IS 13935 : 1993(Reaffirmed 1998)

Edition 1.1(2002-04)

Indian Standard

REPAIR AND SEISMIC STRENGTHENINGOF BUILDINGS — GUIDELINES

(Incorporating Amendment No. 1)

UDC 699.841 : 624.012.45 : 624.042.7

© BIS 2002

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Price Group 9

Earthquake Engineering Sectional Committee, CED 39

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized bythe Earthquake Engineering Sectional Committee had been approved by the Civil EngineeringDivision Council.Himalayan-Naga Lushai region, Indo-Gangetic Plain, Western India and Kutch and Kathiawarregions are geologically unstable parts of the country and some devastating earthquakes of theworld have occurred there. A major part of the peninsular India, has also been visited by moderateearthquakes, but these were relatively few in number and had considerably lesser intensity. It hasbeen a long felt need to rationalize the earthquake resistant design and construction of structurestaking into account seismic data from studies of the Indian earthquakes, particularly in view ofthe heavy construction programme at present all over the country. It is to serve this purpose thatIS 1893 : 1984 ‘Criteria for earthquake resistant design of structures’ was prepared. It lays downthe seismic zones, the basic seismic coefficients and other factors and criteria for variousstructures. As an adjunct to IS 1893, IS 4326 ‘Code of practice for earthquake resistant design andconstruction of buildings’ was prepared in 1967 and revised in 1976 and in 1993. 1976 version,contained some recommendations for low strength brick masonary and stone buildings which havenow been covered in greater detail in IS 13828 : 1993 ‘Guidelines for improving earthquakeresistance of low strength masonary building’.Earthquakes damages to buildings in Himachal Pradesh, North Bihar and hill districts of UttarPradesh emphasized the need to formulate this standard to cover guidelines for repair andstrengthening of these buildings from any future earthquakes.The composition of the technical committee responsible for formulating this standard is given inAnnex A.This edition 1.1 incorporates Amendment No. 1 (April 2002). Side bar indicates modification of thetext as the result of incorporation of the amendment.

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Indian Standard

REPAIR AND SEISMIC STRENGTHENINGOF BUILDINGS — GUIDELINES

1 SCOPE

1.1 This standard covers the selection ofmaterials and techniques to be used for repairand seismic strengthening of damagedbuildings during earthquakes and retrofittingfor upgrading of seismic resistance of existingbuildings.1.2 The repair materials and techniquesdescribed herein may be used for all types ofmasonry and wooden buildings, and theconcrete elements used in buildings.1.3 The provisions of this standard areapplicable for buildings in seismic Zones III toV of IS 1893 : 1984 which are based ondamaging seismic intensities VII and more onModified Mercalli or M.S.K. scales. The schemeof strengthening should satisfy therequirements stipulated for the seismic zone ofIS 1893 : 1984, building categories ofIS 4326 : 1993 and provisions made inIS 13827 : 1993 for earthen buildings andIS 13828 : 1993 for low strength masonarybuilding. No special seismic resistance featuresare considered necessary for buildings inseismic Zone II.

2 REFERENCES

The Indian Standards listed below are thenecessary adjuncts to this standard:

3 TERMINOLOGY

3.0 For the purpose of this guide, the followingdefinitions shall apply.

3.1 Separation Section

A gap of specified width between adjacentbuildings or parts of the same building, topermit movement, in order to avoid hammeringdue to earthquake.3.2 Crumple Section

A separation section filled with appropriatematerial which can crumple or fracture in anearthquake.3.3 Centre of Rigidity

The point in a structure, where a lateral forceshall be applied to produce equal deflections ofits components, at any one level in anyparticular direction.3.4 Shear Wall

A wall designed to resist lateral force in its ownplane. Braced frames, subjected primarily toaxial stresses, shall be considered as shearwalls for the purpose of this definition.

3.5 Space Frame

A three-dimensional structural systemcomposed of interconnected members withoutshear or bearing walls, so as to function as acomplete self-contained unit, with or withoutthe aid of horizontal diaphragms or floorbracing systems.

3.6 Moment Resistant Frame

A space frame capable of carrying all verticaland horizontal loads by developing bendingmoments in the members and at joints.

3.7 Moment Resistant Frame with Shear Walls

A space frame with moment resistant jointsused in combination with shear walls to resistthe horizontal loads.

3.8 Box System

A bearing wall structure without a space frame,the horizontal forces being resisted by the wallsacting as shear walls.

3.9 Band

A reinforced concrete, reinforced brick orwooden runner provided horizontally in thewalls to tie them together and to imparthorizontal bending strength in them.

