SEISMIC RETROFITTING OF STRUCTURES

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45th IEP Convention ‘12SEISMIC RETROFITTING OF STRUCTURESRANJITH DISSANAYAKEDEPT. OF CIVIL ENGINEERING, FACULTY OF ENGINEERING,UNIVERSITY OF PERADENIYA, SRI LANKAABSTRACTMany existing reinforced concrete structures in present world are inadequate for earthquakes.Recent earthquakes which occurred during last decade have indicated that major damageoccurred was not directly due to actions of earthquakes but due to poor performance of structuresduring earthquakes. It is recognized that the most effective method of reducing the risk ofdamaging structure is seismic retrofitting. In recent years, there is a significant improvement ofretrofitting techniques. This study highlights the principals of assessing and retrofitting ofstructures against seismic events. Some of these methods were practically implemented andprocedures are illustrated using a case study of four storey RC building retrofitted for a moderateearthquake. Finite Element Method was used to investigate the performance of the buildingduring the earthquake and to check the behaviour of the structure after applying retrofittingtechniques. The methods such as steel and concrete jacketing and application of fibre reinforcedpolymer (FRP) composites which were used to improve the load bearing capacity of individualstructural elements are highlighted and methods such as shear walls and shear cores which canbe used to improve overall stability are discussed.Keywords: Earthquake, Hazards, Retrofitting, Assessment, Finite element model1

45th IEP Convention ‘121. INTRODUCTIONAn earthquake is a vibration of the earth surface that follows a sudden release of energy in theearth crust. This huge energy generated during an earthquake can be transmitted from its originthrough seismic waves. These seismic waves cause vibration of the ground even far away fromits epicentre. During an earthquake the ground surface moves in all directions. The most criticalfactor for damaging buildings is horizontal movements which cause lateral inertia forces on thestructures. Generally most structures are designed and constructed to carry gravity loads. Mostbuildings do not have good lateral load resisting systems. It is understood that most of thebuildings are not designed or constructed to face earthquake.Concrete has been the most preferred construction material of the twentieth century and unless anew material with spectacular characteristics is invented, it appears to remain this way foranother century. The existing low-rise and medium-rise buildings in the world consist of largenumber of reinforced concrete (RC) frame buildings designed only for gravity loads (Lakshmanan2007). Among various methods of retrofitting, the most suitable method for a particular buildingcan be identified after performing a structural assessment. The decision about the method ofretrofitting is governed by the cost of retrofitting and the required level of performance.2. ASSESSMENT CRITERIABased on current information of the building condition, an assessment procedure was developedto assess the building which is shown in figure 1.Condition assessment procedure consists of preliminary investigation and planning, conditionsurveys, material testing (destructive and non-destructive testing) and field load testing. Thepreliminary investigation involves study of available data, discussions with relevant people, visualinspections, identification of failure modes and necessary testing. Condition survey of all thestructural members must be carried out in order to identify the current condition of the structure.In the condition survey, the surveyor must record present dimensions, deterioration level of eachand every member and the connections. Material testing must be carried out in order to obtaincorrect material properties for the analysis. For this, samples need to be taken from appropriatelocations for laboratory testing and based on requirements non-destructive tests can beperformed on site. The field load testing is decided depending on the measurement required andit could be either dynamic or static load testing or both. Based on the results of aboveinvestigations, the structural analysis is performed.FEMA 356 (Federal Emergency Management Agency, 1997a, Federal Emergency ManagementAgency, 1997b) outlines four different procedures for analysis of the seismic evaluation of astructure: the linear static procedure, the linear dynamic procedure, the nonlinear static procedure(push-over analysis), and the nonlinear dynamic procedure (Bai, JW, year unknown, Core, B. &2

45th IEP Convention ‘12Long, M., 2004). Here, the non-linear dynamic analysis was carried out. The method of retrofittingwas decided based on the outcome of the analysis.3. RETROFITTING OF STRUCTUREThere are many seismic retrofitting techniques available, depending upon the various types andconditions of structures. Some methods are additions of RC structural walls, use of steelbracings, seismic isolation, use of FRP and supplemental energy dissipation devices (dampers).Selection of the most appropriate method(s) and material(s) are based on results of structuralassessment and detailed structural analysis. The structural analysis of a proposed retrofittingscheme is usually based on loads specified by the client. In the detailed analysis the bestperforming method should be selected according to the behaviour of the structure and theelements that need to be retrofitted must be identified. With proposed methods the structure mustbe reanalyzed. If the performance is up to a satisfactory level the above mentioned methods canbe implemented. Otherwise the structure is reanalysed using different retrofitting improvementmethod(s) or material(s) until it satisfies the required improvement of the structure. The flow chartfor structural retrofitting is given in figure 2.4. CASE STUDY4.1 Building DescriptionThis case study is based on a four-storey library building. The structure is a reinforced concretestructure designed for carrying only gravity loads. This is a doubly symmetric building with a floorheight of 3.5m each level. The general view of the building is shown in figure 3.4.2 Numerical InvestigationA non-linear finite element model (see figure 5) was developed to study the behaviour of thestructure during prescribed earthquake loading (CSI Analysis Reference Manual, 1995).Geometric non linearity was introduced to the model by using RESPONCE 2000 package andmoment curvature curve which was used in the analysis is shown in figure 4. Analysis was donefor expected earthquake loading based on details gathered during the initial conditionassessment process. The outcome of the analysis was compared with limitations given inrelevant standards. Deformed shape for peak ground acceleration is shown in figure 6. Secondlyone or more retrofitting techniques were introduced to the model at once and performance wascompared with allowable limits. Based on this analysis the most suitable retrofitting technique(s)was selected for the construction. The final proposal was drawn in such a way that it improves thestrength, ductility and stiffness of the structure and it was taken in to account in terms of the interstorey drift ratio, storey shear force and fundamental period of the structure.3

