بررسي اثرات برهم‌كنش سينماتيك خاك و سازه در بارگذاري لرزه‌يي سازه‌ها بخش دوم: حركت‌هاي ورودي پي

EVALUATION OF THE EFFECTS OF KINEMATIC SOIL STRUCTURE INTERACTION ON SEISMIC LOADING OF STRUCTURES PART II: FOUNDATION INPUT MOTIONS

در اين نوشتار، با درنظرگرفتن اثرات برهم‌كنش سينماتيك خاك و سازه، رابطه‌يي براي تعيين تحريك لرزه‌يي سازه‌هاي مستقر بر دو نوع سيستم پي گسترده و منفرد ارايه شده است. براي اين منظور، ابتدا تاثيرات شكل هندسي پي بر حركت ورودي اين دو نوع سيستم پي به‌صورت پارامتريك بررسي و سپس توابع تبديل متناظر با حركات ورودي اين پي‌ها استخراج شده‌اند. همچنين بحثي پارامتريك براي مقايسه‌ي مولفه‌هاي دوراني و انتقالي ورودي دو نوع سيستم پي گسترده و منفرد انجام شده است. نتايج عددي نشان مي‌دهند كه مولفه‌هاي انتقالي و پيچشي ورودي سازه با سيستم پي منفرد كه داراي طبقات با ديافراگم صلب است، تقريباً برابر با مولفه‌هاي انتقالي و پيچشي سازه‌يي مشابه با پي گسترده‌ي صلب است.

As seismic waves pass through foundations, high frequency waves of free-field motions will be filtered by them. To obtain these filtered motions, called foundation input motions (FIMs), the effects of kinematic interaction between the foundation and surrounding medium, due to the spatial variation of earthquake ground motion (SVEGM), should be considered. For the structures supported on the large mat foundations, considering the effects of kinematic soil structure interaction may lead to filtering of the high-frequency components of the translational response and significant rotational motions. Therefore, the contribution of rotational components may be detrimental in the seismic response of structures, such as vertically irregular structures, short-period (stiff) structures such as nuclear reactors, and even secondary systems or equipment mounted on structures that are sensitive to high frequency components of motion. The main objective of this paper is to present a new procedure for evaluating all six input components of structures supported on two foundation systems: rigid mat and single foundations. To achieve this objective, at first, the transfer functions of foundation input motions are derived. Then, a parametric study is performed on the rigid mat foundation system, in order to estimate the effects of the geometrical shape of such a foundation system on the foundation input motion. Moreover, a numerical study is performed on the two different foundation systems; mat and single, in order to compare relations between input rotational and translational components in the structural loading. The results imply that the input translational and torsional accelerations of structures with rigid floor diaphragms, which are supported on the single foundation system, are approximately equal to the same structures with rigid mat foundation systems.