IS No. Title

456 : 1978 Code of practice for plain andreinforced concrete

1893 : 1984 Criteria for earthquake design ofstructures

4326 : 1993 Code of practice for earthquakeresistant design and constructionof buildings ( third revision )

13827 : 1993 Guidelines for improvingearthquake resistance earthenbuildings

13828 : 1993 Guidelines for improvingearthquake resistance of lowstrength masonry buildings

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3.10 Seismic Zone, and Seismic Coefficient

The seismic Zones II to V as classified and thecorresponding zone factors as specified in 6.4.2(Table 2) of IS 1893 (Part 1).3.11 Zone Factor (Z)

It is a factor to obtain the design spectrumdepending on the perceived maximum seismicrisk characterized by maximum consideredearthquake (MCE) in the zone in which thestructure is located.3.12 Concrete Grades

28 days crushing strength of concrete cubes of150 mm side, in MPa, for example, for M15grade of concrete ( see IS 456 : 1978 ), thestrength = 15 MPa.

4 GENERAL PRINCIPLES AND CONCEPTS

4.1 Non-structural/Architectural Repairs

4.1.1 The buildings affected by earthquake maysuffer both non-structural and structuraldamages. Non-structural repairs may cover thedamages to civil and electrical items includingthe services in the building. Repairs tonon-structural components need to be taken upafter the structural repairs are carried out.Care should be taken about the connectiondetails of architectural components to the mainstructural components to ensure their stability.4.1.2 Non-structural and architecturalcomponents get easily affected/dislocatedduring the earthquake. These repairs involveone or more of the following:

a) Patching up of defects such as cracks andfall of plaster;

b) Repairing doors, windows, replacement ofglass panes;

c) Checking and repairing electricconduits/wiring;

d) Checking and repairing gas pipes, waterpipes and plumbing services;

e) Re-building non-structural walls, smokechimneys, parapet walls, etc;

f) Replastering of walls as required;g) Rearranging disturbed roofing tiles;h) Relaying cracked flooring at ground level;

andj) Redecoration — white washing, painting.

etc.The architectural repairs as stated above do notrestore the original structural strength ofstructural components in the building and anyattempt to carry out only repairs toarchitectural/non-structural elements

neglecting the required structural repairs mayhave serious implications on the safety of thebuilding. The damage would be more severe inthe event of the building being shaken by thesimilar shock because original energyabsorbtion capacity of the building would havebeen reduced.

4.2 Structural Repairs

4.2.1 Prior to taking up of the structuralrepairs and strengthening measures, it isnecessary to conduct detailed damageassessment to determine:

a) the structural condition of the building todecide whether a structure is amendablefor repair; whether continued occupationis permitted; to decide the structure as awhole or a part require demolition, ifconsidered dangerous;

b) if the structure is considered amendablefor repair then detailed damageassessment of the individual structuralcomponents (mapping of the crackpattern, distress location; crushedconcrete, reinforcement bending/yielding,etc). Non-destructive testing techniquescould be employed to determine theresidual strength of the members; and

c) to work out the details of temporarysupporting arrangement of the distressedmembers so that they do not undergofurther distress due to gravity loads.

4.2.2 After the assessment of the damage ofindividual structural elements, appropriaterepair methods are to be carried outcomponentwise depending upon the extent ofdamage. The repair may consist of thefollowing:

a) Removal of portions of cracked masonrywalls and piers and rebuilding them inricher mortar. Use of non-shrinkingmortar will be preferable.

b) Addition of reinforcing mesh on both facesof the cracked wall, holding it to the wallthrough spikes or bolts and then coveringit, suitably, with cement mortar ormicro-concrete.

c) Injecting cement or epoxy like materialwhich is strong in tension, into the cracksin walls.

d) The cracked reinforced cement elementsmay be repaired by epoxy grouting andcould be strengthened by epoxy or polymermortar application like shotcreting,jacketting, etc.

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4.3 Seismic Strengthening

The main purpose of the seismic strengtheningis to upgrade the seismic resistance of adamaged building while repairing so that itbecomes safer under future earthquakeoccurrences. This work may involve some of thefollowing actions:

a) Increasing the lateral strength in one orboth directions by increasing column andwall areas or the number of walls andcolumns.

b) Giving unity to the structure, by providinga proper connection between its resistingelements, in such a way that inertia forcesgenerated by the vibration of the buildingcan be transmitted to the members thathave the ability to resist them. Typicalimportant aspects are the connectionsbetween roofs or floors and walls, betweenintersecting walls and between walls andfoundations.

c) Eliminating features that are sources ofweakness or that produce concentration ofstresses in some members. Asymmetricalplan distribution of resisting members,abrupt changes of stiffness from one floorto the other, concentration of large massesand large openings in walls without aproper peripheral reinforcement areexamples of defects of this kind.

d) Avoiding the possibility of brittle modes offailure by proper reinforcement andconnection of resisting members.

