Study On Seismic Capacity Of Reinforced Concrete Structure With Soft-first Story

The earthquake disaster investigation shows that the Reinforced Concrete(RC) frame structures with soft-first story are damaged seriously in greatearthquakes. The main characteristics of this kind structure is that the first storyis designed and constructed with few infill wall considering the special functionssuch as taking the first story as reception hall, parking garage and store etc,while the upper stories are designed with more infill walls for the function ofapartment. Soft-first story is an unavoidable feature in the modern multistoryconstructions according to the demand of buildings, and the common feature ofsuch buildings is that the stiffness of the first story is less than the upper stories.For this kind of buildings, the upper stories being stiffer undergo smallerinter-story drift, while the inter-story drift in first story is larger and the strengthdemand for the first story is also very high as the shearing force is very large.The seismic performance of this kind building is discussed and analyzed bycomparing with normal RC frame structures, and some suggestions to improvethe seismic design are obtained in the study.The main works and conclusions are summarized as below.(1) Simplified equivalent x-brace model of infill wallThe state of arts of the Reinforced Concrete frame structure with thesoft-first story is reviewed firstly. Researchers usually take x-brace to simulatethe effect of infill walls and explore the effects of the window and door’sopening, herein, we use a reduction factor to compute the width of equivalentbrace in the modeling. Based on the results of other studies, a program forautomatically computing the width of x-brace is developed considering thewindow and door opening. The validity and efficiency are verified by comparingthe computing results with others’ study.(2) Nonlinear static analysis of RC frame structureAccording to the pure RC frame structure, RC frame structure with infillwall in all stories and soft-first story RC frame structure, the static analyticalmodels are designed and constructed. The push-over analysis is performed tofind out the performance point by combining the capacity spectrum with thedemand spectrum. The structural top displacement under the specific seismicexcitation is calculated, and then the structure is pushed to the correspondingvalues to obtain the drift angle which is used to compare with the elastic andelastic-plastic drift angle limits provided by the seismic design code. Moreover,the damage levels are discussed according to the plastic hinges. At last, theseismic performances of three kinds of models are compared and the suggestions for improving structure seismic design are concluded.(3) Nonlinear dynamic time-history analysis of RC frame structureAccording to three structures in the nonlinear static analysis, the finiteelement numerical simulation models are established. The nonlinear dynamictime-history analysis is performed to explore the seismic performance under theexcitation of different strong ground motion input level. As same as thenonlinear static analysis, the drift angle under the action of strong groundmotion is analyzed and compared with the elastic and elastic-plastic drift anglelimits provided by the seismic design code. The damage levels are discussedaccording to the plastic hinges. At last, the seismic performances of three kindsof structures are compared and the suggestions for improving structure seismicdesign are concluded.Finally, the main study works and results are summarized detailedly, andthe study plan and content in the future are analyzed and discussed.