Seismic Assessment of the "Strongback" System

Abstract eng:
Conventional steel braced frames have a tendency to form weak stories during strong earthquake shaking, concentrating damage in a few stories while the rest of the frame contributes little to the structure’s ability to dissipate energy. A two-story, one-bay “strongback” (SB) braced frame was tested under quasi-static cyclic loading conditions to assess the system’s ability to mitigate this weak story behavior as part of a strategy to retrofit an existing building. The SB system employs an elastic mast that is pinned at its base to the foundation and runs over the full height of the structure. The mast imposes a displaced shape that increases linearly with height, resulting in nearly uniform drift demands in all stories. The test specimen arranged the braces in a “lambda” configuration, with a single buckling restrained brace (BRB) in the bottom story that acts as an energy dissipating “fuse” and two HSS braces that are part of a relatively strong vertically-oriented truss, or “mast.” Test results show that the SB test was effective in impeding the formation of a weak story mechanism and in mobilizing the reserve strength of other structural components even after BRB fracture. Numerical results were able to capture the overall response of the frame, including the fracture of the BRB. Based on the results from this experimental test, a three-story SB system was analyzed using OpenSEES to improve understanding this system’s behavior under a suite of 240 ground motions at three different hazard levels for a site in Oakland, CA.

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