4.4 Seismic Retrofitting

Many existing buildings do not meet theseismic strength requirements of presentearthquake codes due to original structuralinadequacies and material degradation due totime or alterations carried out during use overthe years. Their earthquake resistance can beupgraded to the level of the present day codesby appropriate seismic retrofitting techniques,such as mentioned in 4.3.

4.5 Strengthening or Retrofitting vs. Reconstruction

4.5.1 Replacement of damaged buildings orexisting unsafe buildings by reconstruction is,generally, avoided due to a number of reasons,the main ones among them being:

a) higher cost than that of strengthening orretrofitting,

b) preservation of historical architecture,and

c) maintaining functional social and culturalenvironment.

In most instances, however, the relative cost ofretrofitting to reconstruction cost determinesthe decision. As a thumb rule, if the cost ofrepair and seismic strengthening is less thanabout 50 percent of the reconstruction cost, theretrofitting is adopted. This may also requireless working time and much less dislocation inthe living style of the population. On the otherhand reconstruction may offer the possibility ofmodernization of the habitat and may bepreferred by well-to-do communities.4.5.2 Cost wise the building constructionincluding the seismic code provisions in thefirst instance, works out the cheaper in terms ofits own safety and that of the occupants.Retrofitting an existing inadequate buildingmay involve as much as 4 to 5 times the initialextra expenditure required on seismic resistingfeatures. Repair and seismic strengthening of adamaged building may even be 5 to 10 times asexpensive. It is therefore very much safe as wellas cost-effective to construct earthquakeresistant buildings at the initial stage itselfaccording to the relevant seismic IS codes.

5 SELECTION OF MATERIALS AND TECHNIQUES

5.1 General

The most common materials for repair works ofvarious types buildings are cement and steel. Inmany situations suitable admixture may beadded to cement mortar/cement concrete toimprove their properties, such as,non-shrink-age, bond, etc. Steel may berequired in many forms like bolts, rods, angles,beams, 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 in therepair and strengthening operations. Theseshould be selected appropriately depending onthe nature and cost of the building that is to berepaired, materials availability and feasibilityand use of available skills, etc. Some specialmaterials and techniques are described below.5.2 Shotcrete

Shotcrete is cement mortar or cement concrete(with coarse aggregate size maximum 10 mm)conveyed through a hose and pneumaticallyplaced under high velocity on to a preparedconcrete or masonry surface. The force of the jetimpingement on the surface compacts theshotcrete material and produces a dencehomogeneous mass. Basically there are two

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methods of shotcreting; wet mix process anddry mix process. In the wet mix process, all theingredients including water are mixed togetherbefore they enter the delivery hose. In the drymix process, the mixture of damp sand andcement is passed through the delivery hose tothe nozzle where the water is added. The drymix process is generally used in the repair ofconcrete elements. The bond between theprepared concrete surface of the damagedmember and the layer of shotcrete is ensuredwith the application of suitable epoxy adhesiveformulation. The shear transfer between theexisting and new layer of concrete is ensuredwith the provision of shear keys.5.3 Epoxy Resins

Epoxy resins are excellent binding agents withhigh tensile strength. These are chemicalpreparations the compositions of which can bechanged as per requirements. The epoxycomponents are mixed just prior to application.Some products are of low viscosity and can beinjected in fine cracks too. The higher viscosityepoxy resin can be used for surface coating orfilling larger cracks or holes. The epoxy resinsmay also be used for gluing steel plates to thedistress members.5.4 Epoxy Mortar

For larger void spaces, it is possible to combinethe epoxy resins of either low viscosity orhigher viscosity with sand aggregate to formepoxy mortar. Epoxy mortar mixture hashigher compressive strength, higher tensilestrength and a lower modulus of elasticity thancement concrete. The sand aggregate mixed toform the epoxy mortar increases its modulus ofelasticity.5.5 Quick-Setting Cement Mortar

This material is a non-hydrous magnesiumphosphate cement with two components, thatis, a liquid and a dry powder, which can bemixed in a manner similar to cement concrete.5.6 Mechanical Anchors

Mechanical type of anchors employ wedgingaction to provide anchorage. Some of theanchors provide both shear and tensionresistance. Such anchors are manufactured togive sufficient strength.Alternatively, chemical anchors bonded indrilled holes through polymer adhesives can beused.

