Brick Masonry 18

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CHAPTER 18

MASONRY STRUCTURES IN EARTHQUAKE-HIT ZONES IN INDIA

S. Majumdar Department of Civil Engineering,

IIT, Kharagpur.

Part I: Actual Practice and Shortcomings

1. Introduction

Settlers and cultivators introduced masonry structures as dwelling places. These structures havenaturally evolved over thousands of years and are the most tested and adaptable structuresimprovised by man. Depending on the local availability of materials, experience and knowledgethey have gradually developed into present forms. Safety, utility and economy are the threeguiding factors. Except for very specific and isolated reasons, it is very difficult to makeimprovement upon such time tested structures. Till now for poor and lower middle class commonman, economy is a very important factor. Over the years depending on the availability of constructional materials, ingenuity of design capabilities and economic affordability dwellingplaces have developed as an art. These constructions vary from zone to zone. It is observed thatrecent activities of earthquakes are restricted all around the periphery of the Gangetic plane andfoot hills of the Himalayas. The seismic activities in these areas are attributed to the tectonicmovement of the Deccan plate and the main Asian plate.

However, because of its infrequent assault people often overlook to consider the action of earthquakes in the design of buildings. In most regions an individual generally experiences a

strong tremor once in his life time and its devastating effect is forgotten over time. Only over major faults earthquakes are regular features and people are reminded of its looming presencetime to time. Further confusion is caused by the so called experts and researchers, who often inorder to keep the research funds flowing cry wolf all too often. And the government officials,NGOs and professional bodies, instead of being vigilant during the construction step up their activities in distributing relief and poorly managed misused aids.

There are several codes of practices related to masonry and RC Frame cum masonry structures.However, in actual practice structures are quite different. In the present notes actual practices inthe earthquake hit areas in India are explored, their defects are pointed out and remedialmeasures are suggested. Off late RC-cum-brick masonry is fast replacing the old constructionprocedures. However, for economic reasons vast majority of lower-middle class and poor Indianscan not afford such structure and to our assessment will continue to follow the age old

procedures. It is for multitude of this impoverished people common remedial measures andprecautions are suggested.

In section 2 the types of local construction in four major earthquake hit regions have beenclassified. The circumstances under which such particular structures have evolved are discussed.Causes of failures of these structures are explored in section 3. In order to check whether our hypotheses are rational, experimental simulation has been carried out on models in section 4.Section 5 discusses the elementary design considerations, more rigorous finite element analysisthat is yet to come in practice and logical preventive measures. In Section 6 typical codalsuggestions have been discussed. Section 7 discusses the post failure assessment of strength

NPCBEERM, MHA (DM) 1


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c) Latur and Killari, Maharastra

Latur and Killari are situated in the northern extremity of the planes of Deccan plateau. Whilepassing along the highways one can notice coconut sized rounded stones strewn all over thefields on either side of the highway. These stones make an easily available building material inthese parts. One has to just collect and make a wall with locally available limekankar. Walls aregenerally fifteen to 20 inches thick and buildings are some times double storied (Fig.3). As inwestern Gujarat, to avoid summer heat timber floor are laid over with dirt and plastered. Thesefloors are often more than a foot thick. Unlike in Gujarat stone are not flat shaped units. They areround. The binding material, the lime-kankar paste, looses its strength when wet and is unable tosustain the shear forces between the rounded blocks. Thus the blocks roll away and the heavyroof collapses on unaware sleeping inhabitants.

d) Assam in eastern Himalayas

As we travel along the Himalayas from west to east a noticeable change is observed in the rocks.They get weaker and soil content is more. Growth of vegetation is dense and timber is availablein plenty. Along the eastern boundary of Himalayas runs the Naga-fault in the north-southdirection. Occasional tremors are felt in these regions and it is listed as a zone vulnerable toquakes. Thus people being aware they built their houses mostly in timber and wooden planks

(Fig.4). Timber has three excellent properties. It is light weight, yet possesses considerabletensile strength and pliability. It has its own difficulties. It is weak in shear along the grains. It isalso difficult to make joints. Yet, people in eastern Himalayas made ingenuous constructions withtimber and light-weight wall panels made of in-filled plasters. But here also timber is gettingcostlier and wooden buildings are fast being replaced by brick-cum-RC frames.

e) Types and Causes of Failure

i) Vertical cracks at wall junctions

From all the above types of constructions we can summarize different types of failures andanalyze their causes. Let us consider a single room single storey building (Fig.5) . In a way this isthe most vulnerable case. In such a building roof may be sloped or flat, mostly sloped.Earthquake loading acts in the horizontal plane. In Fig.5 , if the periodic motion is in the directionshown it is obvious that the total inertial load developed in the end walls will be borne by thesection AB. If the masonry is not strong enough in tension there will be a vertical crack in thewalls at AB. Once these cracks develop all the wall will be free standing. Mostly roofs are slopedmade of tin overlaid with stone slabs. Free standing cantilever walls will not be able to withstandthe transverse horizontal reaction forces.

