Seismic fragility assessment for mixed concrete/steel buildings considering the appropriate position of the transition story

Abstract

A building that contains two different lateral resisting systems in its height exampling a Reinforced Concrete (RC) moment frame for the lower and a steel moment frame for the upper floors, respectively is called a mixed concrete/steel building. At the intersection of the two parts of RC and steel, there is a floor called a transition story made of both concrete and steel materials. Although such buildings are being constructed in different parts of the world, few efforts have been made to study their seismic behavior. One of the major challenges in these buildings is to find the right location for the transition story. In pursuit of this, 3 groups of buildings with 7, 13, and 19 floors each comprising 5 different models, were analyzed using Incremental Dynamic Analysis (IDA). Fragility curves were generated in four damage states and their Median Collapse Capacities (MCCs) were compared. Once the distribution of Maximum Ductility Demand (MDD) along the elements was determined for the different models, their plastic behavior in the nonlinear domain was studied. The results showed that the plastic behavior of the mixed concrete/steel buildings in stories below the transition level was the same as that of the RC models. Moreover, the value of MDD suddenly increased sharply at the transition level. Among the mixed concrete/steel models, the best position of the transition story was found to be one-third of the upper building height, with the lowest Probability of Exceeding (PoE) under different damage states.