6 TECHNIQUES TO RESTORE ORIGINAL STRENGTH

6.1 General

While considering restoration of structuralstrength, it is important to realise that even

fine cracks in load bearing members which areunreinforced like masonry and plain concretereduce their resistance very largely. Therefore,all cracks must be located and marked carefullyand the critical ones fully repaired either byinjecting strong cement or chemical grout or byproviding external bandage. The techniques aredescribed below along with other restorationmeasures.

6.2 Repair of Minor and Medium Cracks

For the repair of minor and medium cracks(0.50 mm to 5 mm), the technique to restore theoriginal tensile strength of the cracked elementis by pressure injection of epoxy. The procedureis as follows ( see Fig. 1A ):

‘The external surfaces are cleaned ofnon-structural materials and plastic injectionports are placed along the surface of the crackson both sides of the member and are secured inplace with an epoxy sealant. Thecentre-to-centre spacing of these ports may beapproximately equal to the thickness of theelement. After the sealant has cured, a lowviscosity epoxy resin is injected into one port ata time beginning at the lowest part of the crack,in case it is vertical, or at one end of the crack,in case it is horizontal.

The resin is injected till it is seen flowing fromthe opposite sides of the member at thecorresponding port or from the next higher porton the same side of member. The injection portshould be closed at this stage and injectionequipment moved to the next port and so on.

The smaller the crack higher is the pressure ormore closely spaced should be the ports so as toobtain complete penetration of the epoxymaterial throughout the depth and width ofmember. Larger cracks will permit larger portspacing depending upon width of the member.This technique is appropriate for all types ofstructural elements — beams, columns, wallsand floor units in masonry as well as concretestructures. In the case of loss of bond betweenreinforcing bar and concrete, if the concreteadjacent to the bar has been pulverised to avery fine powder (this powder will block theepoxy from penetrating the region). It should becleaned properly by air or water pressure priorto injection of epoxy.’

6.3 Repair of Major Cracks and Crushed Concrete

For cracks wider than about 5 mm or forregions in which the concrete or masonry hascrushed, a treatment other than injection isindicated.

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The procedures may be adopted as follows:a) The loose material is removed and

replaced with any of the materialsmentioned earlier, that is, expansivecement mortar quick setting cement ( seeFig. 1B ).

b) Where found necessary, additional shearor flexural reinforcement is provided inthe region of repairs. This reinforcementcould be covered by mortar to give furtherstrength as well as protection to the

reinforcement ( see Fig. 1C ).c) In areas of very severe damage,

replacement of the member or portion ofmember can be carried out as discussedlater.

d) In the case of damage to walls and floordiaphragms, steel mesh could be providedon the outside of the surface and nailed orbolted to the wall. Then it may be coveredwith plaster or micro-concrete ( seeFig. 1C ).

FIG. 1 STRUCTURAL RESTORATION OF CRACKED MASONRY WALLS

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6.4 Fractured Excessively Yielded and Buckled Reinforcement

In the case of severely damaged reinforcedconcrete member it is possible that thereinforcement would have buckled or elongatedor excessive yielding may have occured. Thiselement can be repaired by replacing the oldportion of steel with new steel using buttwelding or lap welding.Splicing by overlapping will be risky. If repairhas to be made without removal of the existingsteel, the best approach would depend upon thespace available in the original member.Additional stirrup ties are to be added in thedamaged portion before concreting so as toconfine the concrete and enclose thelongitudinal bars to prevent their buckling infuture.In some cases, it may be necessary to anchoradditional steel into existing concrete. Acommon technique for providing the anchorageuses the following procedure:

‘A hole larger than the bar is drilled. Thehole is filled with epoxy expanding cement orother high strength grouting material. Thebar is pushed into place and held there untillthe grout has set.’

6.5 Fractured Wooden Members and Joints

Since wood is an easily workable material, itwill be easy to restore the strength of woodenmembers such as beams, columns, struts, andties by splicing additional material. Theweathered or rotten wood should first beremoved. Nails wood screws or steel bolts willbe most convenient as connectors. It will beadvisable to use steel straps to cover all suchsplices and joints so as to keep them tight andstiff.7 SEISMIC STRENGTHENING TECHNIQUES

7.1 Modification of Roofs or Floors

7.1.1 Slates and roofing tiles are brittle andeasily dislodged. Where possible, they shouldbe replaced with corrugated iron or asbestossheeting.7.1.2 False ceilings of brittle material aredangerous. Non-brittle material, like hessiancloth, bamboo matting or light ones of foamsubstances, may be substituted.7.1.3 Roof truss frames should be braced bywelding or clamping suitable diagonal bracingmembers in the vertical as well as horizontalplanes.

