Document Type : Article
Authors
1
Dept. of Civil Engineering Isfahan University of Technology
2
Technical Office, Governor-General of Esfahan
Abstract
Iran, as a vast country, has frequently witnessed destruction of many of its cities due to earthquake. Many times the occurrence of a short-duration ground motion has affected a society, economically, socially, politically, and
culturally, and essentially changed the destiny of generations. The near-field earthquakes of a Tabas, Bam and Northridge, which displayed destructive effects, differed from other seismic events, and acted as an incentive for many researchers to focus on such kinds of earthquakes. Due to the proximity of active faults to populous regions, the possibility of occurrence of near-field earthquakes is high in Iran, justifying special consideration of this type of earthquake in the design or retrofit of structures. In this research, to assess the safety provided through a code-based design for moment resisting steel structures, first, structures 4, 6, 8, and 10 stories tall are designed for Standard 2800 and Chapter 10 of National Building Regulations. Second, the same buildings are nonlinearly analyzed and evaluated under Tabas, Bam and Northridge earthquakes, for hazard levels with 475 and 2475 return periods. The method of Spectral Balancing is used to scale the time histories of the ground
motion for the two levels of seismic risk. In this method, the response spectrum is displaced vertically to a position where the area between the design and 1.4 times the response spectra are the same between periods 0.2T and 1.5T, with T being the natural period of the building under study. Tables of beam and column responses are provided, in which the number of plastic hinges in each member and the number of hinges with rotations over the code limit are shown. As a result, the buildings investigated are evaluated as weak under
these earthquakes, meaning that damage is beyond allowable limits and that even collapse is observed in certain cases. To modify the behavior and enhance seismic safety, it is proposed to decrease the allowable story drift for
structures subjected to near-field earthquakes. An equation for calculating the modified drift limit is proposed based on regression analysis. The suggested equation gives the value of a reduction factor, as a function of number of stories, to be multiplied to the code-based drift limit. Evaluating the structures designed, according to
the reduced drifts, shows a satisfactory seismic behavior, to the extent that rotations of plastic hinges remain below acceptable limits for each performance level. In addition, where nonlinear behavior may not occur, the section forces do not exceed the member capacities.
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