Non-linear study of the method of transition in mixed concrete/steel structures

A critical zone is formed at the junction of the lower concrete and upper steel sections in mixed concrete/steel structures in terms of gravity and lateral loads caused by the support reactions of the steel frame. There are two common approaches to transition from concrete section to steel one in this critical zone: the first is the direct pin connection of the steel frame to the concrete frame, and the second is the application of a concrete-steel composite story called the transition story. Until now, various studies have been conducted on mixed concrete/steel structures, but no study has investigated and compared the two mentioned approaches. Considering the importance of this issue, 18 models of mixed concrete/steel structures were formed in the present study according to the two approaches and the different places of connection of the concrete and steel sections along the height of the structure. Fragility curves were generated for all models using Incremental Dynamic Analysis (IDA) under 44 far-field ground motion records and probability function at the complete damage state based on HAZUS technical report. A comparison was made from the strength point of view considering the two approaches to transition. Next, to investigate the non-linear response of the structure at the complete damage state, the Peak Inter-Story Drift ratio (PISD) was obtained in the different levels of collapse intensity from IDA for different stories. The results showed that the application of transition story improves the seismic behavior of mixed concrete/steel structures and increases Median Collapse Capacity (MCC) in 7, 13, and 19-story structures up to 24.63%, 19.54%, and 17.99%, respectively. In addition, it makes the displacement of the structure more uniform and reduces the significant difference of drift in the critical zone.