7.1.4 Anchors of roof trusses to supportingwalls should be improved and the roof thrust onwalls should be eliminated.

Figures 2 and 3 illustrate one of the methodsfor pitched roofs without trusses.

7.1.5 Where the roof or floor consists ofprefabricated units like RC rectangular T orchannel units or wooden poles and joistscarrying brick tiles, integration of such units isnecessary. Timber elements could be connectedto diagonal planks nailed to them and spiked toan all round wooden frame at the ends.Reinforced concrete elements may either have40 mm cast-in-situ-concrete topping with 6 mmdia bars 150 mm c/c both ways or bounded by ahorizontal cast-in-situ-reinforced concrete ringbeam all round into which the ends ofreinforced concrete elements are embedded.Fig. 4 shows one such detail.

7.1.6 Roofs or floors consisting of steel joistsflat or segmental arches must have horizontalties holding the joists horizontally in each archspan so as to prevent the spreading of joists. Ifsuch ties do not exist, these should be installedby welding or clamping.

7.2 Inserting New Walls

7.2.1 Unsymmetrical buildings which mayproduce dangerous torsional effects duringearthquakes the center of masses can be madecoincident with the centre of stiffnesses byseparating parts of buildings thus achievingindividual symmetric units and/or insertingnew vertical resisting elements such as newmasonry or reinforced concrete walls eitherinternally as shear walls or externally asbuttresses. Insertion of cross wall will benecessary for providing transverse supports tolongitudinal walls of long barrack-typebuildings used for various purposes such asschools and dormitories.

7.2.2 The main problem in such modificationsis the connection of new walls with old walls.Figures 5, 6 and 7 show three examples ofconnection of new walls to existing ones. Thefirst two cases refer to a T-junction whereas thethird to a corner junction. In all cases the linkto the old walls is performed by means of anumber of keys made in the old walls. Steel isinserted in them and local concrete infilling ismade. In the second case, however, connectioncan be achieved by a number of steel barsinserted in small length drilled holes filled withfresh cement-grout which substitute keys.

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FIG. 2 ROOF MODIFICATION TO REDUCE THRUST OF WALLS

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FIG. 3 DETAILS OF NEW ROOF BRACING

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.

FIG. 4 INTEGRATION AND STIFFENING OF AN EXISTING FLOOR

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FIG. 5 CONNECTION OF NEW AND OLD BRICK WALLS (T-JUNCTION)

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FIG. 6 CONNECTION OF NEW BRICK WALL WITH EXISTING STONE WALL

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FIG. 7 CONNECTION OF NEW AND OLD WALLS (CORNER JUNCTION)

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7.3 Strengthening Existing Walls

7.3.0 The lateral strength of buildings can beimproved by increasing the strength andstiffness of existing individual walls, whetherthey are cracked or uncracked, can be achieved.

a) by grouting,b) by addition of vertical reinforced concrete

coverings on the two sides of the wall, andc) by prestressing wall.

7.3.1 GroutingA number of holes are drilled in the wall (2 to 4in each square metre) ( see Fig. 8 ). First wateris injected in order to wash the wall inside, and

to improve the cohesion between the groutingmixture and the wall elements. Secondly, acement water mixture (1 : 1) is grouted at lowpressure (0.1 to 0.25 MPa) in the holes startingfrom the lower holes and going up.Alternatively, polymeric mortars may be usedfor grouting. The increase of shear strengthwhich can be achieved in this way isconsiderable. However, grouting can not berelied on as far as the improving or making anew connection between orthogonal walls isconcerned.

NOTE — The pressure need for grouting can beobtained by gravity flow from superelevated containers.

7.3.2 Strengthening with Wire MeshMasonry walls with concentration of multiplecracks in the same portion and appearing onboth sides on the wall or weak wall regions maybe repaired with a layer of cement mortar ormicro concrete layer 20 to 40 mm thick on both

sides, reinforced with galvanized steel wirefabric (50 mm × 50 mm size) forming a verticalplate bonded to the wall. The two plates oneither side of the wall should be connected bygalvanized steel rods at a spacing of about 300to 400 mm ( see Fig 9 ).

