Simulated behavior of multi-story X-braced frames

Steel braced frames are a commonly used seismic resisting system and thus, multi-story X-braced frames are frequently used. However, research into the behavior of these systems with midspan gusset plates, as used in practice, is limited. As a result, their seismic performance and the influence of connection design on this performance are not well understood. A comprehensive series of inelastic analyses were undertaken to better understand the nonlinear, cyclic behavior of multi-story X-braced frames and their gusset plate connections. Finite element (FE) analyses were conducted and the FE model was developed and verified by comparing the simulated results with cyclic tests and nonlinear analyses of single story systems, conducted at the University of Washington. The verified analytical model and associated failure estimation procedures were used to predict all yield mechanisms and failure modes, frame deformation capacity, and initial cracking and fracture of critical elements within the frame. A parametric study was performed to examine the influence of the gusset plate, framing members and other structural elements on the seismic performance of multi-story X-braced frames. The results show that the design and detailing of the gusset plate has a significant impact on the seismic performance of the frame. Connections designed with proposed end-rotational clearance models, and with strength and stiffness values balanced to the buckling and tensile yield capacities of the brace provided the best ductility and deformation capacity. In addition, the results suggest that floor slabs, gusset plate stiffeners and framing member sizes affect the frame performance and must be considered in the analysis and design of the system.