Design and Numerical Evaluation of Two Innovative Moment-Free Beam-Gusset-column Connections for BRBFs

Buckling-restrained braces (BRBs) are strong, ductile members used in structural steel frames, called BRBFs, for resisting lateral seismic loads. However, in previous experiments on BRBFs, typical beam-gusset-column (BGC) joints underwent premature fractures, even under cyclic lateral-loadings equivalent to design-basis-earthquakes. The damages were related to the presence of thick gusset-plates, connecting BRB to beam-and-column, which caused unintended joint rigidities and so stress concentrations around gusset-tips. In this paper, two innovative BGC connections are developed to accommodate high story-drifts, by releasing their beam-column fixities. For this purpose, work-point is displaced and the following connection-components are utilized: (1) two bolted flexible wide-plates (WPs) placed on spacer plates, transferring shear forces from beam-web and gusset-plate to column, and a double plate in the middle, carrying horizontal forces; or (2) a few square pins with one vertically and some horizontally slotted holes, connecting only gusset-plate to column. Finite-element models of the connections situated in BRBFs are constructed and their validities are verified against four comparable experiments. Therefore, the cyclic performances of the simulated connections are investigated under cyclic loadings up to 4% drifts. The results demonstrate that the introduced connections possess high rotation capacities and can withstand maximum considered earthquakes (MCEs) without suffering from low-cycle-fatigue. In addition, the important parameters of bolt-pretensioning, bolt-hole clearance, column-section orientation, BRB-strength, bolt-gage, WP-extension, WP-fracture, WP-to-column connection, pin-section, pin-hole arrangement, and work-point location are investigated; and comparisons are made with previous BGC connections incorporating single flexible-plate or double-angles