FIG. 8 GROUT OR EPOXY INJECTION IN EXISTING WEAK WALLS

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FIG. 9 STRENGTHENING WITH WIRE-MESH AND MORTAR

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7.3.3 Connection Between Existing Stone WallsIn stone buildings of historic importance,consisting of fully dressed stone masonry ingood mortar, effective sewing of perpendicular

walls may be done by drilling inclined holesthrough them inserting steel rods and injectingcement grout ( see Fig. 10 ).

7.4 Achieving Integral Box Action

7.4.0 The overall lateral strength and stabilityof bearing wall buildings is very muchimproved, if the integral box like action of roomenclosures is ensured. This can be achieved by(a) use of prestressing (b) providing horizontalbands. Strength of shear walls is achieved byproviding vertical steel at selected locations asdescribed in 7.4.1 and 7.4.2.

7.4.1 Prestressing

A horizontal compression state induced byhorizontal tendons can be used to increase theshear strength of walls. Moreover, this will alsoimprove, considerably, the connections oforthogonal walls ( see Fig. 11 ). The easiest wayof affecting the precompression is to place twosteel rods on the two sides of the wall andstretching them by turnbuckles. Note that,good effects can be obtained by slight horizontalprestressing (about 0.1 MPa) on the verticalsection of the wall. Prestressing is also useful to

strengthen spandrel beam between two rows ofopenings in the case no rigid slab exists.Opposite parallel walls can be held to internalcross walls by prestressing bars as illustratedabove the anchoring being done againsthorizontal steel channels instead of small steelplates. The steel channels running from onecross wall to the other will hold the wallstogether and improve the integral box likeaction of the walls.

7.4.2 External BindingThe technique of covering the wall with steelmesh and mortar or microconcrete may be usedonly on the outside surface of external walls butmaintaining continuity of steel at the corners.This would strengthen the walls as well as bindthem together. As a variation and for economyin the use of materials, the covering may be inthe form of vertical splints located between theopenings and horizontal ‘bandages’ formed overspandrel walls at suitable number of pointsonly ( see Fig. 12 ).

FIG. 10 SEWING TRANSVERSE WALLS WITH INCLINED BARS

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FIG. 11 STRENGTHENING OF WALLS BY PRESTRESSING

FIG. 12 SPLINT AND BANDAGE STRENGTHENING TECHNIQUE

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7.5 Masonry Arches

If the walls have large arched openings inthem, it will be necessary to install tie rodsacross them at springing levels or slightlyabove it by drilling holes on both sides andgrouting steel rods in them ( see Fig. 13a ).Alternatively, a lintel consisting of steel

channels or I-shapes could be inserted justabove the arch to take the load and relieve thearch as shown at Fig. 13b. In jack-arch roofs,flat iron bars or rods shall be provided toconnect the bottom flanges of I-beamsconnected by bolting or welding ( see Fig. 13c ).

FIG. 13 STRENGTHENING AN ARCHED OPENING IN MASONRY WALL

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7.6 Random Rubble Masonry Walls

Random rubble masonry walls are mostvulnerable to delamination and completecollapse and must be strengthened by internalimpregnation by rich cement mortar grout inthe ratio of 1 : 1 as explained in 7.3.1 or coveredwith steel mesh and mortar as in 7.3.2.

Damaged portions of the wall, if any should bereconstructed using richer mortar. In thickwalls, ‘through’ stones or bonding elementsshall be installed, if not present originally, ateach one-third point along the length andheight of wall ( see Fig. 14 ).

FIG. 14 STRENGTHENING OF LONG WALLS BY BUTTRESSES

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7.7 Strengthening Long Walls

For bracing the longitudinal walls of longbarrack type buildings a portal type frameworkmay be inserted transverse to the walls andconnected to them. Alternatively masonrybuttresses or pillasters may be addedexternally as shown in Fig. 14.

7.8 Strengthening Reinforced Concrete Members7.8.1 ColumnsReinforced concrete columns can best bestrengthened by casing, that is, by providingadditional cage of longitudinal and lateral tiereinforcement around the columns and castinga concrete ring ( see Fig. 15 ). The desiredstrength and ductility can thus be built-up.

7.8.2 BeamsA reinforced concrete beam can be encased asshown in Fig. 16 (A). For holding the stirr-up inthis case, holes will have to be drilled throughthe slab. Alternatively it can be jacketed as

shown in Fig. 16 (B), and Fig. 16 (C) whereinholes will need to drilled through web ofexisting beam for the new stirr-ups. Desiredquantity of longitudinal and transverse steelmay be added in each case.

Reinforced concrete beams can also bestrengthened by applying prestress to it so thatopposite moments are caused to those applied.The wires will run on both sides of the web

outside and anchored against the end of thebeam through a steel plate. Loss of prestressdue to creep relation and temperature fall shallbe duly considered.

