Strength Reduction Factor Of Steel Concentrically Braced Frames

The steel concentrically braced frames(CBFs)support structure of the initial anti-lateral stiffness and strength.As reserve system,frame can continue to bear the lateral loadafter the failure of support.Reasonable seismic design plays an important role in the safety of the structure.All previous earthquake damage shows that reserve system directly influence the collapse resistance of CBFs.Therefore,the reserve system of concentrically braced frames has a significant influence on the seismic performance of the structure,which should be considered seriously.In structure design,the design base shear can be calculated by strength reduction factors to reduce elastic strength.Soit is the key to determinethe strength reduction factor of CBFs toconsidersthe reserve coefficientof the systemfor the seismic design.The paper based on the SDOF system of CBFs investigated the influence of the ductility,reserve coefficient and damping ration for the strength reduction factors of the CBFs by dynamic time history analysis method.It is indicated that the strength reduction factor increases significantly with the increase of ductility.Stiffness reserve capacity has an obvious influence for structures with low ductility,and the larger ductility of middle long period structures should consider the influence of the strength reserve capacity.Finally,the R-?-T model of the CBFs is established,which provides a basis for the strength-based design of steel CBFs.In addition,the equivalent single degree of freedom system which is converted from the multi-degree of freedom(MDOF)system to the first mode is analyzed by pushover.Then,a simplified method is proposed for evaluating the displacementbased on the R-?-T model.Compared with traditional time history analysis method,this method can determine the peak displacement of the structure,and verify the applicability of the R-?-T model on MDOF.It is indicated that theR-?-T model for the SDOF of thesteelconcentrically braced frames can be used to evaluate the peak displacement of the structure,which canguide the Seismicdesign and performances evaluationfor theMDOF of CBFs.