Reza DADASHI - Ph.D. Student of Structural Engineering, University of Zanjan, Zanjan, Iran
Kiarash NASSERASADI - Assistant Professor of Structural and Earthquake Engineering, University of Zanjan, Zanjan, Iran

Recently, fragility functions are used to study the vulnerability of structures. Currently, the analytical fragility functions are developed by two-dimensional analysis of structures, and the effect of three earthquake component and three-dimensional analysis is neglected. However, some studies have been conducted to develop fragility function of the structure by 3D analysis. Presumably, three-dimensional analysis of structures was required for the asymmetric and irregular structures, and symmetric structures are exempt from that. However, even in a perfectly symmetric structure, the difference between the earthquake components may cause differences between each direction. Xu et al. compared the fragility function of asymmetric structure which developed by 2D and 3D analysis and showed that the using 2D analysis underestimated the results. In this study, to investigate the effect of 3D analysis on the fragility function of structures, three perfectly symmetrical reinforced concrete structures with moment resisting frame system were investigated in the far-field of the earthquake. The result is then compared with the corresponding fragility curves developed by the twodimensional analysis. Three symmetrical buildings with 3, 6, and 9 stories are designed according to the sixth and ninth volume of the Iranian national building regulations and the fourth edition of Iranian seismic code (see Figure 1). The response of structures in 2D and 3D analysis was evaluated by OPENSEES under 17 pair of far-field time histories which recorded on soil type III. For the 3D model, the two-directional incremental dynamic analysis was performed, and their median IDA is compared with traditional IDA in Figure 2 as a function of spectral acceleration at the period of first structural mode. As it can be seen in this figure, the median of IDA of 3D structure capture higher drifts. Moreover, the maximum drift of taller structures gets higher.

Fragility curve, Regular RC structure, 3D structures, Multi-direction incremental dynamic analysis,Fourth edition of Iranian seismic code