MDOF Effects on Steel Moment Resisting Structures Under Near-fault Earthquakes with Forward Directivity Effect

INTRODUCTIONIn recent years, various studies have been performed on the nonlinear responses of steel moment resisting frames under the near-fault earthquakes. Due to the event of near-fault earthquake, a significant amount of energy is exerted upon the structure, in a very short time. For this reason, the nonlinear distribution of demands differ with respect to those of the far-fault earthquakes. Investigating previous damages due to near-fault earthquakes indicated that significant inter-story drift demands are formed within the structure, which endanger its safety and stability. Near-field earthquakes containing forward directivity effects, due to the pulse in the velocity record, cause significant demands on the steel frames with respect to the ordinary earthquakes. Therefore, investigating the steel frames behavior as well as the higher modes effects under near-fault earthquakes is essential. For this purpose, five intermediate (ductility) steel moment resisting frames with 4, 7, 10, 15 and 20 stories under 20 far and near-fault accelerogram have been investigated. Finally, by examining the elastic responses of the single degree of freedom structure (SDOF) under considered accelerograms, the coefficients for transforming response of the SDOF structure to that of the MDOF structure are presented. The results of this research show that higher modes effects under the far-fault earthquakes are greater in comparison to those of the near-fault earthquakes. Besides, for about 30%-50% of the height of upper stories of the structures, the drift angle resulting from the near-fault earthquakes with the forward directivity effect is greater than that of far-fault earthquakes.RESEARCH METHODValidation of analytical models is one of the most important steps of a study. In numerical studies, especially when a considerable database should be prepared for the extraction of the empirical expressions, lack of certainty concerning the validity of that created model could lead to inaccurate results. To avoid this issue in this article, all models are validated. In order to investigate the higher modes effects, 4, 7, 10, 15 and 20 stories 5-span 2D frames selected. Each model has 4 m story height and 5m span length. The frames are intermediate (ductility) moment resisting frames. The structures being investigated in this research are designed completely based on the ANSI/AISC 341-05 and ASCE/SEI7-05 codes for gravity and seismic loads. Both the equivalent static lateral force and the modal response spectrum analysis were used for the models. ST37-type steel is used in design of the structures with the yield stress of 2400KgCm2and the ultimate stress of 3600KgCm2and the Poisson's ratio is 0.30. The lateral drift values in all the structures are compared with the allowable value in the ASCE/SEI7-05 code. All elements have been chosen as compact sections (limiting local buckling) assuming enough lateral supports.In this study, 10 far-fault accelerograms and 10 near-fault acceleration time history with forward directivity effect have been chosen to be used in the nonlinear time history analysis. The near-fault earthquakes have effects of forward directivityandloweffective duration as wellashighvelocity pulse period, chosenfrom thestations located less than 15 km from the fault. All chosen accelerograms in this research have the moment magnitude greater than 6.5 and the soil properties are of the Class D soil type based on the Fema 356 classification guidelines and are taken from the PEER website. The elastic response spectrum created by Seismosignal software. Besides, all acceleration time history has been normalized to their peak ground acceleration (PGA) before being scaled. All used accelerograms in this research are scaled according to the method presented in the Iranian Seismic Code (Standard 2800) and used in the NTHA method. Nonlinear time history analysis is also conducted by OpenSEES.CONCLUSIONThe higher modes effects under far-fault earthquakes are greater than those of the near-fault earthquakes with the forward directivity effect.By an increase in the structure height (period), the difference in seismic demands values of structures under the far and near-fault earthquakes decreases.Investigating the drift angle over the height of various structures shows that for about 30%-50% of the height of structure, at the upper stories, the response obtained from the near-fault earthquakeswith forward directivity effect is greater than the response obtained from the far-fault earthquakes.The buildings’ lateral load-resisting system is steel special moment-resisting frame. All buildings are 15 m in width.