Document Type : Research Note
Author
Dept. of Civil Engineering Urumia University of Technology
Abstract
Nowadays, in most earthquake resistant design codes, structures are permitted to experience significant inelastic deformations under severe earthquake. Structural experts believe that if structures are designed properly, under severe earthquake, they can deflect in inelastic limits, and, as a result, can dissipate most of the earthquake energy.A general way to evaluate the inelastic behavior of structures is using nonlinear dynamic analyses. These kinds of analysis are usually time consuming and uneconomic and need a high level of knowledge to be undertaken. Thus, it is not always possible for practicing engineers to perform these sorts of analyses. In order to solve the problem, earthquake resistant design codes permit use of a reduction factor (R). This factor, named the response modification factor, reduces the earthquake design force so that the response of the structure is assumed elastic.In this study, the ductility reduction factor of ordinary and special concentrically braced frames, using a combination of V and inverted V bracing systems, is investigated. According to the results, the maximum height of ordinary frames, which are braced using a combination of V and inverted V bracing systems, can increase up to 15 meters. This value is larger than that proposed by ASCE7 (10.7 m). Also, results indicate that using ordinary frames, which are braced by a combination of special inverted V and V braced systems, can have a saving of about 0 to 29 percent using materials for frames from 1 to 16 stories, in comparison with ordinary frames. According to the results of this study, the response modification factor proposed by the Iranian seismic design code (2800 standard fourth edition), (R=5.5), is more logical than the one proposed by ASCE7 (R=6). Unfortunately, for frames braced by a combination of special inverted V and V braced systems, when the stories of the frames increase up to 10, the expected ductility demand cannot be achieved. So, as a result, for frames with more than 10 stories, the lower response modification factor should be used. In addition, frames taller than 10-stories do not experience specified target displacement and collapse before reaching the preferred mechanism. This phenomenon shows the necessity of using a different response modification factor for frames taller than 10 stories.
Keywords
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