Experimental And Analytical Investigation Of Seismic Performace Of Nonductile RC Frames Retrofitted With FRP

In previous earthquake events, it have been reported that a number of existingreinforced concrete (RC) frames experienced severe damage, or even collapsed, dueto insufficient ductility and energy dissipation capacity. Most of these framesdesigned according to pre1970codes which bears gravity loads only, and noneconsidering seismic load, or can not stisfy the present siesmic requirements due tothe revision of design code. All of these RC frames with insufficient can be definedas nonductile RC frames, which is the research object of this study. At persent, thereare a great many of this nonductile RC frames existed in the potential earthquakeregions of our country. All of this nonductile RC frames have a high risk level ofsevere damage or collapses in the furture strong earthquakes. In order to reduce thisrisk, threre is an urgent need to seismic strengthen such existing nonductile frames.Fiber reinforced polymer (FRP) composites have been widely used for sismicstrengthening or rehabilitation of concrete strucutures in the past decades, due totheir light weight, high strength, and ease of application. However, comparing withthe actual application in engineering, the theoretical investigation of seismicperformance of nonductile RC frames strengthened with FRP is obviously laggedbehind. The two key problems of reasonable FRP retrofitted scheme andquantification design are still not solved. At present, there is no seismic designmethod in structural level for RC frmaes retrofitted by FRP. Consequently, theinvestigation of seismic performance and design method of FRP strengthendnonductile RC frames has important theoretical significance and engineering value.It is the basis work that providing technical support for modifieding the seimicdesign code and the strengthen design code for concrete structures of our country.In order to address current research gaps, this thesis firstly on the material levelconducted the investigation of stress-strain behavior of FRP-confined concrete, andthen the seismic performance of FRP strengthened RC columns were studied as thebasic work on member level. Finally, the structural seismic performance of FRPstrengthened nonductile RC frames were investigated, The concept design principlefor FRP seismic retrofitting nonductile RC frames was proposed, so the problem ofhow to determine reasonable FRP retrofitted scheme was solved. Then thedisplacement-based seismic design method for nonductile RC frames retrofittedusing FRP was preliminary proposed, so the problem of how to quantification designthe FRP amount was preliminary solved. The main contents of this thesis aresummarized as follows: (1) On the material level, FRP-confined circular and square RC columns weretested under concentric monotonic or cyclic compression loading regimes, and thenthe influence of cross-sectional shape and size, internal longitudinal and hoop steelreinforcement, number of layers of FRP wrap, and pre-damage level on the axialcompressive behavior of FRP-confined concrete was investigated. Based on the testresults of monotonic loading, the monotonic stress-strian model which consideringsize effect factor and hybrid confinement of FRP and hoops was proposed. Based onthe test results of cyclic loading, the equations of plastic strian and theunloading/reloading paths considering the influence of internal steel reinforcementwere also proposed. Combined with the monotonic stress-strian model and theunloading/reloading model, the cyclic stress-strian model for FRP-confinedreinforced concrete was finally developed. Then the proposed stress-strain modelwas added to the OpenSees software as a uniaxial FRP-confined concrete materialwhich can be used to analysis the seismic behavior of FRP strengthend RC membersand structures.(2) On the member level, quasi-static test of FRP strengthend full scale RCcolumns were conducted firstly. The influence of number of layers of FRP wrap,axial compression ratio, and cross-sectional size on the lateral load-displacementhysteretic behavior of strengthened columns was studied, and the seismicperformance of the RC columns before and after strengthend was evaluated usingthe parameters of stiffness, strength, ductility and energy dissipation. Based on thetest results, the expressions of unloading stiffness and effective stiffness for FRPstrengthened circular and square RC columns were proposed. Analytical simulationof lateral load-displacement hysteretic behavior of FRP strengthened RC columnswas conduceted using OpenSees and the added FRP-confined concrete material. Thesimulation results agree well with test results. On the basis, the Pushover analysiswas conducted considering the influence parameters of cross-sectional shape andsize, aixal compression ratio, shear span ratio, concrete strength, number of layers ofFRP wrap, and steel reinforcement ratio. Then the lateral load-displacementhysteretic models for FRP strengthened circular and suqare RC columns wereproposed. The model can be used to seismic design and analysis of FRP strengthendcircular and square RC columns.(3) On the structural level, shaking table test of4storeys and2bays bare andFRP strengthened nonductile RC frames was conducted to investigate the variationof failure mode, seismic response and performance between bare and FRPstrengthened frames, and the development process from initial damage to failure ofstructures under different eathquake levels was also discussed. Then the finite element model of shaking table specimen and nonlinear dynamic analyses wereconducted using OpenSees software. The simulation results agreeing well with testresults show the correctness of modeling approach and accuracy of analysis results.On the basis, the seismic performance of nonductile RC frames with differentheights and strengthened using three FRP rehabilitation schemes were evaluatedanalycitcally. Nonlinear static Pushover and time history analysis were conductedfor the studied cases. The concept design principle for FRP seismic retrofittingnonductile RC frames was proposed from the analyzing of analytical results.Combined with the seismic concept design principle and the lateralload-displacement hysteretic model of FRP strengthened columns, using the ultimatedrift ratio of FRP strengthened columns as the target diaplacement of structural,preliminary proposed the displacement-based seismic design method for FRPseismic strengthend nonductile RC frames.