بررسي حاشيه‌ي اطمينان در طراحي لرزه‌يي شالوده ها بر اساس دستورالعمل بهسازي لرزه‌يي ASCE41-06

Assessment of the safety margin in the seismic design of foundations based on ASCE 41-06 standard

در طراحي هاي لرزه ‌يي مبتني بر نيرو، جهت تخمين تقاضا متداول است كه پاسخ خطي سازه توسط ضريب كاهش نيرو تقليل داده شود. با توجه به استقبال فراوان طراحان از اين روش، كاليبره ‌كردن مقادير ضريب كاهش نيرو از اهداف اصلي مهندسان در سال هاي اخير بوده است. از طرفي با توجه به روند رو به رشد مقاوم سازي ساختمان ها، محافظه كارانه‌شدن نتايج بهسازي شالوده ها نسبت به طراحي آن ها از ديگر مسايلي است كه مورد توجه پژوهشگران قرار گرفته است. عمده‌ترين دليل اختلاف نتايج دو روش را مي توان تفاوت در نيروهاي لرزه ‌يي مورد استفاده برشمرد. بر اين اساس در اين نوشتار، جهت بررسي مقادير پيشنهادي ضريب كاهش نيرو در دستورالعمل بهسازي لرزه ‌يي، قاب هاي بتـني همراه با ديوار برشي متفاوت در تعداد طبقات در دو حالت پايه‌ي صلب و انعطاف پذير، توسط روش هاي استاتيكي خطي و ديناميكي غيرخطي تاريخچه‌ي زماني مورد تحليل قرار گرفته است. مقايسه‌ي نتايج طراحي و بهـسازي شالوده ها منجر به پيشنهاد رابطه‌ي جديدي جهت تعيين ضريب كاهش نيرو شده است.

The seismic linear demand of structures is usually reduced by employing a force-reduction factor in most force-based seismic regulations. This approach is simple enough to be widely used by designers. However, calibration of the employed force-reduction factor has a high level of importance, since it is supposed that this factor can transfer the structural linear response to the structural nonlinear response. Current force reduction factors in the ASCE 41-06 standard, which is the latest regulation for seismic rehabilitation programs, results in conservative designs in the case of foundations when compared to design regulations, e.g., IBC 2000. This conservation source comes from the fact that the force reduction factor is limited between one and two in the case of force-controlled members. The aim of the current paper is to evaluate the influence of SFSI effects on the force-reduction factor, based on the ASCE 41-06 standard, focusing on force-controlled members. For this purpose, the beam on the nonlinear Winkler foundation approach is used, which is a simple and efficient method. First, a collection of 3, 6, 10 and 15 story concrete moment-resisting frames, founded on soft, medium and hard soil, are designed and analysed for the case of fixed-base and flexible-base assumptions. Eight ground motion records were chosen in order to estimate the median response of the considered frames for a pre-defined seismic scenario. A comparison has been made between the results of the nonlinear response history analysis of each frame, under flexible and fixed base conditions, with the response based on the equivalent linear static approach. The results show that the equivalent linear static approach load combinations, in the case of foundations, can lead to conservative designs. Finally, by adjusting linear to nonlinear results, a set of new force-reduction factors has been proposed in order to deal with this problem.