محاسبه‌ي بار ديناميكي ناشي از برخورد آوار در حالت حدي فروريزش ساختمان‌هاي فولادي

Calculation of Dynamic Debris Impact Load on Steel Buildings in Collapse Limit State

ماهيت پيچيده‌ي بارهاي غيرمترقبه و اثرات اوليه‌ي خرابي و ثانويه‌ي ضربه‌ي ناشي از ريزش آوار، تحليل سازه‌ها را در برابر اين نوع بارها با مشكل مواجه مي‌كند. در اين نوشتار، روش‌هاي محاسبه‌ي بار ناشي از آوار و چگونگي بررسي مسيله‌ي برخورد آوار با اعضاي سازه‌يي مورد مقايسه و بررسي قرار گرفته و نتايج آنها با تجربيات موجود از آزمايش‌هاي اعضا فولادي مقايسه شده است. با تهيه‌ي مدل عددي غيرخطي و انجام مطالعه‌ي پارامتري نمودارهاي طراحي پيشنهادي جهت تخمين ميزان بار ضربه روي قاب‌هاي خمشي فولادي در شرايط مختلف ارايه شده است. همچنين تحليل حساسيت روي شاخص‌هاي مهم آوار انجام و نمودارهاي موردنظر براساس آنها تنظيم شده است. نتايج اين بررسي‌ها نشان مي‌دهند كه برخورد آوار در بسياري از موارد مي‌تواند بارهاي قابل ملاحظه‌يي بسيار بيشتر از پيش‌بيني آيين‌نامه‌ها ايجاد كند و عواقب خرابي زيادي داشته باشد.

An important task in the design of buildings against progressive collapse is to account for the dynamic impact effect of falling debris loads caused by extreme abnormal loading conditions. Extreme loads such as explosions may generate primary debris like bombs or vehicle fragments or secondary debris consisting of blast-borne debris or debris from failed structures near the source. It is necessary to consider this debris in the structural design to reduce hazards to the occupants and prevent the loss of key structural components. Casualty mitigation and fatality prevention is achieved by preventing collapse and by limiting debris caused by the local failure of structural and non-structural elements. Debris reduction is achieved by proper detailing and the provision of ductility in the structure. Generally, since inelastic behavior is permitted in elements subjected to these loads, performance is deformation controlled; i.e., some allowable rotation or ductility (based on minimizing debris) is specified for each structural member type for blast loads. For progressive collapse, deformation limits are specified to ensure some residual capacity exists. Presently, recently published deformation criteria or “response limits” for blast loads are restricted, as contained in the UFC manual. Progressive collapse criteria are presently unrestricted and contained in the GSA Progressive Collapse Guidelines and in UFC. This paper employs elastic dynamic analysis and contact mechanics to determine dynamic impact load factors for a variety of debris loading scenarios possible for beam members in buildings. Parametric studies are performed to investigate the effect of different parameters on dynamic load factor values, and some graphs are proposed as design aids for estimating such factors for engineering applications. The effect of nonlinear inelastic material behavior on impact factor values is discussed through numerical examples. The results show that in some cases, the realistic loads imposed on the structure due to falling debris are much more that the load values proposed by the codes to prevent progressive collapse in building structures.