Research On Seismic Performance Index And Limit Value Of Earthquake Damaged RC Columns Strengthened With FRP

Disaster earthquakes occurred frequently in China recently and caused different damages to building structures.After earthquakes,damaged buildings need to be inspected,evaluated even retrofitted to meet the normal usage requiremnts.The seismic performance of reinforced concrete(RC)columns influences the sesimsic behavior of RC frame structures.However,the in-forced seismic design codes and related studies mainly focus on the RC columns without seismic damage and retrofitting,and there are limited studies on the seismic performance of RC columns strengthened without seismic damage and RC columns strengthened after seismic damage.In this paper,the seismic performance of RC columns without seismic damage and retrofitting,RC columns strengthened without seismic damage and RC columns strengthened after seismic damage are deeply studied.The main contents and results are as follows:1.The available 50 groups of test results of three kinds of RC column specimens have been investigated.It was found that the existing seismic behavior classification methods and index limits for RC columns without seismic damage and retrofitting cannot be used for other two kinds of RC columns directly.Based on the existing codes and related research results,methods for dividing the damage degree of RC columns are analyzed,and the index limits of displacement angle have been proposed.2.24 RC column specimens have been tested to failure to investigate the influence of tensile and compressive stress history on the compressive performance of RC columns,in which 8 specimens were subjected to graded unidirectional cyclic axial compression loading and 16 specimens were subjected to different degrees of tensile and compressive preloading under different initial axial compression ratios,and then they were subjected to graded unidirectional cyclic axial compression loading to failure.The effects of unidirectional cyclic axial compression and tensile and compressive preloading degree on the axial compression performance of RC columns were studied.3.For RC columns without seismic damage and retrofitting,a trilinear momentrotation angle skeleton curve model has been proposed.Given the geometric size,section reinforcement detailing and material property,based on the plane section remain plane assumption,its yield,peak and ultimate limit state condition definitions,the judgment rules and simplified calculation methods for the key point parameters on the skeleton can ben determined and a MATLAB program has been compiled.The predicted results are basically consistent with the experimental results and the reliability of the proposed simplified method is verified.4.The method for determining the size of the damage zone of RC columns based on concrete ultimate compressive strain has been proposed and the MATLAB program is compiled.Based on the function of the model change method in ABAQUS,the numerical technology for simulating seismic performance of RC columns has been proposed.It can simulate the pre-damage of RC columns,the replacement of local peripheral damaged RC,FRP strengthening and bahavior of the RC columns after retrofitting.The numerical simulation of three kinds of RC columns have been carried out.The simulated results agree well with the experimental results.By comparing the simulated skeleton curves of RC columns strengthened under different pre-damage conditions and different FRP reinforcement methods,it was pointed out that the pre-damage degree,reinforcement methods and design parameters are the key factors governing the seismic performance of RC columns after strengthening.5.The reduction coefficient of material property of damaged concrete and yield reinforcement has been proposed.Based on the above work,given the constitutive of FRP restrained damaged concrete and damaged reinforcement,and the equivalent rectangular stress graphic coefficient of FRP restrained RC columns is calculated according to the equivalence principle.According to the plane section assumption and assumptions,the yield,peak and ultimate limit states of a RC column retrofitted by FRP after seismic damage have been analyzed,and the judgment criteria and calculation method of key performance point parameters of trilinear moment-rotation angle skeleton curve model are proposed and a calculation program is compiled with MATLAB.The simplified calculation method is verified.Based on the trilinear skeleton curve,using the displacement angle as the performance index,several performance states and the displacement angle limit of each performance state has been proposed.