Seismic retrofit of H-section beam-to-HSS column connections in existing high-rise steel buildings

The absence of damping devices and the lack of consideration for long-period ground motions (LPGMs) in the design of 1970s high-rise steel moment-resisting frame (SMRF) buildings have raised concerns about potential damage to beam-to-column connections during long-duration LPGMs. This study focuses on retrofitting commonly used H-section beam-to-HSS column connections in these structures and evaluating their seismic performance under LPGMs. A macro-model, incorporating beam-end fractures, was calibrated and validated using prior quasi-static component tests of three retrofitted connection types. A cyclic pushover analysis was then performed to generate the response of a full-scale retrofitted SMRF building tested at E-Defense shake-table in 2009, accurately reproducing the experimental results. The analysis revealed that retrofitted connections significantly reduced brittle fractures at beam ends; increasing the energy dissipation capacity in the wing plate and modified supplemental weld-retrofitted frames by 3.1 and 2.5 times, respectively. Using experimentally validated parameters, nonlinear numerical analyses of a prototype 21-story SMRF building were conducted to quantify the impact of retrofit strategies and their vertical distribution on seismic performance under LPGMs. The results demonstrated that the proposed retrofit strategies substantially reduced the collapse risk of 1970s high-rise SMRFs. Additionally, mainshock-aftershock analyses showed that the retrofit effectively mitigated weak-story collapse mechanisms caused by beam-end fractures, while significantly enhancing the building's energy dissipation capacity.