Retrofit of Steel-Frame Buildings Using Enhanced Gravity-Frame Connections

This paper presents a new retrofit method that employs gravity-frame connections to improve the seismic performance of buildings by enhancing the lateral-force resisting capacity of conventional shear-tab connections. In the retrofit method, proprietary T-stub flange connectors used in the Simpson Strong-Tie Strong Frame Special Moment Frame are installed adjacent to the shear tab connections to create a ductile partially restrained beam-to-column connection. Beam flanges and buckling restraint plates are used to force the reduced-width segment of the T-stub stem into high-mode buckling and to act as a “structural fuse” in an earthquake. The collapse safety and seismic serviceability performance was predicted for a set of archetypical office buildings with ductile or non-ductile (pre-Northridge) moment connections by employing the FEMA P-695 and FEMA P-58 methodologies. The results indicated that the seismic collapse safety and serviceability performance of retrofitted buildings was generally superior compared to conventional buildings. The retrofit method decreased interstory drifts and reduced repair cost and downtime, although retrofitted buildings were stiffer and experienced higher roof accelerations in some scenarios.