تاثير خروج از مركزيت جرمي در فروريزش پيش‌رونده در ساختمان‌هاي 3 طبقه‌ي بتن مسلح تحت اثر بار زلزله

Investigating the Effect of Mass Eccentricity on Progressive Collapse in 3 Story Reinforced Concrete Buildings under Earthquake Loads

يكي از مباحث مهم در سيستم‌هاي سازه‌يي، بررسي حاشيه‌ي ايمني در برابر خرابي پيش‌رونده است. در ساختمان‌هاي پيچشي تخريب به صورت موضعي است و اين امر سبب تشديد تخريب پيش‌رونده مي‌شود. بدين منظور خرابي پيش‌رونده در ساختمان‌هاي 3 طبقه‌ي متقارن و نامتقارن بتن مسلح با قاب خمشي معمولي و خروج از مركزيت‌هاي جرمي مختلف 5، 15 و 25 درصد ناشي از بار زلزله مطالعه و مقايسه شده است. نتايج حاصل از تحليل‌هاي تاريخچه‌ي زماني نشان مي‌دهد كه با افزايش خروج از مركزيت جرمي در ساختمان، پتانسيل خرابي پيش‌رونده در لبه‌هاي نرم و سخت سازه‌ها افزايش مي‌يابد. بررسي دريفت‌ها نشان مي‌دهد كه دريفت‌هاي سازه در مركز جرم رفتاري مشابه با لبه‌ي سخت دارند. همچنين جهت پيش‌بيني خرابي پيش‌رونده مي‌توان به جاي بررسي دشوار تعداد مفاصل شكست‌يافته در ساختمان‌ها به بررسي جابجايي نسبي طبقات پرداخت.

One of the most important issues in earthquake and structural engineering is investigation of safety margins against the potential for progressive collapse. Here, the spread of collapse is investigated by tracking down the location and type of collapsed beam and column elements, from the first element to the entire symmetric/asymmetric building, via comparing the results obtained by nonlinear time history analyses. Damage concentrates locally in asymmetric buildings, which intensifies the progressive collapse mechanism. In this paper, we study 3 story reinforced concrete ordinary moment resisting frame buildings in 3 dimensions, with various levels of 0%, 5%, 15% and 25%, one directional mass eccentricity in the presence of earthquake load, with respect to the behavior of story drifts and the number of collapsed hinge criteria. Results show that mass asymmetrical distributions result in further local damage, which, consequently, leads to a larger progressive collapse in the torsional buildings. Therefore, when mass eccentricity increases, the potential of progressive collapse increases, in both stiff and flexible edges, too. By increasing mass eccentricity, the percentage of increments in the number of collapsed hinges in the entire building are similar to the percentage of increments in the story drifts of the mass centers. These percentages are less than those in the story drifts of the stiff and flexible edges. The behavior of the story drifts in the stiff edge is also similar to that of the mass centers. However, the value of story drifts in the mass centers is less than that in the stiff and flexible edges. Our results demonstrate that, as an alternative to a much difficult-to-calculate local response parameter of the “number of collapsed hinges”, the “story drift”, as a global response parameter, measures the potential of progressive collapse more easily and comfortably.