بررسي آثار برهم‌كنش سينماتيك خاك و سازه در بارگذاري لرزه‌يي سازه‌ها بخش اول: الگوهاي بارگذاري لرزه‌يي سازه‌ها

EVALUATION OF THE EFFECTS OF KINEMATIC SOIL STRUCTURE INTERACTION ON SEISMIC LOADING OF STRUCTURES PART I: SEISMIC LOADING PATTERNS OF STRUCTURES

در اين مقاله الگوهاي بارگذاري لرزه‌يي سازه‌هاي مهندسي ناشي از تغييرات مكاني حركت زمين با درنظرگرفتن آثار مولفه‌هاي دوراني زلزله تعيين شده است. براي اين منظور ابتدا روش‌هاي رايج محاسبه‌ي مولفه‌هاي دوراني در ميدان آزاد بررسي و سپس با اصلاح تابع چگالي طيفي كانايي- تاجيمي رابطه‌يي جديد براي تعيين چگالي طيفي مولفه‌هاي دوراني از طريق مولفه‌هاي انتقالي متناظر ارايه مي‌شود. سپس الگويي براي تعيين تغييرات مكاني حركت نقاط مختلف روي سطح زمين ناشي از تحريكات لرزه‌يي، به‌صورت تاريخچه‌ي زماني ارايه شده است. از الگوي ارايه‌شده مي‌توان به‌منظور بارگذاري لرزه‌يي سازه‌هاي چند تكيه‌گاهي مانند پل‌ها استفاده كرد. همچنين از الگوي به‌دست‌آمده، براي لحاظ‌كردن آثار برهم‌كنش سينماتيك در تعيين مولفه‌هاي دوراني و انتقالي ورودي پي‌هاي گسترده‌ي صلب استفاده شده است. نتايج عددي تحقيق حاضر نشان مي‌دهند كه لحاظ‌نكردن آثار برهم‌كنش سينماتيك در تحريك لرزه‌يي سازه‌هاي گسترده، مي‌تواند منجر به بارگذاري ناايمن آنها شود.

One of the main challenges in structural analysis is the reliable definition of structural loading; especially the loading pattern due to earthquake input acceleration at the base of the structure. During the past four decades, in spite of the fact that six components–three translational and three rotational–are needed to describe strong ground motion (SGM) and excitation due to rotational motion, which are mainly associated with failures of some tall and long structures in the course of past earthquakes, like Kobe and Northridge, structural analysis procedures are still based on the principles of classical earthquake engineering, and the effects rotational components on the seismic loading of structures are ignored. One reasons for such assumption is the complexity of the derivation of simple seismic loading patterns for structures subject to rotational loading. This study addresses research on the effects of earthquake rotational components on the seismic loading patterns of engineering structures, due to spatial variations of earthquake ground motion (SVEGM). To achieve this objective, at first, a new method is derived to obtain the power spectral density function of the rotational ground motion in free-fields in terms of translational components, by modifying the Kanai-Tajimi SDF. Next, a new procedure is presented using a coherency function to obtain the SVEGM pattern in the time domain. The presented technique is applicable for the earthquake loading of multiple support structures, such as bridges, pipelines, and buildings, supported on spread foundations, etc. Moreover, a simple relation is derived in order to estimate the input rotational and translational motions of rigid mat foundations, by considering the effects of kinematic soil structure interaction. The numerical results show that ignoring the effects of kinematic soil structure interaction in the seismic excitation of extended structures may lead to the unsafe loading of these structures.