Assessment of Seismic Behavior of Steel Moment Frames under earthquake Sequences Using Fragility Curves

Research Aim:The purpose of this study is to investigate the seismic performance of special and intermediate moment resisting frames subjected to single and multiple earthquake events. The effect of cyclic deterioration of beam and coloumn plastic hinges, height of structure and type of its design on the seismic response of structure is studied. Research method:Four benchmark perimeter steel moment resisting frames are used for comparative studies. The analyzed models are classified based on their type (intermediate or special) and number of stories (4 and 8-story). The overall width of each frame is 24 meters divided into four bays of equal width. The vertical distance between floors is assumed to be 4.5 meters for the first story and 3.5 meters for other stories. The ST52 steel material are used in design and modeling of structures. The reference models are loaded as per AISC 7-10 for SDC Dmax and designed as intermediate or special moment frames according to provisions of AISC 360-10 and AISC 341-10. Numerical modeling and analysis of the specimens is conducted using open system for earthquake engineering simulation (OpenSees) software. The beams and columns in the frames are modeled using elastic beam-column elements connected by plastic hinge rotational springs (zero-length elements) at the ends. The modified Ibarra-Krawinkler deterioration model is used to model the cyclic hysteretic behavior of these rotational springs. The column panel zones are modeled using the method developed by Krawinkler, which considers the panel zone as an assembly of 8 rigid elements forming a parallelogram. The hysteretic behavior of the panel zone is simulated by a trilinear rotational spring, located in one of the corners of the parallelogram. The columns are modeled with fixed base. The P-delta effects caused by the gravity loads of the interior frames are considered using fictitious bays consisted of pin-ended rigid beam-column elements with co rotational coordinate transformation.Incremental dynamic analysis is conducted for each model to evaluate their seismic collapse capacity under single and multiple record earthquake events. Findings: Neglecting pre-shock and after-shock in the design process will decrease the level of earthquake tolerated by structure. Special moment resisting frames will experience slightly greater decrease of capacity compared to intermediate counterparts. However even after this decrease the special frames demonstrate greater capacity compared to IMFs. Conclusion:Based on the results of nonlinear static (pushover) analysis it can be said that for both 4 & 8 story models, the intermediate frame has greater stiffness and strength compared to the special one. However, the special frames demonstrate greater deformation capacity before losing 20% of their ultimate strength. However, incremental dynamic analysis using single record earthquake events showed that both 4 & 8 story special frames have slightly higher strength and displacement capacity compared with the corresponding IMFs. Incremental dynamic analysis using multi record earthquake events showed that neglecting pre-shock and after-shock in the design process will decrease the level of earthquake tolerated by structure. Even after this decrease the SMFs have higher strength compared to IMFs, but the amount of decrease for SMFs is greater. This can be mainly due to the fact that the intermediate frames rely more on their load carrying capacity than their ductility (compared to special frames). Therefore the sensitivity of these models to ground motion intensity is higher than special frames. On the other hand, as the special frames has to rely on their ductility and hysteric energy dissipation capacity to avoid collapse, they are more sensitive to duration of ground motion.