Shake table test of a two‐story steel building seismically retrofitted using gravity‐controlled rocking braced frame system

Abstract

AbstractThis article describes a shake table test program that was conducted to investigate the seismic behavior of a half‐scale two‐story gravity‐controlled rocking braced steel frame building. In this system, braced frame columns are designed to uplift from the foundation under severe earthquakes to reduce the seismic force demands on the frame members. Self‐centering capacity is solely provided by the gravity loads carried by the rocking frame. Energy dissipative devices are added at the base of the braced frame columns to control drifts. The system can be used for new structures as well as the retrofit of seismically deficient structures. In the test program, the specimen represented a gravity‐controlled rocking frame that had been proposed for seismic retrofit in a previous study. The test structure was subjected to ground motions expected for two site classes in two seismically active regions in Canada. Three different energy dissipative devices located at the rocking interface were studied: friction, friction spring dampers, and steel bars yielding in tension and elastically buckling in compression. The focus of the tests was on peak axial loads in the columns and additional moments and shears in the beams resulting from column impact upon rocking. Axial loads in the braces and columns from higher mode response were also examined. The tests revealed significant increases in beam forces due to column impacts. Large axial forces due to the second vibration mode response were measured in the second story braces. A numerical model is proposed to accurately predict the measured force demands.