FIG. 15 CASING A CONCRETE COLUMN

FIG. 16 INCREASING THE SECTION AND REINFORCEMENT OF EXISTING BEAMS

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7.8.3 Shear WallsThe casing technique could be used forstrengthening reinforced concrete shear walls.7.8.4 Inadequate section of beams, columns andwalls could be strengthened by adding a layerof reinforced concrete (outershell) around themembers with the addition of newreinforcements. Also to the existing steel, newsteel reinforcement bars could be welded toincrease the carrying capacity of the members.In all cases of adding new concrete to the oldconcrete, effective bond should be ensured.Such bond could be created by the applicationof suitable epoxy adhesive formulations on theprepared old concrete surface. In addition tothis, suitable shear connectors in the form ofsteel rods placed in predrilled holes in the oldconcrete at required spacing should beprovided. These rods should also be dipped inepoxy adhesive formulations before placing inposition.7.8.5 In all cases of adding new concrete to oldconcrete the original surface should beroughened, grooves made in the appropriatedirection for providing shear transfer. The endsof the additional steel are to be anchored in theadjacent beams or columns as the case may be.7.9 Strengthening of FoundationsStrengthening of foundations before or after

the earthquake is the most involved task sinceit may require careful underpinning operations.Some alternatives are given below forpreliminary consideration of the strengtheningscheme:

a) Introducing new load bearing membersincluding foundations to relieve thealready loaded members. Jackingoperations may be needed in this process.

b) Improving the drainage of the area toprevent saturation of foundation soil tooviate any problems of liquefaction whichmay occur because of poor drainage.

c) Providing apron around the building toprevent soaking of foundation directly anddraining off the water.

d) Adding strong elements in the form ofreinforced concrete strips attached to theexisting foundation part of the building.These will also bind the various wallfootings and may be provided on bothsides of the wall ( see Fig. 17 ) or only oneside of it. In any case, the reinforcedconcrete strips and the wall have to belinked by a number of keys inserted intothe existing footing.

NOTE — To avoid disturbance to the integrity of theexisting wall during the foundation strengtheningprocess proper investigation and design is called for.

FIG. 17 STRENGTHENING EXISTING FOUNDATION (R. C. STRIP ON BOTH SIDES)

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ANNEX A( Foreword )

COMMITTEE COMPOSITION

Earthquake Engineering Sectional Committee, CED 39Chairman Representing

DR A. S. ARYA 72/6 Civil Line, Roorkee

Members

SHRI O. P. AGGARWALSHRI G. SHARAN ( Alternate )

Indian Roads Congress, New Delhi

DR K. G. BHATIADR C. KAMESHWARA RAO ( Alternate )SHRI A. K. SINGH ( Alternate )

Bharat Heavy Electricals Ltd, New Delhi

SHRI S. C. BHATIADR B. K. RASTOGI ( Alternate )

National Geophysical Research Institute (CSIR), Hyderabad

DR A. R. CHANDRASEKARANDR BRIJESH CHANDRA ( Alternate )DR B. V. K. LAVANIA ( Alternate )

Department of Earthquake Engineering, University of Roorkee, Roorkee

DR S. N. CHATTERJEESHRI S. K. NAG ( Alternate )

Indian Meterological Department, New Delhi

SHRI K. T. CHAUBALDR B. K. PAUL ( Alternate )

North Eastern Council, Shillong

DR A. V. CHUMMARDR S. K. KAUSHIK ( Alternate )

Indian Society of Earthquake Technology, Roorkee

DIRECTOR EMBANKMENT (N & W)DIRECTOR CMDD (NW & S) ( Alternate )

Central Water Commission (ERDD), New Delhi

DIRECTOR STANDARDS (B & S), RDSO Railway Board, Ministry of RailwaysJOINT DIRECTOR STANDARDS (B & S)

CB-I, RDSO, LUCKNOW ( Alternate )KUMARI E. DIVATIA

SHRI C. R. VENKATESHA ( Alternate )National Hydro-Electric Power Corporation Ltd, New Delhi

SHRI I. D. GUPTASHRI J. G. PADALE ( Alternate )

Central Water & Power Research Station, Pune

SHRI V. K. KULKARNISHRI P. C. KOTESWARA RAO ( Alternate )

Department of Atomic Energy, Bombay

SHRI V. KUMARSHRI R. S. BAJAJ ( Alternate )

National Thermal Power Corporation Ltd, New Delhi

SHRI M. Z. KURIENSHRI K. V. SUBRAMANIAN ( Alternate )

