بررسي اثرات اندركنش خاك- پي- سازه در ضريب اصلاح تقاضاي عضو در دستورالعمل بهسازي لرزه‌يي ساختمان‌هاي موجود

Effects of Soil Foundation Structure Interaction on the Component Demand Modifier Factor based on Instructions for Seismic Rehabilitation of Existing Buildings

در اين مطالعه، اثرات انعطاف پذيري پي در نحوه‌ي اعمال ضريب اصلاح تقاضاي عضو در دستورالعمل بهسازي لرزه‌يي ساختمان‌هاي موجود بررسي شده است. بدين منظور از روش ساده و كارآمد و در عين حال با دقت مناسب تير مستقر بر خاك غيرخطي وينكلر استفاده شده است. در اين راستا، قاب‌هاي 3 ، 6 ، 10، و 1? طبقه‌ي بتني مستقر بر خاك هاي نرم، متوسط، و سخت طراحي و نتايج تحليل هاي غيرخطي تاريخچه‌ي زماني هر يك در حالت هاي پايه‌ي صلب و انعطاف پذير با نيروهاي پيشنهادي روش استاتيكي خطي دستورالعمل مقايسه شده است. نتايج نشانگر آن است كه تركيبات بارگذاري دستورالعمل براي اجزا كنترل‌شونده توسط تغييرشكل با بارهاي ثقلي قابل توجه (براي نمونه‌ي تيرها) مي تواند به برآورد غيرواقع‌گرايانه‌ي تقاضاي لرزه ‌يي منجر شود. در انتها، يك تركيب بارگذاري جديد، به منظور رفع اين نقيصه، پيشنهاد شده است.

Soil-Foundation-Structure Interaction (SFSI) in a structural modelling procedure can change seismic structural response. However, SFSI effects are mostly ignored in the analysis procedure of structures, due to a general engineering belief regarding its conservative effects. This conservativeness is not always the case, although the period and the damping of the structure changes by considering the SFSI effects and, consequently, seismic demand decreases. Consideration of SFSI effects still contains some level of difficulty due to the needed in advance modelling procedures, but this issue can be solved by employing advanced finite element modelling programs, e.g; OpenSees software. The aim of the current paper is to evaluate the influence of SFSI effects on the component demand modifier factor, based on Iranian instructions for the seismic rehabilitation of existing buildings. 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 storey 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 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-base 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 for the elements with significant gravity loads that are controlled by deformation actions (e.g. beams) can lead to non-conservative prediction of the seismic demand. Finally, a new load combination has been proposed in order to limit the influence of the demand modifier factor only on the seismic loads. This new proposed load combination can be used to improve the equivalent linear static approach in the instructions for seismic rehabilitation of existing buildings.