Abstract
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.
Introduction
Many lateral force-resisting systems for buildings can be designed economically to provide life safety during a design-level earthquake, but most of these systems are expected to be difficult to repair after a moderate or larger event. Therefore, alternative self-centering systems are being developed to avoid damage and residual deformations. One such system is a controlled rocking steel braced frame (CRSBF), which has been studied experimentally (Midorikawa et al. 2006; Tremblay et al. 2008a; Ma et al. 2010; Sause et al. 2010; Wiebe et al. 2013a, b; Eatherton et al. 2014) and has also been implemented in practice (Gledhill et al. 2008; Mar 2010; Latham et al. 2013; Tait et al. 2013). Part I (Wiebe and Christopoulos 2014) proposes a general framework for the design of CRSBFs and validates a method for designing the base rocking joint to achieve a predetermined maximum response, assuming that all members of the CRSBF were adequately designed to remain linear elastic.
Conclusion
The peak forces in the members of a controlled rocking steel braced frame may be dominated by the higher modes of seismic response. Higher mode effects are likely to be particularly significant for tall frames, especially if the system is designed with a large forcereduction factor, as recommended in Part I (Wiebe and Christopoulos 2014). This paper presented a method to estimate the elastic force demands on the elements of a CRSBF, including the influence of higher mode effects. This method can be implemented in a spreadsheet easily, and it can be used for preliminary design purposes because it requires only building properties that are known before a trial design has been developed.
