ارزيابي لرزه يي ساختمان هاي جداسازي‌شده با استفاده از تحليل هاي بارافزون

Seismic Evaluation of Base-Isolated Structures using Pushover Analyses

در اين مطالعه دقت روش هاي تحليل بارافزون در ارزيابي لرزه‌يي ساختمان هاي جداسازي‌شده‌ي پايه مورد بررسي قرار گرفته است. بدين منظور 2 سازه‌ي قاب خمشي فولادي متوسط با ارتفاع كوتاه و متوسط (3 و 9 طبقه) در نظر گرفته شده و با استفاده از جداسازها‌ي لاستيكي با هسته‌ي سربي (LRB) جداسازي شده‌اند. براي هر سازه، 3 نوع جداساز با سختي‌هاي مختلف، شامل جداسازها‌ي سخت (H)، جداسازهاي متوسط (N) و جداسازهايي نرم (S) در نظر گرفته شده است. تحليل‌هاي تاريخچه‌ي زماني غيرخطي با استفاده از ركوردهاي حوزه‌ي دور با شدت هاي مختلف و همچنين تحليل‌هاي بارافزون با به‌كارگيري 4 توزيع بار مختلف براي ساختمان هاي جداسازي‌شده اجرا شده‌اند. همچنين به منظور در نظرگرفتن اثر مودهاي ارتعاشي بالاتر در سازه‌ي جداسازي‌شده با ارتفاع متوسط، روش تحليل بارافزون مودال (MPA) اعمال شده است. نتايج تحليل ها و همچنين دقت تحليل هاي بارافزون در محاسبه‌ي پاسخ هاي لرزه‌يي مختلف به تفصيل در اين نوشتار بررسي شده است.

Base isolation is one of the modern strategies for reduction of the seismic responses of structures, which reduces the seismic forces rather than increasing the resistance capacity of structures. In recent decades, the efficiency of this system has been confirmed in many research investigations and it has been used in real projects. Nonlinear time history analysis (NTHA) is known as a robust and accurate approach for seismic evaluation of structures, but because of its complexity, the nonlinear static procedure, (NSP) based on pushover analysis (POA), is being increasingly used instead of NTHA. In recent investigations, POA has been mostly used for the seismic evaluation of fixed-base structures and little attention has been paid to the applicability of POA for base-isolated building structures. Therefore, in this study, we attempt to assess the accuracy of pushover analysis methods for base-isolated buildings. For this purpose, two steel moment frame structures, including low- and medium-rise (3 and 9-story) frames, were considered. The structures were isolated with different lead rubber bearing (LRB) isolation systems. Three types of isolator with different stiffness, including hard (H), normal (N) and soft (S) isolators, were taken into consideration. Totally, six isolated structures were obtained. For base-isolated structures, nonlinear time history analyses and pushover analyses were performed. For nonlinear time history analyses, three different seismic intensities were considered, i.e. the records were scaled to 0.4g, 0.6g and 0.8g. For pushover analyses, four different lateral load distributions were used. Also, to account for the effect of higher modes in the case of a medium-rise isolated building, modal pushover analysis (MPA) was implemented. The results show that for the three-story base-isolated structure, in which the superstructure remains elastic, a triangular load distribution gives better estimation of story drifts. It is illustrated that all load distributions do not provide sufficient accuracy in the case of medium-rise base-isolated buildings. It is shown that the MPA method can provide accurate predictions of seismic demands for base-isolated building frames with hard isolators, but is unable to accurately estimate the seismic responses in the case of buildings with normal and soft isolators.