Conventional concentrically braced steel frames are not capable of redistributing large unbalanced vertical forces caused by brace buckling through the system. This creates many design challenges to engineers. In order to retain the advantages of providing efficient stiffness and strength to limit inter-story drifts, new concentrically braced steel frame configurations are developed. This paper will focus on the seismic evaluation of the suspended zipper frame configuration.
Unlike the conventional Chevron braced frame, the suspended zipper frame is designed to distribute the unbalanced vertical forces along its height using the zipper column, a vertical structural element connecting the gusset plates at beam mid-span points from the second to the top story of the frame. The theory and analytical simulations demonstrate that the intended force redistribution is, indeed, occurring. However the inelastic behavior of the entire frame depends strongly on the brace hysteresis and the interaction of the zipper columns. Due to the nature of the geometry, the braces provide most of the lateral stiffness until they buckle. Once the braces buckle, a large reduction in the brace stiffness will cause drastic force redistributions in the frame. Since the process is highly nonlinear, tracing the force redistribution is very complex. Thus, a hybrid simulation test has been conducted to evaluate the performance of the suspended zipper frame. The hybrid simulation method combines the analytical finite element model and the experimental element to capture the structural response under external excitation. The complex Chevron brace buckling behavior, which is difficult to model analytically, is captured using a specimen. The results presented in this paper indicate that the hybrid simulation method is an excellent tool to evaluate the seismic performance of structural systems with complex substructures, such as the suspended zipper frame.