45th IEP Convention ‘125. RETROFITTING METHODS OF STRUCTURESAmong various methods of retrofitting techniques available, for this case three types wereselected: Introducing a shear core in a suitable location, applying FRP at selected structuralelements and constructing bracing dampers across the selected places. Analysis was done byintroducing one method at first and introducing the next in cumulative manner. Maximum andminimum values of displacement of these techniques are tabulated in table 1. Based on theresults of the numerical analysis the final retrofitting proposal was developed. Other than thatconcrete and steel jacketing was done in columns in order to increase the load carrying capacity.The load bearing capacity of RC slabs were increased by introducing steel girders.5.1 Shear CoreThe overall stability of the structure was increased by introducing shear cores at well identifiedlocations. Introduction of shear walls for low-rise buildings can improve its lateral load bearingcapacity due to the increase of lateral stiffness. The construction of the shear core is shown infigure 7.5.2 Fibre Reinforced PolymerThis is an innovative retrofitting method currently used all around the world in various retrofittingactivities. It has properties such as high specific stiffness, low weight compared to some othermaterials (concrete) and superior environmental durability. In this case fibre reinforced polymer isapplied at columns, beams, slabs and beam column joints.Application of these, results inimproving structural performances such as stiffness, load carrying capacity and ductility.Application of FRP at beam column joint and slabs is shown in figure 8 and 9.6. CONCLUSIONLaterial variable load carrying capacity is important for structures in order to resist earthquakeloading. It is the responcibility of designers to analyse structures applying the effect of lateralloads in a proper manner during the design stage.Structures that are not designed for lateral variable loads are vulnerable for earthquake loadingand unsafe. Therefore, such structures must be subjected to appropriate retrofitting. Determiningthe most suitable (with respect to structural integrity, possibility of applying on the structure andcost) retrofitting technique, material and method could be done after analysing a finite elementmodel of the structure using computer based softwares.The case study shows that the lateral load carrying capacity of existing structures that are notoriginaly designed for variable lateral loads could be improved applying retrofitting techniques.4

45th IEP Convention ‘127. DISCUSSIONThis paper presented a case study of retrofitting of a four storey library building. In the originaldesign (existing structure) there was no provision for later load resistance except the primarywalls. Therefore the client insisted on getting a detailed analysis and proposal for retrofitting thestructure to enhance its lateral load carrying capacity. The prior assessment helped optimizingthe cost as well as the project duration.The paper described the application of numerical methods to evaluate the performance of theexisting structure and retrofitted structure. It also discuss about various materials and severaltechniques in the retrofitting field. From the study it was understood that rather than introducingone technique at once it is efficient go for different techniques in the same structure whereapplicable.Retrofitting of structures is not an easy task. It requires testing, structural analysis and costanalysis for deciding on a proper retrofitting scheme.5

45th IEP Convention ‘128. REFERENCESApplied Technology Council. Report ATC-40:1996. Seismic Evaluation and Retrofit of ConcreteBuildings. Redwood City, U.S.A.Bai JW. Seismic Retrofit for Reinforced Concrete Building Structures: Final Report. Texas: A&MUniversity Consequence-Based Engineering (CBE) Institute; xxxx.Core B, Long M. Analysis of Rotational Column with Plastic Hinge. Rice University and BatesCollege. 2004.CSI Analysis Reference Manual for SAP2000 : ETABS. Computers and Structures, Inc.California, USA. 1995.Federal Emergency Management Agency. FEMA 273:1997a. NEHRP guidelines for the seismicrehabilitation of buildings. Washington, D.C.Federal Emergency Management Agency. FEMA 274:1997b. NEHRP Commentary on theguidelines for the seismic rehabilitation of buildings. Washington, D.C.Lakshmanan N. Seismic Evaluation and Retrofitting of Buildings and Structures: [the annuallecture 26th ISET]. Structural Engineering Research Centre; Texas A&M University. 2007.National Research Council of Canada. NRC/IRC:1993. Guidelines for Seismic Evaluation ofExisting Buildings. Ottawa, Ontario.Institute for Research in Construction.Seismic Safety Commission. Report SSC 96-01:1996. Seismic Evaluation and Retrofit ofConcrete Buildings. Sacramento, U.S.A.6

45th IEP Convention ‘12StructuralAssessmentStructural RetrofittingStructural AssessmentPreliminaryinvestigation stingField rentmechanicalpropertiesandchemicalcompositionof d etc.Detail StructuralAnalysisPropose sStructuralAnalysisImprove RetrofittingMethod(s) or ChangeValidationStructural AnalysisEndEndFig 1. Assessment ProcedureFig 2. Retrofitting Procedure7End

45th IEP Convention ‘12Fig 3. General View of theBuildingFig 4. Moment Curvature CurveFig 6. Deformed shape at peakaccelarationFig 5. Finite element model8

45th IEP Convention ‘12Fig 7. Construction of a shear coreFig 8. FRP at a beam-columnjointFig 9. FRP at a slabFig 10. Steel bracing9

45th IEP Convention ‘12Table 1. Comparison of peak displacements before and after applyingretrofittingDisplacement /mmjointDirectionUXOriginalmodelShear coreShear core,FRPShear core,FRP 9.816.116.6185(top floor)10