Seismic Stability of Special Concentrically Braced Frames in a Moderate Seismic Region

The special concentrically braced frame (SCBF) system is common in high seismic regions due to its ductile behavior that is enabled by capacity design and detailing requirements. However, SCBFs are used much less frequently in moderate seismic regions, and low-ductility braced frames are more prevalent. To provide a broader perspective on braced frame design and behavior in moderate seismic regions, SCBFs are investigated and compared to the low-ductility systems more typically used in moderate regions, namely the R = 3 CBF and the ordinary concentrically braced frame (OCBF). Nonlinear static and dynamic analyses are used to assess system behavior considering variations in system ductility, frame height, and brace configuration. Global static response is characterized using base shear vs. roof drift response to compare strength, drift capacity and elastic and post-elastic stiffnesses. All CBFs reached comparable levels of base shear in the elastic regions but differed in their post-elastic responses, where the low-ductility CBFs relied on system reserve capacity, and the SCBFs exhibited considerable ductility. The post-elastic stiffness ratios of the R = 3 CBFs and SCBFs were comparable for low-rise frames, whereas taller low-ductility CBFs developed lower post-elastic stiffness ratios than the SCBFs. A FEMA P695 collapse performance assessment is conducted for all CBF model variations. The results are evaluated with two measures: (1) the adjusted collapse margin ratio (ACMR) compared to the required ACMR for 10% probability of collapse and (2) a CBF system weight-normalized performance index (PI). The SCBFs consistently demonstrated lower probabilities of collapse compared to the low-ductility CBFs. Furthermore, the chevron configurations consistently exhibited lower probabilities of collapse than their split-x counterparts for all frames studied. For the SCBFs, the split-x brace configurations had higher PIs than the chevron brace configuration, whereas the low-ductility frames with chevron configuration outperformed the split-x configurations in most cases. For the low-rise frames, the SCBFs had significantly higher PIs than the low-ductility frames, whereas for the taller frames, the PIs were in a narrower range.

View content