Viscoelastically Enhanced Buckling Restrained Braces for P-Delta Cancellation and Reduced Residual Drifts

An innovative viscoelastic buckling-restrained brace (VE-BRB) is proposed as an enhancement to conventional buckling-restrained braces (BRBs). This brace mitigates P-delta effects, introduces a recentering capability, and provides enhanced inherent damping, thus enhancing the seismic performance of tall steel buildings in high-seismic regions, especially under subduction interface seismic events. The VE-BRB enhances a conventional BRB with viscoelastic dampers mounted directly on the BRB restrainer, functioning in parallel with the BRB core element. The viscoelastic dampers are designed with sufficient elastic stiffness to counteract P-delta effects and induce recentering without the need to amplify story design forces using the code-prescribed stability coefficients. The proposed brace is described in detail, including design equations tailored for multistory steel buildings. To evaluate its effectiveness, the braces were implemented as seismic-force resisting systems in 10- and 20-story prototype buildings located in Vancouver, BC. These buildings were compared with conventional BRB frames designed with amplified design story shear forces using the inelastic stability coefficient, as prescribed in the Canadian Building Code. Nonlinear response history analyses were conducted using seismic records from crustal, in-slab, and subduction interface sources. The VE-BRB system demonstrated superior performance compared with conventional counterparts, achieving more-uniform drift distributions along the building heights and controlling residual drifts within repairable limits. Detailed finite-element (FE) models were developed to validate the proposed design assumptions at the brace component level, including restrainers, connections, bolts, and gusset plates. Conventional BRB models were calibrated against past cyclic test data, followed by models incorporating the viscoelastic dampers. Full-scale FE models of the 6th and 10th stories of the 10- and 20-story VE-BRB frames were subjected to quasi-static cyclic pushover analyses, because these floors correspond to available matching calibration experimental test data. The results confirmed the adequacy of the proposed design assumptions and demonstrate robust and stable performance of the VE-BRB system under extreme drift demands.