Tata Consulting Engineers, Bombay

SHRI A. K. LALSHRI T. R. BHATIA ( Alternate )

National Buildings Organization, New Delhi

SHRI S. K. MITTAL Central Building Research Institute, RoorkeeSHRI S. S. NARANG Central Water Commission (CMDD), New DelhiSHRI A. D. NARIAN

SHRI O. P. AGGARWAL ( Alternate )Ministry of Transport, Department of Surface Transport (Roads Wing),

New DelhiSHRI P. L. NARULA

SHRI A. K. SRIVASTAVA ( Alternate )Geological Survey of India, Calcutta

RESEARCH OFFICER Irrigation Department, Govt of Maharashtra, NasikDR D. SENGUPTA

SHRI R. K. GROVER ( Alternate )Engineers India Ltd, New Delhi

DR R. D. SHARMASHRI U. S. P. VERMA ( Alternate )

Nuclear Power Corporation, Bombay

COL R. K. SINGHLT-COL B. D. BHATTOPADHYAYA ( Alternate )

Engineer-in-Chief’s Branch, Army Headquarters, New Delhi

DR P. SRINIVASULUDR N. LAKSHMANAN ( Alternate )

Structural Engineering Research Centre (CSIR), Madras

SUPERINTENDING ENGINEER (D)EXECUTIVE ENGINEER (D) II ( Alternate )

Central Public Works Department, New Delhi

DR A. N. TANDON In personal capacity ( B-7/50 Safdarjung Development Area, New Delhi )SHRI J. VENKATARAMAN,

Director (Civ Engg)Director General, BIS ( Ex-officio Member )

Secretary

SHRI S. S. SETHI

Director (Civ Engg), BIS

( Continued on page 22 )

IS 13935 : 1993

22

( Continued from page 21 )

Earthquake Resistant Construction Subcommittee, CED 39 : 1

Convener Representing

DR A. S. ARYA (72/6 Civil Lines, Roorkee)

Members

SHRI N. K. BHATTACHARYA Engineer-in-Chief’s Branch, New DelhiSHRI B. K CHAKRABORTY

SHRI D. P. SINGH ( Alternate )Housing and Urban Development Corporation, New Delhi

SHRI D. N. GHOSAL North Eastern Council, ShillongDR SUDHIR K. JAIN

DR A. S. R. SAI ( Alternate )Indian Institute of Technology, Kanpur

SHRI M. P. JAISINGH Central Buildings Research Institute, RoorkeeJOINT DIRECTOR STANDARDS (B & S) CB-I Railway Board (Ministry of Railways)

ASSISTANT DIRECTOR (B & S) CB-I ( Alternate )SHRI V. KAPUR

SHRI V. K. KAPOOR ( Alternate )Public Works Department, Government of Himachal Pradesh, Simla

SHRI M. KUNDU Hindustan Prefab Limited, New DelhiSHRI A. K. LAL

SHRI T. R. BHATIA ( Alternate )National Buildings Organization, New Delhi

DR B. C. MATHURDR (SHRIMATI) P. R. BOSE ( Alternate )

University of Roorkee, Department of Earthquake Engineering, Roorkee

SHRI G. M. SHOUNTHU Public Works Department, Jammu & KashmirDR P. SRINIVASULU

DR N. LAKSHMANAN ( Alternate )Structural Engineering Research Centre (CSIR), Madras

SHRI SUBRATA CHAKRAVARTY Public Works Department, Government of Assam, GauhatiSUPERINTENDING ENGINEER (DESIGN) Publing Works Department, Government of GujratSUPERINTENDING SURVEYOR OF WORKS (NDZ)

SUPERINTENDING ENGINEER (D) ( Alternate )Central Public Works Department, New Delhi

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Bureau of Indian Standards

BIS is a statutory institution established under the Bureau of Indian Standards Act, 1986 to promoteharmonious development of the activities of standardization, marking and quality certification of goods andattending to connected matters in the country.

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Review of Indian Standards

Amendments are issued to standards as the need arises on the basis of comments. Standards are alsoreviewed periodically; a standard along with amendments is reaffirmed when such review indicates that nochanges are needed; if the review indicates that changes are needed, it is taken up for revision. Users ofIndian Standards should ascertain that they are in possession of the latest amendments or edition byreferring to the latest issue of ‘BIS Catalogue’ and ‘Standards : Monthly Additions’.

This Indian Standard has been developed from Doc : No. CED 39 (5270)

Amendments Issued Since Publication

Amend No. Date of Issue

Amd. No. 1 April 2002

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