi-nation of sand and portland cement whichmixed pneumatically and conveyed in drystate to the nozzle of a pressure gun, wherewater is mixed and hydration takes placejust prior to expulsion. The material bondsperfectly to properly prepared surface ofmasonry and steel. In versatility of appli-

cation to curved or irregular surfaces, itshigh strength after application and goodphysical characteristics, make for an idealmeans to achieve added structural capa-bility in walls and other elements. Thereare some minor restrictions of clearance,thickness, direction of application, etc.

9.3.2 Epoxy resinsEpoxy resins are excellent binding agentswith high tensile strength. There are chemi-cal preparations the compositions ofwhich can be changed as per requirements.The epoxy components are mixed just priorto application. The product is of low vis-cosity and can be injected in small crackstoo.

The higher viscosity epoxy resin can beused for surface coating or filling largercracks or holes. The epoxy mixture strengthis dependent upon the temperature of cur-ing (lower strength for higher temperature)and method of application.

9.33 Epoxy mortarFor larger void spaces, it is possible to com-bine epoxy resins of either low viscosity orhigher viscosity, with sand aggregate toform epoxy mortar. Epoxy mortar mixturehas higher compressive strength, highertensile strength and a lower modulus ofelasticity than Portland cement concrete.Thus the mortar is not a stiff material forreplacing reinforced concrete. It is also re-ported that epoxy is a combustible mate-rial. Therefore it is not used alone. The sandaggregate mixed to form the epoxy mortarprovides a heat sink for heat generated andit provides increased modulus of elasticitytoo.

5


9.4.1 Small cracksIf the cracks are reasonably small (openingwidth = 0.075 cm), the technique to restorethe original tensile strength of the crackedelement is by pressure injection of epoxy.The procedure is as follows, Fig 9.1 (a) and(b).

The external surfaces are cleaned ofnon-structural materials and plastic injec-tion ports are placed along the surface ofthe cracks on both sides of the member andare secured in place with an epoxy sealant.The centre to centre spacing of these portsmay be approximately equal to the thick-ness of the element. After the sealant hascured, a low viscosity epoxy resin is injectedinto one port at a time, beginning at the low-est part of the crack in case it is vertical or atone end of the crack in case it is horizontal.

The resin is injected till it is seen flow-ing from the opposite sides of the memberat the corresponding port or from the nexthigher port on the same side of member. Theinjection port should be closed at this stageand injection equipment moved to the nextport and so on.

The smaller the crack, higher is the pres-sure or more closely spaced should be theports so as to obtain complete penetrationof the epoxy material throughout the depthand width of member. Larger cracks willpermit larger port spacing, depending uponwidth of the member. This technique is ap-propriate for all types of structural elements� beams, columns, walls and floor unitsin masonry as well as concrete structures.Two items should however be taken care ofin such type of repair:

9.3.4 Gypsum cement mortarIt has got rather limited use for structuralapplication. It has lowest strength at fail-ure among these three materials.

9.3.5 Quick-setting cementmortarThis material is patented and was origi-nally developed for the use as a repair ma-terial for reinforced concrete floors adjacentto steel blast furnaces. It is a non-hydrousmagnesium phosphate cement with twocomponents, a liquid and a dry, which canbe mixed in a manner similar to portlandcement concrete.

9.3.6 Mechanical anchorsMechanical type of anchors employ wedg-ing action to provide anchorage. Some ofthe anchors provide both shear and ten-sion resistance. Such anchors are manu-factured to give sufficient strength.

Alternatively chemical anchors bondedin drilled holes polymer adhesives can beused.

9.4 TECHNIQUES TO RESTOREORIGINAL STRENGTHWhile considering restoration work, it isimportant to realise that even fine cracksin load bearing members which areunreinforced, like masonry and plain con-crete reduce their resistance very largely.Therefore all cracks must be located andmarked carefully and the critical ones fullyrepaired either by injecting strong cementor chemical grout or by providing externalbandage. The techniques are described be-low along with other restoration measures.

6

IAEE MANUAL

(i) In the case of loss of bond betweenreinforcing bar and concrete, if theconcrete adjacent to the bar has beenpulverised to a very fine powder, thispowder will dam the epoxy from

saturating the region. So it shouldbe cleaned properly by air or waterpressure prior to injection of epoxy.

