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REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGS

1 REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGSC hapter 9 REPAIR, RESTORATION ANDSTRENGTHENING OF 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.

9.2.3 Strengthening of existing buildings The seismic behaviour of old existing build-ings is affected by their original structural inadequacies, material degradation due to time, and alterations carried out during use over the years such as making new open-ings, addition of new parts inducing dis-symmetry in plan and elevation, etc.

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Transcription of REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGS

1 1 REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGSC hapter 9 REPAIR, RESTORATION ANDSTRENGTHENING OF 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.

2 (ii)Just after a damaging earthquake,temporary supports and emergencyrepairs 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 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.

3 Thus, decisions are fre-2 IAEE MANUAL quently based on gross conservative as-sumptions about actual 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 REPAIR, RESTORATIONAND STRENGTHENINGCONCEPTSThe underlying concepts in the three op-erations are stated 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.

4 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, 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 RestorationIt is the restitution of the strength the build -ing had before the damage occurred.

5 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 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 process may involve inserting tempo-rary supports, underpinning, etc. Some ofthe approaches are stated below:3 REPAIR, RESTORATION AND STRENGTHENING OF BUILDINGS (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 alternatives have been used.

6 (iii)Injecting epoxy like material, whichis strong in tension, into the cracksin walls, columns, beams, 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 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, 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 is an improvement overthe original strength when the evaluationof the building indicates that the strengthavailable before the damage was insuffi-cient and RESTORATION alone will not be ad-equate in future 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.

7 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 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.

8 (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 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 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, the above, special materials andtechniques are available for best results inthe repair and STRENGTHENING are described 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.

9 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, Epoxy resinsEpoxy resins are excellent binding agentswith high tensile strength. There are chemi-cal preparations the compositions ofwhich can be changed as per epoxy components are mixed just priorto application. The product is of low vis-cosity and can be injected in small 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 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.

10 Epoxy mortar mixturehas higher compressive strength, highertensile strength and a lower modulus ofelasticity than Portland cement 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 , RESTORATION AND STRENGTHENING OF Small cracksIf the cracks are reasonably small (openingwidth = cm), the technique to restorethe original tensile strength of the crackedelement is by pressure injection of procedure is as follows, Fig (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 centre to centre spacing of these portsmay be approximately equal to the thick-ness of the element.


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