ارزيابي پتانسيل خرابي پيش‌رونده در ساختمان‌هاي فولادي طرح‌شده براساس آيين‌نامه‌ي ايران

ASSESSMENT OF PROGRESSIVE COLLAPSE POTENTIAL OF STEEL STRUCTURES DESIGNED BASED ON IRANIAN CODE

خرابي پيش‌رونده، عبارت از گسترش خرابي در سازه در اثر يك خرابي موضعي اوليه است، و اولين بار پس از واقعه‌ي ساختمان رونان پوينت در انگليس مطرح شد، و از آن زمان تاكنون تحقيقات وسيعي در اين خصوص انجام شده است و كشورهاي مختلفي چون انگليس، كانادا، كشورهاي اروپايي، و امريكا به آن پرداخته‌اند. بارهاي غيرعادي، تغييرشكل‌هاي بزرگ ناشي از حرارت و ... از عوامل خرابي موضعي‌اند. با توجه به مسكوت‌ماندن اين بحث در ايران، در اين مطالعه 46 مدل سازه‌ي فولادي منطبق بر آيين‌نامه‌هاي ايران طرح، و پتانسيل خرابي پيش‌رونده در آنها بررسي شد و نتايج نشان از وجود پتانسيل خرابي در اين سازه‌ها و لزوم واردشدن اين بحث در آيين‌نامه‌هاي ايران را دارد.

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 the Ronan Point apartments in 1968 initiated this subject in Britain, and the events of 9/11/2001 have instigated a comprehensive debate among 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 for 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, which, therefore, 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 this, there are still many issues regarding the progressive collapse event which are unknown. Iranian codes have not yet considered this subject. Because of this, a study was undertaken on 46 models of structures with differences in height and structural systems, which were designed according to Iranian codes, and which were then assessed for potential of progressive collapse due to the removal of columns at different positions on the first floor, based on the direct method. Finally, it was found that these structures have a potential for progressive collapse. This study also shows that many features are effective in progressive events, like the number of stories, position of removed elements, structural system, and indetermination due to extra elements. It is, finally, concluded that progressive collapse should be taken into consideration in the Iranian code.