(ii) It has been stated that cracks smallerthan about 0.75 mm may be difficult

Fig 9.1 Strengthening of existing masonry (continued on next page)

7


to pressure inject. So cracks smallerthan this should not be repaired bythis method.

9.4.2 Large cracks and crushedconcreteFor cracks wider than about 6 mm or forregions in which the concrete or masonryhas crushed, a treatment other than injec-

tion is indicated. The following proceduremay be adopted.

(i) The loose material is removed andreplaced with any of the materialsmentioned earlier, i.e., expansive ce-ment mortar, quick setting cement orgypsum cement mortar, Fig 9.1 (c).

Fig 9.1 Strengthening of existing masonry

8

IAEE MANUAL

(ii) Where found necessary, additionalshear or flexural reinforcement isprovided in the region of repairs.This reinforcement could be coveredby mortar to give further strength aswell as protection to the reinforce-ment, Fig 9.1 (d).

(iii) In areas of very severe damage, re-placement of the member or portionof member can be carried out as dis-cussed later.

Fig 9.2 Roof modification to reduce thrust of walls

(iv) In the case of damage to walls andfloor diaphragms, steel mesh couldbe provided on the outside of the sur-face and nailed or bolted to the wall.Then it may covered with plaster ormicro-concrete, Fig 9.1 (d).

9.4.3 Fractured, excessivelyyielded and buckledreinforcementIn the case of severely damaged reinforcedconcrete member, it is possible that the re-inforcement would have buckled, or elon-

9


enclose the longitudinal bars to preventtheir buckling in future.

In some cases it may be necessary toanchor additional steel into existing con-crete. A common technique for providingthe anchorage uses the following proce-dure:

A hole larger than the bar is drilled. Thehole is filled with epoxy, expanding cement,or other high strength grouting material.

gated or excessive yielding may have oc-curred. This element can be repaired by re-placing the old portion of steel with newsteel using butt welding or lap welding.

Splicing by overlapping will be risky. Ifrepair has to be made without removal ofthe existing steel, the best approach woulddepend upon the space available in theoriginal member. Additional stirrup ties areto be added in the damaged portion beforeconcreting so as to confine the concrete and

Fig 9.3 Details of new roof bracing

10

IAEE MANUAL

The bar is pushed into place and held thereuntil the grout has set.

9.4.4 Fractured woodenmembers and jointsSince wood is an easily workable material,it will be easy to restore the strength ofwooden members, beams, columns, strutsand ties by splicing additional material.The weathered or rotten wood should firstbe removed. Nails, wood screws or

steelbolts will be most convenient as con-nectors. It will be advisable to use steelstraps to cover all such splices and jointsso as to keep them tight and stiff.

9.5 MODIFICATION OF ROOFS(i) Slates and roofing tiles are brittle and

easily dislodged. Where possiblethey should be replaced with corru-gated iron or asbestos sheeting.

Fig 9.4 Integration and stiffening of an existing floor

11


(ii) False ceilings of brittle material aredangerous. Non brittle material likehesian cloth, bamboo matting, orlight ones of foam substances maybe used.

(iii) Roof truss frames should be bracedby welding or clamping suitable di-agonal bracing embers in the verti-cal as well as horizontal planes.

(iv) Anchors of roof trusses to support-ing walls should be improved andthe roof thrust on walls should beeliminated.

Figs 9.2. and 9.3 illustrate one of themethods.

(v) Where the roof or floor consists ofprefabricated units like RC rectangu-

Fig 9.5 Details of inserted slab

12

IAEE MANUAL

lar, T or channel units or woodenpoles and joists carrying brick tiles,integration of such units is neces-sary. Timber elements could be con-nected to diagonal planks nailed tothem and spiked to an all roundwooden frame at the ends. RC ele-ments may either have 40 mm cast-in-situ-concrete topping with 6 mmφ bars 150 mm c/c both ways or ahorizontal cast-in-situ RC ring beamall round into which the ends of RCelements are embedded.Fig 9.4.shows one such detail.

(vi) Roofs or floors consisting of steeljoists and flat or segmental archesmust have horizontal ties holdingthe joists horizontally in each archspan so as to prevent the spreadingof joists. If such ties do not exist, theseshould be installed by welding orclamping.

9.6 SUBSTITUTION ORSTRENGTHENING OF SLABS(a) Insertion of a new slabA rigid slab inserted into existing wallsplays an important role in the resistingmechanism of the building keeping thewalls together and distributing seismicforces among the walls.

