Performance-Based Seismic Design of Controlled Rocking Steel Braced Frames. I: Methodological Framework and Design of Base Rocking Joint

Controlled rocking steel braced frames (CRSBFs) are being developed as a seismic force resisting system that can be constructed economically to avoid structural damage and residual deformations following an earthquake. In a CRSBF, selected columns are permitted to uplift from the foundation in response to severe seismic loading, and posttensioning and energy dissipation are selected to control the magnitude of the rocking response. Despite extensive experimental testing to demonstrate that this behavior is stable and repeatable, there has been a lack of comprehensive guidance for potential designers of CRSBFs. This paper proposes a performance-based design methodology for CRSBFs, which consists of defining the performance objectives, designing the base rocking joint based on a single-degree-of-freedom (SDOF) model, and capacity protecting the rest of the structure for the maximum forces expected during the rocking response. This paper focuses on the design of the base rocking joint, while Part II proposes a method for capacity designing the rest of the structure with due consideration of higher-mode effects. This paper shows how to design the base rocking joint to achieve a targeted response based on the results of 30,492 analyses of SDOF systems with flag-shaped hystereses. The SDOF results suggest that the rotation demands on the rocking joint depend on the initial stiffness of the system. For structures with initial periods less than about 0.4 s, the base rocking joint requires smaller force reduction factors than are codified for ductile seismic force-resisting systems, but much larger force reduction factors could be used for structures with longer initial periods, provided that the frame members are capacity designed and that there is sufficient deformation capacity in the posttensioning and energy dissipation elements. These conclusions are validated by considering the peak and residual displacement response of three example structures at two seismic hazard levels.