عنوان مقاله [English]
Progressive collapse is used to refer to the spread of an initial local failure within a structure this local failure is triggered by the loss of one or more load carrying members and leads to partial or total collapse of the structure
in a manner analogous to a chain chemical reaction. The partial collapse of Ronan Point apartment in 1968 initiated this subject in Britain, and events of 11 September 2001 have instigated a comprehensive debate among the structural engineering communities regarding the performance of tall buildings under extreme loading conditions such as blast, impact or fire. A wide range of explanations were given in different countries like England, Canada, USA and EU Meanwhile two specific codes were offered for progressive collapse in the USA.
These require that all new and existing buildings of three stories or more be designed to avoid progressive collapse. Abnormal loading, large deformation due to fire and etc are reasons of first local failure. The two general approaches currently employed for mitigating the risk of progressive collapse include: 1) the indirect design method, and 2) the direct design method. The former is a prescriptive approach of providing a minimum level of connectivity between various structural components, and therefore it can be readily implemented in structural design without the need for any additional analysis. Instead of calculations investigating the effects of extreme loads on the building, the designer employs an implicit design approach that incorporates measures typically related to strength, continuity and ductility to enhance the overall robustness of the structure. On the other hand, the direct methods rely heavily on structural analysis and, in fact, can significantly benefit from sophisticated analysis techniques, such as nonlinear and/or dynamic analysis, which are not commonly used in routine design practice. Despite all of them there are many problems around progressive collapse event which are unknown.
Furthermore, Iranian codes have not yet considered about this subject. Because of these, this study was done on 46 models of structures with differences in heights and the structural systems, which were designed according to Iranian codes, and then they were assessed for potential of progressive collapse due to removed column in different positions in the first floor based on the direct method. Finally it was found that these structures have potential of progressive collapse. This study also shows that many features are effective in
progressive events like the number of stories, position of the removed element, the structural system, indetermination due to extra elements, and finally it is concluded that the progressive collapse should be taken consideration in Iranian code.