B

A

Fig.5 Inertial forces on Section



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ii) 45 o bi-directional cracks in wall panels

A second type of crack appears in the wall panels and could be seen in many buildings in Bhujarea. That is stress concentration at the window corners. Because of the horizontal shear exertedby the inertial forces of roof and walls diagonal cracks appear in the wall panel. These cracks aredue to diagonal tensile forces arising from shear. (continuous lintel and Reinforced Bricks).

Primary failures are in lintels and bi-directional shear cracks in walls without window openings.Problems with massive construction are due to the failure in arch actions. Arches are extremelyvulnerable to support motions.

Fig.6 Diagonal cracks originating at opencorners

ing

Apart from that the flooring system is also complex. The floor area is successively divided intosmaller simply supported spans which are then covered with stone slabs or wooden planks.

iii) Lintels and beams

Piece lintels are not adequate for earthquake resistance. Cracks generate at the brick wall andlintel interface.

Failures of beams are observed but main reason of failures of beams is loss of support reaction,resulting from wall collapse. Thus often slabs and beams must be cut open to rescue buriedpeople. Thus integrity of beam slab complex often becomes a hindrance to rescue operations.Thus wall and column supports should be strong enough not to allow the roofs to collapsealtogether.

iv) Brick columns

Brick pillars supporting a roof over verandah often collapse and subsequently bring down the roof it supports.

v) Masonry Arches

Another structural feature is common in brick buildings, these are arches. In order to span a largeopening without a lintel arch is the only alternative. Arches carry load primarily in compression.However, the supports must be rigid and immovable. Vulnerability of arches to support motionmakes them totally unsuitable under earthquake motion.



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3. Lessons from Experimental Model Simulations

The best lesson one learns is from actual failures in real life. These lessons are invaluable, but atthe same time painful and expensive. Quakes are rare occurrences and only one example out of many possibilities. Thus, to complement the actual field observation experiments should becarried out in the laboratories. These experiments are scaled down versions and often all theparameters may not be represented accurately. Yet they may be repeated many times under closely controlled environment and a lot may be learnt from these experiments. Particularly if theexperiments exhibit the same trend as in the field, a lot may be inferred from these repeatedexperiments. Such an attempt was made by Suryawanshi (1994) in his masters thesis. A shakingtable is built from a discarded lathe machine. A circular disk is mounted on a thick plate onrollers. The disk can be oriented in any direction to simulate incident ground motion from anydirection relative to a idealized masonry building.

On this platform, executing a harmonic motion with different amplitude and frequency (frequencyof course is limited to only 3 levels because of the limitation of the old lathe machine), 1 and 2-room buildings were assembled with brick like units made of mild steel (to scale up inertial forces)and a weak mortar of sand, mud and cements (to scale down the bond). Buildings are subgectedto harmonic base motions from different directions. Each experiment is repeated several timesand they show a consistent failure patterns. Results (type of failures) on the parametric spaceexhibit a clustering tendency.

Objective of this experimental program is to investigate the possibility of predicting the failurepattern in a fuzzy environment. To achieve this, the inputs are considered to be crispdeterministic variables, whereas outcomes are qualitative and fuzzy.

PART-II: Analysis and Design Considerations

4. Analysis and Design of Masonry Structures

Main constraint in designing masonry buildings is that it can hardly sustain tension. Tensionappears in the structure as a consequence of shear, arising from horizontal inertial forces,settlement of support, stress concentration at the corners of door and window openings,horizontal support motion, etc. It will be quite clear if we consider a few typical cases throughMohrs circles. Consider a wall with door and window openings and the wall itself being supportedon an elastic foundation. The wall is subjected to a horizontal inertial force at the floor and roof levels. Generally elementary strength of material approach is adopted for an approximateanalysis.

