Seismic collapse safety assessment of suspended zipper-braced frames

Abstract

Seismic evaluation of suspended zipper-braced frames, which are an alternative to inverted-V-braced frames to improve their seismic behavior, is of greatest significance to determine the level of confidence in this type of seismic system during severe earthquakes. The arrangement and design parameters of these frames are mentioned in some references, but there is no probabilistic assessment of collapse risk based on various collapse uncertainties. To evaluate the probability of collapse and margin of safety, eighteen suspended zipper-braced frames with different geometry parameters in the most severe seismic design category (D max) have been designed. The designed frames were modeled in OpenSees software by considering the effect of gusset plate connections and evaluated by performing more than 15,800 dynamic and nonlinear static pushover analyses using FEMA P695 methodology. Total collapse uncertainty is considered in the evaluation of the probabilistic behavior of frames. The results show that the adjusted collapse margin ratio (ACMR) of designed frames by considering the total collapse uncertainty of 0.726 and 0.529 is 27% and 64% higher than the acceptance criteria, respectively. The results also indicate that a response modification coefficient of much more than 6 can be used for the economic design of long-period suspended zipper-braced frames.