دانلود رایگان مقاله محاسبه عوامل افزایش بار برای ارزیابی پتانسیل فروپاشی پیشرونده در سازه های فلزی قاب شده

Abstract

Progressive collapse of building structures is a relatively rare event. However, the consequences of progressive collapse may be catastrophic in terms of injuries and loss of lives. In addition, in many parts of the world including the United States of America, Europe, Asia, and recently, United Arab Emirates, there is a trend to build taller and more structurally complicated buildings with adventurous load paths. Therefore, structural design that takes into account the potential for progressive collapse is becoming critical. This paper outlines and discusses the process of estimating the load increase factor (LIF) needed for progressive collapse resistant design of steel building structures that takes into account the effects of component ductility on structural response following the initiation of collapse. LIF are used to account for the dynamic effects of column/wall removal when the designer opts for linear or nonlinear static analysis to assess the potential for progressive collapse. The approach recognizes the difference in response associated with deformation-controlled compared to force-controlled response quantities and structural elements. Emphasis in this paper is on the Alternate Path (AP) approach which is the most commonly used approaches for progressive collapse resistant design of building structure that fall under Occupancy Category II.

Summary

 The current approach in progressive collapse resistant design benefited from lessons learned in earthquake resistant structural design. Structural demand and capacity are estimated taking into consideration ductility (or lack thereof) of components.  Linear and nonlinear static analysis procedures are permitted for a large class of structures. Dynamic effects associated with removal of columns are accounted for through magnified gravity loads. This magnification is applied only in areas tributary to the notionally removed column. The procedure for calculating the magnification factor involves the determination of a factor ‘‘m’’ known as component or element demand modifier, as demonstrated in this paper. The demand modifier originated in the earthquake design research and practice [3].  Structures with long spans are particularly vulnerable to progressive collapse, especially when corner columns are notionally removed. Stiffening the structural elements may not be practical either. In this case study, challenging spans were used in a typically loaded regular structure. It was shown that the DoD [1] load combinations impose significant demand on the structural system, which may not be able to withstand the loads on its own. Alternate structural solutions, such as outrigger systems [7] may be necessary to resist progressive collapse loads.  This paper demonstrated the procedure for calculating the magnified gravity loads in areas adjacent to notionally removed columns. It is clear that the number of analyses to capture the entire response is large as the procedure must be applied to several perimeter and interior columns. Furthermore, the procedure must be repeated for each floor for deformation-controlled actions as well as force-controlled actions. This procedure must be automated or otherwise simplified if progressive collapse design based on UFC 4-023-03 [1] is to be adopted by other building codes.