Evaluation of the fragility curve for steel structures with linked columns frame system under sequences of critical earthquakes

Fragility curves as a powerful tool assess the possible damage to existing structures and estimate the probability of structural damage exceeding a given level of seismic damage for several levels of seismic ground motions. This paper tries to evaluate the seismic vulnerability of steel structures containing relatively new Linked Column Frame (LCF) system—which are not considered by most of the seismic design codes—using the fragility curves under the critical multiple earthquakes. Therefore, 10 LCF steel frames with shear/flexural behavior and length of 2 m have been designed and implemented in OpenSees software. After selecting consecutive critical earthquakes, the fragility curves have been calculated for single and consecutive scenarios. For more comprehensively evaluation of the mentioned structures, the effect of various parameters including the number of stories and behavior of the linked beams has been investigated. Despite what is often assumed by the seismic design codes about singularity of the design earthquake, the results indicate an increase in the probability of collapse under successive shocks compared to a single earthquake. This increase is more obvious in the performance level corresponding to the collapse threshold. So that the collapse prevention of 7-story frame with flexural linked beam exposed to consecutive earthquakes is 17% higher than the single case. Performance comparison of the steel structures with shear/flexure link beams at height showed that the failure exceedance probability has decreased with the increasing the number of stories especially in steel frames with shear link beams. Moreover, structures with shear link beams have higher failure exceedance probability than flexure performance.