تحليل ديناميكي غيرخطي اندركنش شمع- خاك- روسازه در ساختمان هاي بلند

Nonlinear Dynamic Analysis of Pile-Soil-Superstructure Interaction in High Rise Buildings

در اين نوشتار، پاسخ فونداسيون شمعي و روسازه‌ي يك ساختمان بلندمرتبه تحت بار لرزه‌يي و اثرات سينماتيكي و اينرسيال آن، به‌وسيله‌ي برنامه‌ي اجزا محدود سه بعدي ABAQUS بررسي شده است. اهداف اين پژوهش عبارت اند از: الف) ايجاد يك مدل اجزا محدود كاملاً يك‌پارچه از روسازه‌ي يك ساختمان بلندمرتبه، فونداسيون شمعي آن و خاك پيراموني شمع ها؛ ب) انجام تحليل ديناميكي ضمني روي مدل ايجادشده براي ارزيابي اثرات اندركنش سينماتيكي و اينرسيال، و ج) ارزيابي اثرات ضخامت كلاهك گروه شمع در پاسخ سرشمع . براي ارزيابي اثرات اندركنش سينماتيكي، پاسخ ميدان آزاد خاك و پاسخ سرشمع ها حاصل از تحليل سيستم گروه شمع- خاك با هم مقايسه شده است. براي ارزيابي اثرات اندركنش اينرسيال، پاسخ روسازه‌ي حاصل از دو تحليل كاملاً يك‌پارچه و تحليل جداگانه‌ي روسازه و سيستم شمع- خاك با هم مقايسه شده است و همچنين براي ارزيابي اثرات ضخامت كلاهك گروه شمع، پاسخ سرشمع در 3 ضخامت مختلف كلاهك با هم مقايسه و ملاحظه شده است كه اندركنش سينماتيكي، اندكي حركت سنگ بستر را در مقايسه با ميدان آزاد تقويت مي كند و اندركنش اينرسيال اغلب اثر پاسخ روسازه را بر حسب جابجايي نسبي، نيروي برشي، ممان خمشي و دامنه‌ي فوريه كاهش مي‌دهد. همچنين با افزايش ضخامت كلاهك گروه شمع، اثرات پاسخ سرشمع افزايش مي‌يابد.

During recent destructive earthquakes, the performance of pile-supported structures has shown the need for further research into revamping the design codes and setting regulations in order to prevent further catastrophic loss in terms of human life and economic assets. The damage to pile foundations during these earthquakes has resulted in understanding the importance of considering SSPSI (Superstructure Pile Soil Interaction). SSPSI can be categorized into four basic modes: inertial interaction, kinematic interaction, physical interaction and radiation damping. As these modes bring complexity to the analysis of SSPSI, they are not considered in the ordinary methods of analysis and design of pile foundations and structures. Besides, based on the pile group configuration, the soil profile and the natural frequency of the superstructure, the result of system response might be underestimated or overestimated in real conditions, which could fundamentally change the design criteria. The present study investigates the response of the pile foundation and the superstructure of a high-rise building under seismic loading, considering its kinematic and inertial interactions, using the three-dimensional finite element program “ABAQUS”. The objectives of this study are 1) to develop a finite element model of the superstructure of a twelve-floor high-rise building, with its foundation pile and soil surrounding piles, which could accurately model the inertial and kinematic interactions of the fully-coupled system, these include considering the nonlinear behavior of the soil and the superstructure, the discontinuity condition at the pile-soil interface, energy dissipation, and wave propagation, 2) to have a fully-coupled analysis for the developed model, by applying ground motion, in order to evaluate the inertial effects on the response of the pile head and the kinematic interaction effects on the response of the superstructure, and 3) to evaluate the pile cap thickness effects on the response of the pile head. Earthquake excitation was applied in the form of a time history of acceleration over the base of the finite element mesh as the bedrock. In order to evaluate the effects of kinematic interaction, the responses of both the free-field model (with no piles) and the pile-soil model were compared. To evaluate the effects of inertial interaction, the response of the pile head was calculated from the pile-soil model. This pile head movement is then used as the input excitation to calculate the response of the structure. This response for the structure was compared with the response obtained from the fully-coupled analysis. Also, in order to evaluate the effects of the pile cap thickness, the responses of the pile head were compared in three pile cap thicknesses, using fully coupled analysis. It is found that the kinematic interaction of the piles group has slightly amplified the motion of the bedrock, once compared with the response of the free-field. Inertial interaction often attenuates the effects of the responses of the superstructure, in terms of drifts, shear forces, bending moments, and Fourier amplitudes. In addition, increasing the thickness of the pile cap amplifies the effects of the responses of the pile head.