The slab has to be properly connectedto the walls through appropriate keys.Fig 9.4 shows typical arrangement to beadopted while in Fig 9.5 some details areshown.

(b) Existing wooden slabsIn the case in which the existing stab is notremoved the following actions have to beundertaken:

Stiffening of the slabThis can be achieved either by planks nailedperpendicularly to the existing ones, Fig 9.6or by placing a RC thin stab over the oldone, Fig 9.7.

In this case a steel network is nailed tothe wooden slab and connected to the wallsby a number of distributed steel anchors.These can be hammered into the intersticesof the wall and a local hand cement grout-ing has to be applied for seating.

Connection of the slab to the wallsA proper link can be obtained by means ofthe devices shown in Figs 9.8. and 9.9.

They consist of flat steel bars nailed tothe wooden supporting beams and to thewooden slab. Holes drilled in the walls toanchor them have to be infilled with ce-ment. If a steel mesh has been used, the con-nection can be made as shown in Fig 9.5,i.e., inserting a small RC band into the ex-isting walls, the band has to be keyed atleast each 3 m.

9.7 PLANNAR MODIFICATIONSAND STRENGTHENING OFWALLS9.7.1 Inserting new wallsIn the case the existing buildings showdissymetries which may produce danger-ous torsional effects during earthquakes,the center of masses can be made coinci-dent with the center of stiffness by separat-ing parts of buildings, thus achieving indi-vidual symmetric units and/or, insertingnew vertical resisting elements such asnew masonry or reinforced concrete wallseither internally as shear walls, or exter-nally as buttresses. Insertion of cross wall,

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Fig 9.6 Stiffening of wooden floor by wooden planks

Fig 9.7 Stiffening wooden floor by reinforced concrete slab and connection to wall

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will be necessary for providing transversesupports to longitudinal walls of long bar-rack type buildings used for various pur-poses such as schools and dormitories.

The main problem in such modifica-tions is the connection of new walls withold walls. Figs 9.10. and 9.11. show twoexamples of connection of new walls to ex-isting ones. The first case refers to aT-junction, the second figure to a cornerjunction. In both cases the link to the old

walls is performed by means of a numberof keys made in the old walls. Steel is in-serted in them and local cement infilling ismade. In the second case however connec-tion can be achieved by a number of steelbars inserted in small length drilled holeswhich substitute keys.

9.7.2 Strengthening existingwallsThe lateral strength of buildings can beimproved by increasing the strength and

Fig 9.8 Connection of floor to wall

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stiffness of existing individual wallswhether they are cracked or uncracked.This an be achieved (a) by grouting; (b) byaddition of vertical reinforced concrete cov-erings on the two sides of the wall (c) bypre-stressing walls.

(a) GroutingA number of holes are drilled in the wall,Fig 9.1. (2 to 4 m2). First water is injected inorder to wash the wall inside and to im-

prove the cohesion between the groutedmixture and the wall elements. Secondly acement water mixture (1:1) is grouted at lowpressure (0.1 to 0.25 MPa) in the holes start-ing from the lower holes and going up.

Alternatively, polymeric mortars may beused for grouting. The increase of shearstrength which can be achieved in this wayis considerable. However grouting cannotbe relied on as far as the improving or con-

Fig 9.9 Connection of floor with wall

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Fig 9.10 (a) Connection of new and old brick walls (T-junction) (b) Connection of new brick wall with existing stone wall

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nection between orthogonal walls is con-cerned. Note that pressure needed for grout-ing can be obtained by gravity flow fromsuper-elevated tanks.

(b) Strengthening with wire meshTwo steel meshes (welded wire fabric withan elementary mesh of approximately50 × 50 mm) are placed on the two sides of

the wall, they are connected by passingsteel each 500 to 750 mm apart, Fig 9.12. A20 to 40 mm thick cement mortar or micro-concrete layer is then applied on the twonetworks thus giving rise to two intercon-nected vertical plates. This system can alsobe used to improve connection oforthogonal walls.

Fig 9.11 Connection of new and old walls (corner junction)

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(c) Connection between existingstone wallsIn stone buildings of historic importanceconsisting of fully dressed stone masonryin good mortar effective sewing of perpen-dicular walls can be done by drilling in-clined holes through them, inserting steelrods and injecting cement grout, Fig 9.13.

