Comparison between RCS and Concrete Moment Frames under Near-Fault Earthquakes

INTRODUCTIONThe composite RCS moment resisting frame, including concrete columns and steel beams, has some advantages in comparison with the ordinary steel and concrete moment resisting frames. Previous studies have shown that these systems, in case of preserving sufficient strength and needed ductility in seismic condition, could be effective in design and construction. Notwithstanding the previous research, during the 1970s and 1980s, the use of this composite structural system was limited to areas with low seismic hazard in the US, and it was used as a replacement for steel moment resisting frames and high rise buildings. Besides, in Japan, it was used instead of concrete frames in low rise and short span buildings. In the previous research on these structures, studies were made on the frame and RCS connections, but there has not been any study on the seismic assessment of the RCS structure with time history and for the near-fault earthquakes. Investigating the structure behavior under near-fault earthquake, due to the special nature and characteristics of these earthquakes in comparison to the far-fault earthquakes, seems to be essential. In this research, the seismic demand of composite RCS and concrete structures under the near-fault earthquakes are investigated in comparison with the far-fault earthquakes. For this purpose, five composite RCS intermediate moment resisting frames with 4, 7, 10, 15 and 20 stories and five spans were designed, and then, nonlinear dynamic analysis was performed on the structures using the OpenSees software under 10 far-fault and 10 near-fault accelerographs. The obtained results in this research showed that the stories displacement demand due to the far and near-fault earthquakes in composite structures is lower with respect to the stories displacement of concrete structure due to the same record, and by increasing the number of stories, the values of this difference increase. The effectiveness of near-fault records on the composite structures is greater than the ordinary concrete structures. It seems that the assessment of high rise composite structures with respect to high rise concrete structures yields a better displacement response. Finally, the effect of steel beam in reducing displacement due to both of the records is observed.RESEARCH METHODIn this research, the seismic demand of RCS and RC moment frames under the near-fault earthquakes are investigated with respect to the far-fault earthquakes. For this purpose, five RCS and RC intermediate moment resisting frames with 4, 7, 10, 15 and 20 stories and five spans were designed and then nonlinear dynamic analysis was performed on the structures using the OpenSees software. Then, 10 far-fault and 10 near-fault accelerographs were used respectively. All used accelerograms that have been received from the site of Peer, had a view to soil type of III on the basis of regulations of seismic design code of Iran (2800) or dirt Class of D based on the classification guidelines of FEMA. To draw the wholereactionaryresponse,thesoftwareof SeismoSignalwas used andall accelerogramsbefore scaling had theirequalmaximumwithacceleration(PGA).Fornonlineardynamic analysis on intended frames, OpenSees software was used and the results of story displacement, drift angle and story shear were provided in full paper. Selected records in this study were applied to the models and finally the decision has been made among the obtained responses. For scaling, accelerograms used method for scaling of the Fourth Edition of 2800 guideline. Finally, the effect of steel beam in reducing displacement due to both of the records is observed.CONCLUSIONDisplacementand drift angle of RC structures under near-fault records was more than far-fault records. However, with increasing the number of stories, drift angle of RC structures under far-fault records was more than near-fault recordsBy an increase in the number of stories, the RCS frames' drift angle due to near records is less than RC frames. The drift angle in RCS tall buildings under near-fault records and in RC tall buildings under far-fault records are more critical.Base shear of RCS structures under near-fault records was more than the base shear under far-fault records and by an increase in number of stories, the difference was reduced.In RCS frames, by an increasein the number of stories, steel beams cause a decrease in effectiveness of drift angle and displacement under near-fault records.