Preliminary performance assessment of special concentrically-braced frames for moderate seismic regions

Steel concentrically-braced frames (CBFs) are widely used lateral force resisting systems with high strength, stiffness, and material efficiency. Special concentrically-braced frame (SCBF) systems are a type of CBF commonly used in high seismic regions because they are designed to permit large inelastic drifts. Ductile detailing and capacity design requirements ensure braces are the main source of energy dissipation in the system. While the use of SCBFs in high seismic regions is prevalent, they are not frequently designed when seismic demands are lower. Low-ductility steel CBFs, the typical system used in low to moderate seismic regions, do not have the same detailing or proportioning requirements as SCBFs, which permits lighter system weights at the expense of ductile frame behavior. Structural behavior of low-ductility CBFs relies on reserve capacity, or secondary stiffness and strength, after initial brittle limit states. This study considers SCBFs designed for moderate seismic regions as a direct comparison to recent assessments of frame behavior and economy of widely used low-ductility CBFs, namely the R=3 CBF and the ordinary concentrically-braced frame (OCBF). Numerical models for a suite of three story braced frames are analyzed to characterize system behaviors in regions with moderate seismic demands. Pushover analyses are conducted on the models to study the benefit of inherent system ductility on collapse mitigation, to consider the effect of frame type on system economy, and to evaluate the efficacy of low-ductility CBFs widely used in practice.