ارزيابي رفتار لرزه‌يي سازه‌هاي فولادي با مهاربندي كمانش تاب به روش تحليل ديناميكي غيرخطي مقياس‌شده

Seismic Evaluation of Buckling Restrained Braced Frames by Utilization of the Scaled Nonlinear Dynamic Procedure

در اين پژوهش عملكرد لرزه‌يي، قاب‌هاي فولادي با مهاربندهاي كمانش تاب به روش تحليل ديناميكي غيرخطي مقياس‌شده كه به تازگي معرفي شده و همين‌طور تحليل استاتيكي غيرخطي مورد بررسي قرار گرفته است. براي اين‌ كار تعدادي قاب مهاربندي‌شده با پيكربندي‌هاي مهاربندي قطري، ضربدري دوبخشي، و شورون طراحي شده‌اند. مقادير پاسخ غيركشسان سازه‌ها شامل جابجايي، جابجايي نسبي طبقات و بام، و برش طبقات حاصل از تحليل‌ها با يكديگر مقايسه شده‌اند. در نهايت، اثر تعداد ركوردهاي زلزله در تحليل ديناميكي غيرخطي مقياس‌شده در پراكندگي نتايج مورد بررسي قرار گرفته است. نتايج نشان مي‌دهند كه روش تحليل استاتيكي غيرخطي در برخي مواقع مقادير پاسخ ساختمان را دست‌بالا تخمين مي‌زند. ضمناً افزايش تعداد ركوردهاي زلزله، تاثير چنداني در بهبود پراكندگي نتايج در روش ديناميكي مقياس‌شده ندارد.

Strong earthquakes during the last two decades and also post-earthquake field investigations have revealed that steel braced frames exhibit non-ductile behavior due to the buckling of braces during severe ground motion. Recently, modified braces, namely; “buckling restrained braces” (BRB), have been proposed, in which the buckling of braces due to compression forces is prevented. Thus, such braces exhibit the same behavior in tension and compression. Since these braces have recently been proposed, more investigations into their seismic performance are needed. Nowadays, the computational effort of computers has facilitated the implementation of nonlinear analysis procedures in structural engineering. Nonlinear dynamic analysis is the most exact method for seismic evaluation of structures. However, it is time-consuming and complicated to be utilized in engineering practice. In contrast, static pushover analysis (SPO) is more practical, but, it has several limitations. Recently, a scaled nonlinear dynamic procedure (scaled NDP) has been proposed. In this procedure, dispersion of the responses of nonlinear dynamic analysis is reduced by scaling a few ground motion records. Ground motion records are scaled to a value equal to roof target displacements, which are obtained by a static nonlinear procedure. Therefore, this procedure is suitable for cases in which a sufficient number of ground motion records are not available. Recently, scaled NDP has been recognized by FEMA440 as an effective tool for the seismic evaluation of structures since it utilizes the benefits of both nonlinear dynamic and nonlinear static procedures. In this study, the seismic behavior of buckling restrained braced frames is investigated using scaled NDP and a nonlinear static procedure. Two groups of BRBF structures including 3-story and 12-story frames with diagonal, x-split and chevron bracing configurations are designed, and seismic responses, including displacements, story drifts and story shears, are compared by scaled NDP and SPO analyses. To evaluate the accuracy of the scaled NDP procedure, the effect of the number of earthquake ground motion records on the dispersion of the results is also investigated. Results show that nonlinear static analysis is able to estimate relatively good results, although it overestimates the responses in some cases. Also, an increase in the number of earthquake ground motions does not considerably improve the dispersion of results.