Study On Longitudinal Seismic Behavior Of Multi-tiered Steel Concentrically Braced Portal Frame Structure

Column bracing is the core member of the longitudinal seismic resistance of the portal frame structure.In tall open building,it is common to use a multi-tiered bracing configuration which can provide laterally support in the plane of the frame at every strut level for column of portal frame structure and also reduces the steel tonnage.However,the seismic design method for multi-tiered bracing configuration in our country has not been clearly explained and there are few researches on the longitudinal seismic behavior of portal frame structures with multi-tiered bracing configuration.In view of the above problems,this paper has done the following works:In order to study the longitudinal seismic performance of multi-tiered steel concentrically braced portal frame structure,a finite element analysis of portal frame structure was performed to simulate the overall elasto-plastic time-history process with finite element program ABAQUS to determine some seismic properties,including mechanical behavior,failure modes.The results showed that the structure meet the requirements of structural longitudinal degeneration under frequent earthquakes and may exhibit unsatisfactory seismic response including evenly drift and column instability under under serve earthquake.Nolinear static analyse was used to study mechanical behavior and failure mode of two-tiers braced frames in the lateral displacement process.The results showed that column will be buckling due to bending moment in-plane of bracing.So this paper proposed a recommended design method that column is designed to resist the axial loads acting in combination with biaxial bending moment.Then the nolinear staticanalyse and hysteresis analysis were carried out for the new specimen designed according to the recommended design method.The results showed that the recommended design method can effectively avoid the buckling of columns and improve the energy dissipation capacity of the specimens.On the basis of the time history analysis and nolinear static analysis of the multi-tiered steel concentrically bracing,the paper proposed two design objectives for multi-tiered steel concentrically bracing: avoiding buckling phenomenon of the column that may lead to the collapse of the structure and reducing drift in the critical brace tier that may lead to the failure of bracing.Then two new specimens were designed by the recommended design method,and the time history analysis under serve earthquake action was carried out.The results showed that the new specimens achieve a more uniform and prevent buckling failure of the column,the design targets of the multi-tiered steel concentrically bracing is reached.This validates the effectiveness of the recommended design method.