Effect of Boundary Conditions on Buckling Restrained Braced Frame Seismic Performance

Abstract

This study evaluates the effect of gusset plate behavior on the seismic performance of Buckling Restrained Braced Frames (BRBFs). The expected failure mode of these frames is yielding of the Buckling Restrained Brace (BRB) core, and although other BRB-related failure mechanisms are commonly reported, the performance and potential failure of BRB-gusset connections are rarely addressed. In this study, finite element (FE) models of  BRBFs with diagonally-oriented BRBs braces were first calibrated in the nonlinear interval using the results of experimental BRBFs subjected to quasi-static cyclic loading. The tested specimens consisted of two BRBFs with different unbraced top gusset lengths, each one with two different beam connections (continuous and spliced beams). The calibrated FE models were then used to predict BRBF performance up to failure, considering monotonic loading. Thereafter, a parametric study assess the effect of several gusset plate parameters on the BRBF capacity. The evaluated parameters included the size, thickness, and initial imperfection of the gusset plate, as well as the presence of lug stiffeners. The study shows that cruciform lug stiffeners enhance the buckling capacity of gusset plates by more than 50% because BRB end plates in the out-of-plane (OOP) direction significantly reduce displacements in this direction at the gusset plate free end. The results indicate that the effective length factor used in analytical equations needs to be increased for frames with BRBs without cruciform lug plates. In addition, large gusset plate imperfections and thinner plates decrease the buckling capacity and may even modify the critical failure mode, but design practices and imperfection limits imposed by current codes and standards should prevent a significant decrease in BRBF capacity.