Experimental and Numerical Investigation of Strongback Braced Frame System to Mitigate Weak Story Behavior

Multistory steel braced frames tend to concentrate damage in a few weak stories in response to severe earthquake shaking. This paper presents the numerical and experimental results for a strongback system, a modification of the conventional braced frame that uses an essentially elastic truss that vertically spans the structure's height to delay or prevent weak-story behavior. A cyclic test is performed on a nearly full-scale two-story specimen in which the strongback is introduced as a seismic retrofit of an existing vintage-era concentrically braced frame. The retrofit design is composed of two halves: an inelastic truss using a buckling-restrained brace (BRB) to dissipate seismic input energy and an elastic vertical truss designed to control weak-story behavior. The strongback specimen prevented formation of a weak-story mechanism and mobilized the reserve strength of other structural components, even after the core of the BRB ruptured. Test results establish that the strongback system can be a successful method of mitigating weak-story behavior. A simple numerical model incorporating a low-cycle fatigue material is able to predict the global response of the frame, including the rupture of the BRB.