عنوان مقاله [English]
Since the lateral load pattern along the height proposed in seismic codes such as standard 2800-4th has been developed on the basis of elastic behavior without paying attention to the type of earthquake, it seems the story lateral force under near and far fault motions during inelastic behavior differ from the code load pattern. This difference affects the distribution of strength and stiffness. In the previous studies, less attention has been paid to the effect of a near fault earthquake on the story load pattern, although pulse type vibrations with long period, large amplitude and short duration are important key parameters and may affect the load pattern. Therefore, in this study the exact load pattern has been assessed for four steel moment frames with 4, 8, 12 and 18 stories under two sets of earthquakes (20 near and 20 far fault motions) and two earthquake hazard level (DBE and MCE). Moreover, the 8-story model has been redesigned with the exact load patterns. Based on this study assumption, results show that exact load pattern differs from what has been proposed by standard 2800. This difference depends on earthquake type, hazard level and the height of the structure. In lower and upper stories, exact load pattern is larger than the code value due to the higher mode and P-delta effects. However, for the middle part, the code load pattern is conservative. For the far fault motions higher mode effects and for the near fault earthquakes, the P-delta effects are the main source of aforementioned differences. Furthermore, the result of redesigned 8-story models shows that depending on the earthquake type and hazard level, using exact load pattern can change parameters such as stress ratio, element weights and inelastic drifts at the lower and upper parts in comparison with the code based load pattern.
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