Mitigation of Soft-Story Failures in Multi-Story Steel Concentrically Braced Frames through Implementation of Stiff Rocking Cores

This paper focuses on investigation of a new seismic rehabilitation method to mitigate soft-story failures in deficient, multi-story, steel, concentrically braced frames (CBFs). The considered method consists of a sufficiently stiff rocking core (RC) that is pinned to the foundation and connected to an existing deficient CBF building to re-distribute seismic forces along its height, creating more uniform inter-story drift and ductility demand distributions. Two benchmark steel CBF buildings - including one 3-story and one 6-story - designed for Los Angeles, California, were selected and retrofitted using the considered method. Nonlinear static pushover analyses were conducted to demonstrate the beneficial contribution of RC in mitigating non-uniform, inter-story, drift distribution. It is shown that the RC is effective in reducing inter-story drift concentration in both buildings when they reach the maximum inter-story drift limits associated with collapse prevention, lift safety and immediate occupancy recommended in FEMA 356 for performance-based seismic design.