Seismic Performance Evaluation Of Zipper Braced Steel Frames Under Mainshock-Aftershock Sequences

One or more aftershocks often occur after an earthquake.The damage of building structure will be increased after aftershocks,and even cause it to collapse.The zipper braced frames(ZBF)have been widely studied and applied because of more uniform stress,smaller weak story number,smaller steel consumption and better economy under earthquake action.At present,the research shows that the ZBF has excellent seismic performance under single earthquake,but the seismic performance under mainshock-aftershock sequences is still unclear,and especially the studies on the seismic fragility,collapse resistance performance and seismic resilience of the ZBF is few.Based on different engineering demand parameter indexes,the seismic performance of the ZBF under mainshock-aftershock sequences was studied.The concrete work and main conclusions are as follows:(1)The incremental dynamic analysis(IDA)of the ZBF under real and artificial(repeated synthesis method and random combination method)mainshock-aftershock sequences were carried out.The results show that,the structural response under artificial mainshock-aftershock sequences is larger than that under the real mainshock-aftershock sequences,and the additional damage caused by the aftershock is more significant.(2)Based on the different engineering demand parameter indexes including the peak drift,the residual drift and the modified Park-Ang damage index,the applicability of different indexes of the ZBF under real and artificial(repeated synthesis method and random combination method)mainshock-aftershock sequences were studied.(3)Using the peak drift as engineering demand parameter index,the IDA and seismic fragility analysis under mainshocks and mainshock-aftershock sequences(repeated synthesis method)were performed for the ZBF and the chevron braced frames(CBF).The results show that,the aftershocks increase the seismic fragility of the structures,and their effect becomes more pronounced with increasing seismic intensity.Compared to the CBF,the 5-storey,10-storey and 15-storey ZBF models have lower failure probability for extensive damage and collapse limit states under high-intensity earthquakes(mainshocks and mainshock-aftershock sequences),better deformation coordination ability,and superior collapse resistance performance.However,the failure probability for each limit state of the 20-storey ZBF models under mainshocks is lower than that of the CBF,and it is higher under mainshock-aftershock sequences,explaining the aftershocks have more effect on the ZBF.(4)By using FEMA P-58 methodology,the seismic losses of the ZBF under mainshock and mainshock-aftershock sequences(real method and repeated synthesis method)were compared.The results show that,when subjected to rare earthquakes,the seismic losses including repair cost,repair time and casualties under mainshock-aftershock sequences,are greater than that under mainshock.(5)Based on the Standard for Seismic Resilience Assessment of Buildings and the results of seismic losses,the seismic resilience of the ZBF under mainshock and mainshock-aftershock sequences(real method and repeated synthesis method)was assessed and compared.The results show that the aftershocks have little effect on seismic resilience of the ZBF in terms of the star-class assessment of seismic resilience.