طراحي مستقيم براساس تغييرمكان با لحاظ‌كردن تاثير اندركنش خاك و سازه

Direct Displacement-Based Design Considering the Effects of Soil-Structure Interaction

در اين نوشتار روشي براي طراحي مستقيم براساس تغييرمكان با لحاظ‌كردن اثر اندركنش خاك و سازه ارايه شده است. روش پيشنهادي شامل 2 مرحله است: در مرحله‌ي اول با فرض صلب‌بودن تكيه‌گاه سازه‌ي برش پايه با استفاده از طيف تغييرمكان، تغييرشكل‌هاي موردنظر براي طراحي در سطح عملكرد مربوط، و سيستم يك درجه‌ي آزاد معادل محاسبه شده است. در مرحله‌ي دوم، تغييرمكان‌هاي نسبي اضافي ناشي از دوران پي به تغييرمكان‌هاي محاسبه‌شده در مرحله‌ي اول اضافه شده و برش پايه‌ي طراحي اصلاح‌شده با احتساب اندركنش سازه و خاك به‌دست آمده است. در اين روش تغييرمكان‌هاي طراحي براساس نيازهاي طرح و سطوح عملكرد مورد انتظار از سازه قابل انتخاب‌اند و كنترل خسارت به صورت مستقيم انجام مي‌شود. براي ايجاد درك بهتر، مراحل كامل روش با ارايه‌ي مثال‌هايي تشريح و نتايج طراحي با روش آيين‌نامه‌يي مرسوم مقايسه شده است.

Displacement-based design is a procedure for attaining an acceptable level of damage during earthquakes, which provides a new tool for performance-based design. In this paper, a method of direct displacement-based design considering the effects of soil-structure interaction is proposed. The suggested method is in two stages. First, the base of the structure is assumed to be rigid. In a sequence of different steps, the base shear is calculated for the assumed fixed-base condition. In these steps, first, the yield displacement of the system is estimated using empirical relations. Then, the basis displacement, which is the maximum displacement of an equivalent single degree of freedom system that will be subsequently used for calculation of the lateral displacement profile, is determined. The ductility factor is calculated using the yield and basis displacements and an equivalent damping ratio is computed next. The effective mass of the system is determined using the displacements profile. Using a design displacement spectrum, the effective period and then the effective lateral stiffness are calculated. The fixed-base base shear comes as the effective lateral stiffness multiplied by the basis displacement. In the second stage, the base shear is modified due to the flexibility of the base. For this purpose, the additional displacement due to rotation at the base is calculated and added as a modification to the basis displacement. This modified deformation is used in a similar way as above to compute the modified base shear. In this method, the design displacements can be selected based on the design needs and expected performance levels, and seismic damage control is directly applicable. For clarity of presentation, the steps of the method are explained through an illustrative example, and the results are compared with those of a conventional design code. It is shown that the proposed displacement-based design method can result in heavier or lighter structural members compared with the code-based sections based on the structural dynamic properties.