j Seismic Building Code of Pakistan (Masonry)

8/13/2019 j Seismic Building Code of Pakistan (Masonry)

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CHAPTER 9

MASONRY


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Definitions Section 9.1 & 9.2 deals with the description of symbolic notations and

terminologies used in the documents, such as; ColumnAn isolated vertical load bearing member having length to

width ratio not more than 4.0.

PierA thickened section forming integral part of a wall placed atintervals along the wall length, to increase the stiffness of the wall or to

carry a vertical concentrated load. Single-leaf wal ls The effective thickness of single-leaf walls of either

solid or hollow units is the specified thickness of the wall.

Mult i leaf wal ls The effective thickness of multileaf walls is thespecified thickness of the wall if the space between leafs is filled withmortar or grout. For walls with an open space between leafs, theeffective thickness shall be determined as for cavity walls.

Types of Walls: a) Cavity wal l A wall comprising two leaves, each leaf being built of masonry

units and separated by a cavity and tied together with metal ties or bonding unitsto ensure that the two leaves act as one structural unit, the space between theleaves being either left as continuous cavity or filled with a non-load bearinginsulating and water-proofing material.

b) Faced Wall A wall in which facing and backing of two different materials are

bonded together to ensure common action under load. c) Veneered w allA wall in which the facing is attached to the backing but not so


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9.4 - Materials 9.4.1 Masonry Units

Masonry units may compose of:

a) Burnt clay building bricks b) Stones (in regular sized units) c) Concrete blocks (solid and

hollow)

d) Burnt clay hollow blocks e) Gypsum partition blocks

The compressive strength ofbricks may not be less than1200psi for solid bricks and 800psi for hollow blocks.

9.4.2

Mortar The compressive strength of

masonry mortar in seismic Zones2, 3 and 4 shall not be less than4.10 MPa (595 psi) and notgreater than 75% of the

compressive strength of themasonry unit in the direction

Can not be used in Zone 2, 3

and 4.


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Masonry

The design of masonry structures is recommended to becarried out under the working stress method, for all loads

such as: Self Weight

Live Loads

Seismic and Wind Lateral Loads

Snow & Vibration loads Vertical load dispersion at not more than 30angle is not

allowed. Whereas, arching action is permitted at lintel andbeam joints.

Bearing length of lintel should not be less than 100mm orone-tenth of the span.

Bearing length of lintel 100 mm or 1/10 ofspan

Span


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9.6.6.1 Permissible Compressive

Stresses

The basic compressive stress is further to be multiplied bystress reduction factor, area reduction factor and shapemodification factor as to convert to permissible stressvalues.


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9.6.6.1. Reduction Factors

9.6.6.1.2 Area Reduction

Factor: This factor takes into

consideration smallness of the

sectional area of the element

and is applicable when sectional

area of the element is less than

0.2 m2(2.15 ft2). The factor ka

= 0.7 + 1.5A, A being the area

of section in m2.

9.6.6.1.3 Shape ModificationFactor: This factor is applicable

for units for cursing strength up

to 2200 psi.


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9.6.6.2 Permissible Tensile stresses In general tensile stresses are not permitted in masonry

structures. But when considering out of plane bending, underlateral loads, the following stresses may be permitted: Grade M1 or better mortar:

10 psi for bending in the vertical direction where tension developed isnormal to bed joints.

20 psi for bending longitudinal direction where tension developed is parallelto bed joints provided crushing strength of masonry units is not less than1450 psi.

Grade M2 mortar: 7.25 psi for bending in the vertical direction where tension developed is

normal to bed joints.

14.5 psi for bending in the longitudinal direction where tension developed isparallel to bed joints provided crushing strength of masonry unit is not less

than 1100 psi.

NOTES: 1. No tensile stress is permitted in masonry in case of water retaining

structures in view of water in contact with masonry. Also no tensile stress ispermitted in earth-retaining structures, in view of the possibility of presenceof water at the back of such walls.

2. Allowable tensile stress in bending in the vertical direction may beincreased to 14.5psi for M1 mortar and 10psi for M2 mortar in case of


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Permissible Shear Stress &

Reinforcement 9.6.6.3

Permissible Shear Stresses:

Shear stresses shall be given by the following formula, notexceeding a maximum of 75 psi; fs= 0.1 + fd/6

fd= Compressive stress due to dead loads in MPa.

If there is tension in any part of a section of masonry the area under

tension shall be ignored while working out shear stress on the section. 9.6.6.4 Permissible stresses in Reinforcement

Tensile Stresses: Deformed bars: Fs = 0.5 fy , 24 Ksi max

Wire Reinforcement: Fs = 0.5 fy , 30 Ksi max

Ties, anchors, smooth bars: Fs = 0.4 fy , 20 Ksi max

Compressive Stresses: Deformed bars in columns: Fs = 0.4 fy , 24 Ksi max

Deformed bars in flexural members: Fs = 0.4 fy , 24 Ksi max

Deformed bars in shear walls, confined by ties: Fs = 0.4 fy , 24 Ksi max


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9.6.7 Design TH. / Cross

Section

Wall and columns bearing vertical loads shall be designed on the basis or permissiblecompressive stress.