4.1 Basic design procedures for masonry structures

A general fundamental procedure for designing masonry structures are specified in IS Codes.However, stipulations are general and a designer has to use his discretion to come up with arational design. The most basic underlying principle in earthquake resistance design is to

increase ductility , particularly of walls and columns. Some of the important elements arediscussed below.

1) A continuous ductile lintel band made of reinforced concrete, timber, reinforced brick, etc.should be provided, preferably at 3 levels, plinth, lintel and roof. As such masonry walls show anextremely brittle behavior under earthquake actions. Because of its cyclic nature bidirectionalcracks develop in walls. These cracks join up to produce a set of orthogonal cracks in themasonry. These cracks are evenly spaced through out the wall and break the latter up intorubbles, which ultimately looses all its strength. The reinforced concrete floors then collapse andinhabitants are trapped between floors. In order to avoid such situations one has to make sure of



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the structural integrity of walls. Lintel bands provide the required ductility and help in reducingdiagonal tension cracks by intercepting the crack field. In order to determine the size of astructural element and the percentage of reinforcement (of a lintel, say) following criteria shouldbe kept in mind.

a) Reinforcement should be such that the longitudinal strain generated by the maximum groundhorizontal acceleration (assumed to be 0.3g here) must not exceed the allowable strain inconcrete, that is, 0.003. Thus, for a total mass of the wall of length L (Fig.7(a)) , height H,thickness t and the specific mass , the force generated is

)(0.18000)0.3)(0.2000)(250.0)(0.3)(0.4()3.0(... N gt H LF ===

The size of a typical lintel (150 mm x 200mm) with 4-10 and modular ratio 10, the reinforcementundergoes a strain of

{ }4)10(2727.0

)10(0.75)0.200)(0.150(0.000,18 =

+=

c E

The second criterion is such that due to transverse load the amplitude of displacement in thetransverse direction should not be more than the nominal eccentricity in a column or be less than1/500 (Say). This stipulation will not allow too much eccentricity of the load in the brick column.

The uniformly distributed inertial load is q = W/L. Thus the transverse displacement should be

002.000385.0)10)(9.3(3844)0.18000(5

3845

6

34

>=== EI

qLmm (Unsatisfactory)

Thus, both the size of the lintel and/ or the reinforcement should be increased.

b) Corner L-type reinforcement in walls junctions at layers at regular intervals

As has been mentioned earlier ground motion may come from any direction. It depends on theorientation of the building or a particular wall. Quite often the inertial force coming on an entirewall is borne by the brick walls (Fig. 7(b)) in transverse direction and causes vertical cracks near the corners. In that case walls of a rooms do not act as a unit. Rather they stand in isolation andloose stability. In such cases L- shaped reinforcements are provided in the horizontal layers.These reinforcements should be of mild steel and should be integrated into the structure throughmortar bond. Area requirement of the reinforcing steel is computed through elementarycalculations.

Assume that these reinforcements are provided at an interval of m layers. Thus the inertial forcesustained by a r- bar is

)(0.1350)3)(0.2000)(0.4)(250.0)(3(075.0)3.0.(...075.0 N g Lt mF ===

Since the force produced by this force is in tension the area of steel at each end should be

2350.1500675

)0.250(2mm

F As ===

Thus area appears to be too small.. However, strain also should be less than the allowable strainin brick (assume 0.002). From this consideration,



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0015.0)10(866.0)10(5.0)2500.0)(4)(075.0(2

0.650).075.0(2

410


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be repeated and replace most conventional and traditional constructions. Particularly in the hillscovered ground area comes at premium and framed buildings with isolated footings are quitecommon. Here we will like to draw the attention to a common construction procedure that shouldbe modified. Because of ease of construction frame is first completed and then the brick wall arelaid to fill in the gaps. Thus the dead load of brick walls are entirely borne by the frame. Except for the shear resistance offered by the filled in brickwork, the brick does not share any load. Instead,in almost a reverse procedure, reinforced brick wall should be constructed first and the corner reinforcements should be embedded in the columns when it is cast later. Similarly, the top of brickwork should act as the shuttering to the beam bottom surface of the beam. This type of construction will ensure uniform sharing of load both by the RC frame and the brickwork. It willalso provide an extra bond between the brickwork and the RC columns.

h) Arches

Before reinforced concrete was introduced main difficulty faced by a designer was availability of amaterial which could stand bending moment. Thus arches were the only option which could spana gap and could sustain the load by axial compression. However, a steel bar should be added atthe springing points in order to prevent support displacements. This measure is particularlyessential at end panels.

i) Caution against compound structuresMost of the buildings in earlier times adopted different materials for different portions, such asfoundations, walls, lintels, beams, slabs and roofs. It is difficult to integrate together a varied typeof materials with different strength, stiffness and ductility. Particularly their interactions are oftenlittle understood and almost impossible to develop a general software for their design.

