Research On Seismic Behavior Of RC Columns Strengthened By FRP Bars And FRP Sheets Based On Post-yield Stiffness

More and more people pay attention to the reparability performance of post-earthquake structures,which is an important part of the performance-based seismic design method.Residual deformation is an important indicator of the post-earthquake rehabilitation performance of structures,so it is significant in the theory and practial engineering aspects for the recoverability of the post-earthquake structures to effectively reduce the residual deformation.However,compared to the structures without post-yield stiffness,it is found that the residual displacement of structures with stable post-yield stiffness will be reduced significantly after the earthquake.For this purpose,this dissertation employes the Fiber reinforced Polymer(thereafter called FRP)combination reinforcement technique,which is the combination of Near-Surface-Mounted(NSM)FRP bars and with external bonded BFRP sheets to improve the seismic performance of bridge columns.The main features of this technique include as follows:(a)The structures strengthened by using the NSM FRP bars have the stable post-yield stiffness to improve the loading capacity as well as the reparability;(b)The ductility and energy dissipation of structures can be effectively enhanced by wrapping the FRP sheets on the surface of the structures.In order to study the reparability performance and seismic performance of bridge piers strengthened by FRP combination reinforcement technique,this dissertation carried out pseudo-static experiment on six columns,in which one is reinforced concrete(thereafter called RC)column and the others are strengthened columns by using NSM FRP bars and BFRP sheets.Furthermore,a comparison was made between the experimental results and the numerical results and theoretical analyse.The main works and results are summarized as follows:(1)The assessment method of the reparability performance is investigated on the piles or columns.The professional dearthquake wave analytical software,i.e.,Bispec is firstly used to simulate the dynamic time-series analysis for the columns.Experimental results from shaking table test are used to validate the efficiency of the softare,and the results find that the software is effective and is able to precisely simulate the residual displacement and maximum elasto-plastic diaplacement for the columns.On this basis,the dynamic time-series analysis is performed for different single degree of freedom(SDOF)systems,and the residual displacement ratio spectra(thereafter called RDRS)are established based on the results by inputting 40 ground motions to the Clough hysteretic model.Furthermore,the effect on RDRS is also studied,such as the yield stffness ratio,bilinear factor,site types,the fundamental period and displacement ductility.This dissertation proposes an assessment method of reparability performance based on residual displacement ratio spectra,which can quantitatively calculate the residual deformation and assess the post-earthquake rehabilitation performance of SDOF structures.A column or pile with deficit in seismic performance is regarded as an example to describe the details of the proposed analytical and assessment methods.It is found that the lateral displacements meet the seismic design code requirements in the allowable maximum displacement for the RC column strengthened by using NSM FRP bars or steel bars when the seismic intensity increases from 0.1g to 0.15g,but compared to the column strengthened by using the NSM steel bars,the column strengthened by using the NSM FRP bars reduce 32.7%in the residual displacement.This indicates that the FRP combination method with FRP bars and sheets has good rehabilitation performance.(2)Six RC columns carried out strengthening by NSM FRP bars and BFRP sheets and performed the measurement under cyclic loads in laboratory.The emphasis is investigated on tfailure modes,bearing capacity,displacement ductility factor,stiffness degradation,energy dissipation capacity and reparability performance(including post-yield stiffness,residual displacement and damage index).The results show that compared to the RC column,the strengthened column(RC1)increases 31.4%,36%,160%and 47.2%in the carrying capacity,ductility,and energy dissipation capacity,respectively.(3)The performance of the strengthened columns is simulated numerically by the OpenSees software and a comparison is made between the anlatical values and the experimental results.A good agreement is found between them.On this basis,parameter analyses like aspect ratio,axial compression ratio,reinforcement ratio and the ratio of reinforcement BFRP bars are conducted by the developed numerical model.Finally,the calculation formula of the post-yield stiffness is fitted by using the numerical results.(4)Via theoretical analyses,it is established the load-displacement restoring force model of RC columns strengthened by NSM FRP bars and BFRP sheets.The results show that the theoretical results are in good agreement with the experimental data.In terms of theoretical analyses,a simplified retrofitting design method of the RC columns strengthened by NSM FRP bars and BFRP sheets and the corresponding construction measures are initially proposed for practical engineering application finally.