Performance-Based Seismic Design of Controlled Rocking Steel Braced Frames. II: Design of Capacity-Protected Elements

Controlled rocking steel braced frames (CRSBFs) are intended to have a self-centering response that avoids damage to main structural elements. To ensure that all nonlinearity is confined to the intended elements at the rocking joint, the frame must be adequately capacity designed. This requires accurate predictions of the peak forces that are likely to develop in all members of the frame while the rocking mechanism reaches its peak rotation. Previous studies have shown that the peak forces in CRSBF members are likely to be strongly influenced by higher mode effects, but these effects can be mitigated by designing multiple nonlinear mechanisms. This paper proposes methods for estimating the peak forces in frame elements, designing an additional mechanism if it is desired to mitigate higher mode effects, and predicting the reduction in response that will be achieved by adding this mechanism. The methods are validated by designing buildings with two, six, and 12 stories, including three alternative designs that use multiple mechanisms to mitigate the higher mode effects. The six frames are modeled using OpenSees and are subjected to 44 ground motions at the maximum considered earthquake level. The peak forces in the taller frames without additional mechanisms are dominated by higher mode effects, but these effects can be estimated using the proposed method. These forces can also be reduced by designing multiple mechanisms, and the proposed method provides a reasonable design-level prediction of this force reduction.