Design thickness of solid walls should be as per layout, in consideration with mortarjoints and layout.

Thickness of each leaf of cavity walls should not be less than 75mm. The leaf should be supported at most at a height of 10m.

In faced walls, the permissible load per length of wall shall be taken as the product ofthe total thickness of the wall and the permissible stress in the weaker of the twomaterials.

Veneered walls are not to be accounted in calculations for load carrying purposes.

Free standing walls, subjected to wind pressure or seismic forces shall be designed onthe basis of permissible tensile stress in masonry or stability.

Normally masonry of retaining walls shall be designed on the basis of zero-tension andpermissible compressive stress.

Walls subjected to in-plane bending and vertical loads that is shear walls shall be


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9.7 Empirical Design Provisions

Storey height ofmasonry buildings shall

not be more than 10ft.

9.7.1.6Load Bearing Walls

1. Masonry materials to be used in theconstruction of load-bearing walls shall benatural stone, solid burnt clay brick orconcrete blocks, hollow bricks or concrete

block.2. Natural stone load-bearing walls shall beused only in the basement and groundstories of masonry buildings. The use ofundressed stones is not allowed. Stones

shall at least be semi-dressed.


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Load Bearing Walls (Min TH.)The ratio of the total length ofmasonry load-bearing walls in eachof the orthogonal directions in plan, togross floor area shall not be less 0.85ft/ft2.

Unsupported length of a load-bearingwall between cross walls in theperpendicular direction shall notexceed 18 ft in Seismic zones 3 & 4and 23 ft in other seismic areas.

Otherwise provide intermediatecolumns at a spacing of 13 ft up to amaximum of 50ft length.The following rules shall be followed to provide openings in load-bearing

walls:a) Plan length of the load-bearing wall segment between the corner of the building and

the nearest window or door opening shall not be less than of the height of opening

but not less than 24 in.

b) The total length of openings shall not exceed 50% of length of wall between

consecutive cross walls in single storey construction, 42% in two storey construction

and 33% in three storey buildings.c) The vertical distance from opening to opening directly above it shall not be less than


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9.7.2 Seismic Strengthening The masonry buildings shall be strengthened using

seismic bands and vertical bars or by utilizing

provisions for confined masonry.

9.7.2.1.1

Lintel Bands Bearing of window and door lintels on the walls shall not

be less than 15% of lintel clear span nor less than 8 in.

9.7.2.1.2

Horizontal Bands Reinforced concrete bands shall be provided

continuously through all load-bearing longitudinal and

transverse walls at plinth, lintel and roof-eave levels and

also at the top of gables.


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9.7.2.1.3 Vertical Reinforcement in

Walls

Steel bars shall beinstalled at the criticalsections (i.e. thecomers of walls,

junctions of walls, and

jambs of doors) andwell anchored into thefoundation concrete.

The vertical steel atopenings may bestopped by embeddingit into the lintel band,but the vertical steel at

the comers andunctions of walls must


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9.7.2.2 Confined Masonry The horizontal and vertical confining elements shall be bondedtogether and anchored to the elements of the main structural

system.

Masonry walls shall be tied to the vertical confining elements byproviding horizontal reinforcement. 2 - in. diameter bars spacedat a maximum spacing of 24 in.

The cross-sectional dimensions of both horizontal and verticalconfining elements shall not be less than 6 in.

Vertical confining elements should be placed at the free edges of astructural wall, within the wall (not exceeding a spacing of 15ft) , atthe intersections of structural walls and at both sides of any wallopening.

Horizontal confining elements shall be placed in the plane of thewall at every floor level and in any case with a vertical spacing ofnot more than 10 ft.

The longitudinal reinforcement of confining elements shall not havea cross-sectional area less than 0.4 in2 , nor more than 1% of thecross-sectional area of the confining element.

Stirrups not less than in. in diameter and spaced not more than 6in. shall be provided around the longitudinal reinforcement.


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Slabs, Roofs & Non Bearing

Walls Slabs and Roofs shall be used as RCC slabs orjoist floors, or as timber or steel trussed roofs,with the cantilever balconies being the extensionof the floor slab.

9.7.4 Non Bearing Walls The thickness of non-bearing partition walls should

be at least brick size or 4 in.

Should be constructed with load bearing walls atboth ends.

Height of parapet walls should not be more than 24in.

Height of garden walls shall not be more than 3 ft

from pavement level.


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j Seismic Building Code of Pakistan (Masonry)