Study On Seismic Behavior Of RC Frame Structures With Few Steel Braces

Seismic damage of reinforced concrete(RC) frame structures during earthquakes has indicated that they may be lack of sufficient redundancy under earthquake action. If few steel braces are installed, the RC frame will become multi-fortification, and its safety margin and collapse-resisting capacity will be improved. In current in-force China design codes, RC frame structure with few shear walls has been clearly defined, while RC frame structure with few steel braces hasn’t been mentioned yet. Definition, collapse-resisting capacities and design criteria of RC frame structure with few steel braces are studied. The main work and achievements are as follows:(1) Verification of finite element model. A finite element model has been developed based on Opensees platform and proved appropriate for elastic-plastic analysis of RC frames with and without steel braces, according to verification of some published test results.(2) Definition of RC frame structure with few steel braces. With seismic precautionary intensity of 8(0.20g), a typical RC frame structure and 3 RC frame structures with different steel braces have been studied and the definition of RC frame structure with few steel braces based on the overturning moment carried by the frame part and lateral stiffness increment has been proposed.(3) Evaluation on collapse-resisting capacity of RC frame structure with few steel braces. With few steel braces, the collapse-resisting capacities of a RC frame structure is proved increased compare to bare RC frame structure and has little chance to collapse under earthquake stronger than designed, through a capacity spectrum method and a collapse fragility analysis based on an incremental dynamic analysis(IDA).(4) Performance requirement and experimental study on seismic behavior of steel braces. The performance requirement of no fracture under seldomly occurred earthquake is derived for tublar steel braces and Buckling-Restrained Braces separately. Based on experimental study, the effect of cross-section shape, width–thickness ratio(w/t ratio), and concrete fill on the seismic behavior and fracture ductility of tubular steel braces are examined and design criteria is proposed. A new type of fabricated Buckling-Restrained Braces with large working segment length is proposed and proved effective through experimental study. It can also used for in-situ retrofitting of tublar steel braces.(5) Summary of design criteria of RC frame structure with few steel braces. To satisfy the performance requirement of withstanding earthquake stronger than designed, design criteria of RC members and steel bracing members of a RC frame structures with few steel braces is summarized based on theoretical analysis and experimental study.