(d) PrestressingA Horizontal compression state induced byhorizontal tendons can be used to increasethe shear strength of walls. Moreover thiswill also improve considerably the connec-tions of orthogonal walls, Fig 9.14. The easi-est way of affecting the precompression isto place two steel rods on the two sides of

Fig 9.12 Strengthening with wire mesh and mortar Fig 9.13 Sewing transverse walls withinclined bars

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the wall and strengthening them byturnbuckles. Note that good effects can beobtained by slight horizontal prestressing(about 0.1 MPa) on the vertical section ofthe wall. Prestressing is also useful tostrengthen spandrel beam between tworows of openings in the case no rigid stabexists.

9.7.3 External bindingOpposite parallel walls can be held to in-ternal cross walls by prestressing bars asillustrated above, the anchoring being doneagainst horizontal steel channels insteadof small steel plates. The steel channels run-ning from one cross wall to the other will

Fig 9.14 Strengthening of walls by prestressing

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hold the walls together and improve theintegral box like action of the walls.

Fig 9.15 Splint and bandage strengthening technique

Fig 9.16 Strengthening an arched opening in masonry wall

The technique of covering the wall withsteel mesh and mortar or micro-concretemay be used only on the outside surface ofexternal walls but maintaining continuityof steel at the corners. This wouldstrengthen the walls as well as bind themtogether. As a variation and for economy inthe use of materials, the covering may be inthe form of vertical splints between open-ings and horizontal bandages overspandrel walls at suitable number of pointsonly, Fig 9.15.

9.7.4 Other points(i) Masonry arches If the walls have

large arched openings in them, it willbe necessary to install tie rods acrossthem at springing levels or slightlyabove it by drilling holes on bothsides and grouting steel rods inthem, Fig 9.16 (a). Alternatively, a lin-

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tel consisting of steel channels or I-shapes, could be inserted just abovethe arch to take the load and relievethe arch as shown at Fig 9.16 (b). Injack-arch roofs, flat iron bars or rodsmay be provided to connect the bot-

tom flanges of I-beams, connected bybolting or welding.

(ii) Random rubble masonry walls aremost vulnerable to complete collapseand must be strengthened by inter-

Fig 9.17 Strengthening of long walls by buttresses

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nal impregnation by rich cementmortar grout in the ratio of 1:1 as ex-plained in 9.7.2 (a) or better still cov-ered with steel mesh and mortar asin 9.7.2 (b). Damaged portions of thewall, if any, should be reconstructedusing richer mortar.

(iii) For bracing the longitudinal wallsof long barrack type buildings, a por-tal type framework can be inserted

Fig 9.18 Jacketing a concrete column

Fig 9.19 Increasing the section and reinforcement of existing beams

transverse to the walls and con-nected to them. Alternatively, ma-sonry buttresses or, pillasters may beadded externally as shown inFig 9.17.

(iv) In framed buildings, the lateral re-sistance can be improved by insert-ing knee braces or full diagonalbraces or inserting infill walls.

9.8 STRENGTHENING RCMEMBERSThe strengthening of reinforced concretemembers is a task that should be carriedout by a structural engineer according tocalculations. Here only a few suggestionsare included to illustrate the ways in whichthe strengthening could be done.

(i) RC columns can best be strengthenedby jacketing, and by providing addi-tional cage of longitudinal and lat-eral tie reinforcement around the col-umns and casting a concrete ring,Fig 9.18, the desired strength andductility can thus be built-up.

(ii) Jacketing a reinforced concrete beamcan also be done in the above man-ner. For holding the stirrup in thiscase, holes will have to be drilledthrough the slab, Fig 9.19.

(iii) Similar technique could tie used forstrengthening RC shear walls.

(iv) Inadequate sections of RC columnand beams can also be strengthenedby removing the cover to old steel,welding new steel to old steel andreplacing the cover.

In all cases of adding new concreteto old concrete, the original surfaceshould be roughened, groves made

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Fig 9.20 Improving a foundation by inserting lateral concrete beams

in the appropriate direction for pro-viding shear transfer. The ends of theadditional steel are to be anchoredin the adjacent beams or columns asthe case may be.

(v) RC beams can also be strengthenedby applying prestress to it so that op-posite moments are caused to thoseapplied. The wires will run on bothsides of the web outside and an-chored against the end of the beamthrough a steel plate.

9.9 STRENGTHENING OFFOUNDATIONSSeismic strengthening of foundations be-fore or after the earthquake is the most in-volved task since it may require careful un-derpinning operations. Some alternativesare given below for preliminary considera-tion of the strengthening scheme.