5.1 FEM modeling of a brick walls

In general even basic building plans have many irregularities in the form of door and windowopenings, different types of materials, an analytical approach is not possible. However to assessthe forces closely a Fem base numerical approach is a tolerably acceptable alternative. For adetail analysis of brick and mortar interaction and also to study the effect of reinforcement in brickwalls a FEM model has been constructed here. The model treats each individual brick as a 20-noded brick element and the mortal joints as 3-noded interface elements. Fig.8 shows an isolated5 thick brick wall. The wall is subjected to in-plane and out-of-plane forces. The edges of the wallare free to deform in the plane of the paper, but the transverse displacements are restricted at theedges.

5. Codal Provisions

There are several codes on masonry buildings. Two main references are IS: 4326:1993Earthquake Resistant Design and Construction of BuildingsCode of Practice andIS:13828:1993, Improving Earthquake Resistance of Low Strength Masonry BuildingsGuidelines.

Codes are mainly guidelines. Since they address the general situation and play a conservative

role they are often restrictive or silent on specific issues. The present author is unaware of theglossaries leading to IS-codes. However, committee reports leading to all codes are available andmay be referred to. In the present notes some guidelines for quantitative design have alreadybeen suggested. A concise procedure for design of earthquake resistant masonry structure basedon Uniform Building Code may be found in Williams (2003). However, such masonryconstructions are rare in India. Particularly the material standards are no where near the UScodes. Thus this design procedures are irrelevant in Indian context.



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6. Post-failure Strength Assessment and Remedial Measures

One more important aspect remains to be discussed. In spite of all precautions and designfailures do occur because of gradual disintegration of brick-mortar bond, corrosion of reinforcements, decay of wooden supports and gradual shifting of masonry blocks. In such casesmaintenance

7. Conclusion

Heterogeneity of material and widely varying construction practices make masonry design achallenging task. It is very difficult to standardize the design procedure. As has been pointed outin the text, four earthquake zones have totally different design concepts. Fortunately, however,most old types of constructions are fast being replaced by standard RC frame-cum-brickmasonry. Yet in India quality of bricks and construction procedures are non-standard. Thus ageneral recommendation should be avoided. Instead a designer should follow a few basicprinciples, such as, ductility, strength and stability as basic principles. However, with FEMtechniques fast developing a more rational analysis is possible and its applications to designshould be explored.

Refernces:1. Dowrick, D. J., (1977), Earthquake resistant design, John-Wiley and Sons.2. IS:13828:1993, Improving Earthquake Resistance of Low Strength Masonry Buildings

Guidelines.3. Sharma, R. P. (2003), Dynamic Response of Blocky Rock Mass: An experimental and

Numerical Study (Submitted for Doctoral Degree to the Dept. of Civil engineering, IIT,Kharagpur.)

4. Suryawanshi, R. K., (1994), Fuzzy Damage Prediction for Stone Masonry using NeuralNetwork, M.Tech Thesis, Dept. of Civil Engineering, IIT Kharagpur.

5. Williams, A.,(2003), Seismic design of Building and bridges, Oxford University Press.

Fig. 1 Stone masonry and arches, Bhuj, Gujrat



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Rounded stonesBonded with weak lime-kankar mortar

Dirt Fillwith floor

Wooden planks

Timber joists

Fig. 2 Timber-stone-sandstone slab structure,UttarKashi, Uttaranchal.

Fig.3 Stone masonry made of small ( 6 ) roundedboulders, Killari, Maharastra.



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Typical light-weight plasteredpanel units

Fig. 4 Timber Buildings in Assam

Timbers

Bamboo lath withfloor finish

Timbers of structuralframe



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(a) RC bands at plinth, linteland roof levels

(b) L-reinforcements atfree corners

c) Reinforced Brick, expandedmetal

Fig. 7 Structural Details for Masonry Constructions



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Fig. 8 FEM Discretisation for a 5Brick Wall


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