Design Concepts for Damage-Free Seismic-Resistant Self-Centering Steel Concentrically Braced Frames

Abstract

Conventional concentrically-braced frame (CBF) systems have limited drift capacity before brace buckling and related damage leads to deterioration in strength and stiffness. Under the NSF funded NEESR-SG research program, a new type of CBF is being developed with increased drift capacity before damage and decreased permanent drift under seismic loading. These self-centering CBF (SC-CBF) systems are intended to provide significant non-linear drift capacity while limiting damage and residual drift, and are motivated by the goal of minimizing structural damage under seismic loading. At low levels of lateral load, the response of an SC-CBF is similar to that of a conventional CBF. At higher levels of lateral load, the fundamental lateral load behavior of the SC-CBF system is rocking about the base of the compression column, which occurs when the column under tension from overturning moment decompresses and uplifts at the foundation. To control the uplift, high strength post-tensioning (PT) bars, oriented vertically over the height of the SC-CBF, prestress the frame to the foundation. The SC-CBF is designed to decompress at the base at a selected level of lateral loading, initiating a rigid-body rotation (rocking) of the frame. The PT bars provide a restoring force to return the CBF to the foundation (to self-center the CBF). The rocking response substantially influences the member forces that develop in the frame members (beams, columns, and braces) and induces deformation and eventually yielding of the PT bars. To minimize structural damage before PT bar yield, the frame members are designed to resist the internal forces that develop at PT yield. The paper presents the results of an analytical study of SC-CBFs. Dynamic analysis results demonstrate the expected drift capacity and self-centering behavior of the system. Modal decomposition of these analysis results shows that the first mode response is effectively limited by the rocking behavior; however, the higher modes are also excited by the rocking response. Based on these analysis results an improved design method is presented that accounts for the higher mode contributions to the member forces of the SC-CBF.