(i) Introducing new load bearing mem-bers including foundations to relievethe already loaded members. Jackingoperations may be needed in thisprocess.

(ii) Improving the drainage of the areato prevent saturation of foundationsoil to obviate any problems of liq-uefaction which may occur becauseof poor drainage.

(iii) Providing apron around the build-ing to prevent soaking of foundationdirectly and draining off the water.

(iv) Adding strong elements in the formof reinforced concrete strips attachedto the existing foundation part of thebuilding. These will also bind thevarious wall footings and may beprovided on both sides of the wall,Fig 9.20. To avoid digging the floor

inside the building, the extra widthcould be provided only on the out-side of external walls. The extrawidth may be provided above the ex-isting footing or at the level of the ex-isting footing. In any case the rein-forced concrete strips and the wallshave to be linked by a number of keys,inserted into the existing footing.

Note: To avoid disturbance to the in-tegrity of the existing wall during thefoundation strengthening process,proper investigation and design iscalled for.

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APPENDIX I

The following definitions are used in the scale:

(a) Type of structures (buildings)Structure ABuildings in field-stone, rural structures,unburnt brick houses, clay houses.

Structure BOrdinary brick buildings, buildings of the largeblock and prefabricated type, half timberedstructures, buildings in natural hewn stone.

Structure CReinforced buildings, well built wooden struc-tures.

(b) Definition of quantitySingle, few About 5 percent

Many About 50 percent

Most About 75 percent

(c) Classification of damage tobuildingsGrade 1 Slight damageFine cracks in plaster; fall of small pieces ofplaster

Grade 2 Moderate damageSmall cracks in walls; fall of fairly large piecesof plaster, pantiles slip off; cracks in chimneys;parts of chimney fall down.

Grade 3 Heavy damageLarge and deep cracks in walls; fall of chim-neys.

Grade 4 DestructionGaps in walls; parts of building may collapse;separate parts of the building lose their cohe-sion; and inner walls collapse.

Grade 5 Total damageTotal collapse of building.

(d) Intensity scaleI Not noticeable

II Scarcely noticeable (very slight)

III Weak, partially observed only

IV Largely observed

V Awakening

VI FrighteningDamage of Grade 1 is sustained in single build-ings of Type B and in many of Type A. Damage

MSK INTENSITY SCALE (AS RELATED TO BUILDINGS)

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in few buildings of Type A is of Grade 2.

VII Damage of buildingsIn many buildings of Type C damage of Grade1 is caused; in many buildings of Type B dam-age is of Grade 2. Most buildings of Type Asuffer damage of Grade 3, few of Grade 4. Insingle instances landslips of roadway on steepslopes; cracks in roads; seams of pipelines dam-aged; cracks in stone walls.

VIII Destruction of buildingsMost buildings of Type C suffer damage ofGrade 2, and few of Grade 3. Most buildings ofType B suffer damage of Grade 3, and mostbuildings of Type A suffer damage of Grade 4.Many buildings of Type C suffer damage ofGrade 4. Occasional breaking of pipe seams.Memorials and monuments move and twist.Tombstones overturn. Stone walls collapse.

IX General damage to buildingsMany buildings of Type C suffer damage ofGrade 3, and a few of Grade 4. Many buildingsof Type B show damage of Grade 4, and a fewof Grade 5. Many buildings of Type A suffer

damage of Grade 5. Monuments and columnsfall. Considerable damage to reservoirs; un-derground pipes partly broken. In individualcases railway lines are bent and roadway dam-aged.

X General destruction of buildingsMany buildings of Type C suffer damage ofGrade 4, and a few of Grade 5. Many buildingsof Type B show damage of Grade 5; most oftype A have destruction of Grade 5; criticaldamage to dams and dykes and severe dam-age to bridges. Railway lines are bent slightly.Underground pipes are broken or bent. Roadpaving and asphalt show waves.

XI DestructionSevere damage even to well built buildings,bridges, water dams and railway lines; high-ways become useless; underground pipes de-stroyed.

XII Landscape changesPractically all structures above and belowground are greatly damaged or destroyed.

•••

Documents

Guidelines for Earthquake Resistant Non … for Earthquake Resistant Non-Engineered Construction Revised edition of “Basic concept of Seismic codes” Volume I part 2, 1980 INTERNATIONAL