Seismic Behavior And Design Method Of Reinforced Concrete Frames Retrofitted With Buckling-Restrained Braces

Deficient existing reinforced concrete(RC) structures are vulnerable to severe damage or even complete collapse under earthquakes. It is necessary to retrofit these existing structures to satisfy current seismic specification. For traditional anti-seismic method, it is difficult to control the structural damage level due to depending on inelastic deformation of main structural members to dissipate input energy under unpredicted earthquake. Nevertheless, novel energy dissipation and structural controlling technique overcomes the disadvantage of traditional anti-seismic method. It is mostly using energy dissipative dampers to dissipate the input energy instead of main structural members. Thus, severe damage of main structural members is prevented. Buckling-restrained brace(BRB) not only overcomes the problem that conventional brace may globally or locally buckle in compression under severe earthquake, but also acts as energy dissipative damper to protect the main structural members. This type brace has become very attractive to researchers and engineers in earthquake engineering. However, currently, few studies have been conducted on RC frames retrofitted with BRBs.This study focuses on the seismic behaviors and design methods of RC frame retrofitted with BRBs, and it includes the following five parts:(1) A hybrid seismic retrofit scheme, which utilizes BRBs to enhance lateral strength, stiffness and energy dissipation capacity of a seismically-weak RC frame and steel caging to improve the flexural strength and deformation capacity of the deficient RC columns, is presented to solve the problem that existences of BRBs may subject the surrounding main structural members unfavorable load conditions. A RC frame representing one bay at the bottom story of a Hebei provincial library building was selected as the test model. Cyclic quasi-static tests were performed on two half-scale one-story, and single-bay RC frames to evaluate the efficiency of the hybrid retrofit technique. One of the frames is unretrofitted, and the other one is retrofitted using the proposed retrofit scheme. It shows that the proposed hybrid seismic retrofit scheme can effectively prevent damage of RC columns and beam-column joints. The BRB significantly increased the lateral load resistance, lateral stiffness and energy dissipation capacity of the frame, and the steel caging could effectively change failure mechanism of the RC frame.(2) Numerical simulations on seismic behaviors of RC frame retrofitted with BRBs are conducted based on Open Sees program. Analytical models of unretrofitted frame and retrofitted frame are firstly developed, respectively. Then the cyclic test results are used to verify the developed analytical models. It shows that reasonable agreements are achieved between numerical and experimental results. The developed analytical models can predict seismic behaviors of unretrofitted frame and retrofitted frame with acceptable accuracy.(3) Some requirements of current seismic specification on an energy dissipation system may subject the system to be not cost-effective. A simplified method for calculating interstory drift capacity of a RC frame is proposed based on weakest plastic hinge to solve this problem. The interstory drift limit state for a RC frame is firstly defined. Then the weakest interstory plastic hinge is determined, based on the beam-column subassemblage failure modes. Finally, the corresponding interstory drift capacity of the RC frame is calculated. It shows that the interstory plastic drift capacity of a RC frame mainly depends on plastic rotation capacity of the interstory weakest plastic hinge, through verifying with previous RC frame test. The proposed calculating method can predict interstory drift capacity of the RC frames with acceptable accuracy.(4) The effects of BRB and gusset plate on deformation capacity of main structural members are investigated. Then a simplified method for calculating interstory drift capacity of a RC frame retrofitted with BRBs is proposed. Simplified methods for evaluating interstory drift demand of a RC frame retrofitted with BRBs are presented. In order to prevent locally strengthen each structural members using BRBs intervention technique, a practical design method of a RC frame retrofitted with BRBs is proposed, based on two design step presented in current seismic specification.(5) Seismic retrofit design of 3 story existing RC frame of Hebei provincial building is conducted to evaluate the efficiency of the proposed practical design method. Analytical results show that longitudinal reinforcement area of lots of columns at ground story and second story of existing frame is insufficient, and elastic interstory drift demand is beyond the limit value under frequent earthquake. Morever, elasto-plastic interstory drift demand of seismically-weak story is beyond limit value under severe earthquake. It is not cost-effective if every deficient column of existing frame is strengthened. Then a cost-effective retrofit scheme is adopted, in which the proposed hybrid seismic design method is utilized to retrofit the existing frame. Analytical study on the retrofitted frame is conducted. It shows that load resistance of structural members and interstory drift demand of the retrofitted frame satisfy requirements of current seismic specification.