Proportioning energy-dissipating members in strongback braced frames

Many structural systems are susceptible to soft-story instabilities during earthquakes that are lifethreatening and can lead to damage that is too costly to repair. One way to mitigate damage and reduce the potential for soft-story instability is through the addition of an elastic spine that distributes drifts across the height of a structure. One such system is the strongback braced frame, which replaces one side of a buckling-restrained braced frame with a strongback truss. With the strongback providing vertical continuity, an expanded design space is made available for the arrangement of buckling-restrained braces (BRBs) or other energy-dissipating members. An example of this expanded design space is that a designer could opt to not include BRBs at every story. Methods for proportioning the energy-dissipating resistance in strongback braced frames have been proposed. However, most methods don't allow exploitation of the full design space. The objective of this work is to propose and evaluate a potential method of proportioning energy-dissipating members for arbitrary vertical arrangements within strongback braced frames. For a prototypical building, the BRBs are designed in various configurations using existing methods and with the new method. Nonlinear time history analyses of the resulting designs coupled with a rigid strongback are performed and the results are compared. The impacts of overstrength and P-Δ effects are quantified. The findings support the proposed method of BRB design that enables exploration of the wide design space made available by the strongback.

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