Research Of Performance-based Design Method For Seismic Retrofit Of Low-ductility Masonry Infilled Reinforced Concrete Frames Using FRP

In In recent earthquake events,a large number of masonry-infilled reinforced concrete(RC)frame structures were severely damaged or even collapsed due to the lack of enough ductility and energy dissipation capacity.These low-ductility infilled RC frame structures include structures designed according to the pre-1970 standards without seismic design,or existing structures which can not meet the current seismic design code because of the revision of design code.At present,there are a great number of low-ductility infilled frame structures in the potential earthquake regions of our country.Due to the effect of infill walls,the structural global ductility and deformation capacity would be greatly decreased and these low-ductility frames are very likely to be collapsed under the future strong earthquakes.Hence,there is an urgent need to conduct pre-earthquake seismic strenthening.Fiber reinforced polymer(FRP)composites have been widely used for seismic rehabilitation of concrete structures because of their light weight,high strength,good durability,small disturbance on the structure,and ease of application.But the relevant studies are not systematic and sufficient.The codes from different countries have just provided the load bearing capacity of slabs,beams and columns strengthend with FRP.The research on the design method for global seismic retrofit of structures is deficient.For existing low-ductility infilled RC frames,improving the seismic performance of the whole structure,establishing the evaluation method in the probabilistic sense from the failure of whole structure and building a quantitative performance-based seismic retrofitting design theory system,have become a key scientific problem for the continued development in the seismic strengthening field.In view of the current shortage of research on seismic performance of low-ductility infilled RC frames strengthened with FRP,this thesis firstly performed the numerical modeling and verification for FRP-strengthened infilled frames.Then,the classification method and quantitative index for seismic performance of low-ductility infilled frames before and after FRP retrofitting was proposed.Based on this,the seismic vulnerability of masonry infilled frames with different heights was studied and the rehabilitation sheme with the minimum global failure probability was given.Finally,the performance-based design method for FRP seismic retrofit of masonry infilled frames was put forward.The main research contents are as follows:(1)Based on Open Sees(Open System for Earthquake Engineering Simulation)platform,the infilled wall material used for seismic analysis of co mponents and structures was developed.Through the numerical simulation of previous tests,the accuracy and reliability of the finite element models for FRP-retrofitted infilled RC frames was verified.The above work layed a foundation for subsequent seismic performance analysis of a large number of masonry infilled frames with and without FRP rehabilitaion.(2)Taking into account the variation of parameters such as the column size,axial compression ratio,shear span ratio,longitudinal reinforcement ratio,concrete strength,yield strength of longitudinal steel,compressive strength,height-to-width ratio and thickness of the infill wall,and etc,the parameter statistical investigation for seismic performance of infilled frames with and without FRP rehab ilitaion was conducted.The variation tendency of interstory drift ratio was therefore discussed,and the classification method and quantitative index for seismic performance of infilled frames before and after FRP retrofit was established.(3)According to out-of-date seismic codes of our country,the five-,ten-,and fifteen-storey infilled RC frames were designed to represent existing low-,medium-,and high-rise low-ductility infilled frame structures,respectively.Based on the maximum likelyhood method,the seismic fragility of infilled frames with different heights before and after FRP strengthening was assessed.The seismic performance evaluation of infilled frame structures using different rehabilitaion schemes in the probabilistic sense was finished.Finally,the concept design principle for FRP seismic retrofit of low-,medium-,and high-rise infilled frames was proposed.(4)The effect of different structural characteristic parameters and rehabilitation schemes on seismic responses of infilled frames was studied.The strength reduction factor spectra for infilled frames with and without FRP rehabilitation,which are applicable to Chinese code,were developed.The simplified expressions of strength reduction factor spectra were also given.The performance-based capacity spectrum method for FRP seismic retrofit of infilled RC frames was finally proposed.