Parametric Study On Steel Concentrically Braced Frames Based On Seismic Ductility Demand

Steel concentrically braced frame (CBF) is a typical kind of dual system in steel structure. It is widely used for its large initial lateral stiffness, and the secondary strength and stiffness after braces fracture plays an important role in seismic collapse prevention. The CBF system displayed poor performance during the past earthquakes due to the buckling behavior of braces. It is important to proportionally design parameters of strength and stiffness between brace and frame, to ensure the structure would not collapse. In this paper, the following aspects of seismic performance of CBF systems were studied:(1) Single-degree-of-freedom (SDOF) systems were employed to represent CBFs system, the strength and stiffness of structure is assumed to be uncoupled and two parameters, α and β, which relate the reserve system to the primary system were put forward. Reserve capacity of CBF structure can be quantified by the value of a and β. And the structure seismic performance can be evaluated by these parameters.(2) Time history analyses of SDOF CBF systems were carried out based on the ductility governing equation of motion. The ductility demand spectra were presented. The threshold periods and parameter values of CBF systems were explored and identified based on ductility demand and structural ductility. The conclusion can provide references to the design of the CBF system.(3) The influence of P-Delta effect and site effect were taken into account. Numerical results show that second-order effects have little influence on the CBF system in elastic stage. When the structure come into plastic, second-order effects must be considered. Only in short period range of structures, site effects have greater impact on structural response. The softer construction site would attract greater structural response.(4) The multi-degree of freedom (MDOF) system has a distinct failure process from the SDOF system. The failure modes and reserve system of MDOF CBF systems were analyzed. To evaluate the seismic performance and design parameters of multistory steel CBF, the first mode dominated MDOF system is simplified to the SDOF system. Parameter study is conducted and threshold periods are identified based on simplified SDOF system. Pushover analysis for the MDOF CBF system can be used to confirm the design parameters, a and β. The method is effective, and proposed to practical application of engineering design.