樁筏基礎靜態與地震行為之案例研究

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Title:	樁筏基礎靜態與地震行為之案例研究
Other Titles:	Case studies on static and seismic behaviors of piled raft foundations
Authors:	洪明揚;Hong, Ming-Yang
Contributors:	淡江大學土木工程學系碩士班張德文
Keywords:	樁筏基礎;靜態行為;地震行為;物理模型;實際案例;三維有限元素分析;piled raft foundation;static behavior;seismic behavior;physical Model;Case Study;three-dimensional finite element analysis
Date:	2016
Issue Date:	2017-08-24 23:48:45 (UTC+8)
Abstract:	本研究係以有限元素軟體MIDAS/GTS進行樁筏基礎結構之三維力學分析，觀察樁筏基礎數值模型在靜態(垂直向和水平向荷載)與受水平地震力之行為。研究案例包括：1. 物理模型實驗結果，2. 大型煤倉樁筏基礎，3. 環狀群樁基礎。研究首先以有限元素分析模擬物理模型實驗結果，另藉有限元素分析觀察實際樁筏基礎之靜態與受震行為。基礎靜態行為除參考前期成果和本研究內容加以印證外，其受震行為和耐震性能評估則另以一維波動方程EQWEAP分析以及PBEE法做為比較和評估依據，並據以討論大型煤倉基礎和環狀群樁基礎在不同地震等級下之基樁耐震性能。相關地震紀錄係依據建築物耐震設計技術規範之中小地震、設計地震與最大考量地震所對應的案例場址最大地表加速度，以中央氣象局資料庫所收集之原始地震資料進行修正而生成。研究結果顯示：採加速度譜修正法(Design Spectrum Based Calibration)所生成地震資料其位移特性並不合理，而採最大地表加速度縮放法(PGA Based Calibration)所修正的地震資料可保有合理的地表振動特性。由物理模型之靜力分析可知，有限元素分析結果將與材料參數設定密切相關，其中介面元素勁度和材料強度參數選擇均會影響分析結果。由大型煤倉樁筏基礎與環狀群樁基礎之靜力分析可知：其在均佈力作用下的版基和基樁位移與力分配性將受基樁配置方式和作用力方向影響，樁體軸力、摩擦力與樁底反作用力可藉由內力之後處理計算而得，相對軟弱場址的樁基礎所受之總力比例將遠較較堅硬地層的樁基礎受力為高，水平向又較垂直向為高。採用實體元素模擬基樁和採梁元素和實體元素混合方式模擬基樁所產生的差異甚有限。從大型樁筏基礎和環狀群樁基礎的水平受震位移歷時分析可知：基樁在此運動下將為結構系統之耐震關鍵，環狀群樁基礎之基樁將較大型單一樁筏基礎之基樁更受其所在位置影響，其中縱向側基樁位移較橫向側基樁為大。根據耐震性能評估，大型煤倉樁筏基礎之基樁在中小和設計地震下安全無虞，但在最大考量地震影響下，樁體在地層勁度明顯差異處將有破壞現象產生。環狀群樁基礎無論在各式地震等級下，均無破壞之虞。樁頂和筏基交接處若以剛性端視之將導致過大彎矩，不利於耐震性能評估。 This study intends to discuss the case studies on static and seismic behaviors of piled raft foundations using the three-dimensional finite element analyses based on the MIDAS/GTS program. The numerical models of case studies include: 1. Physical model test, 2. Mega-size piled raft foundation, 3, Ring shaped pile group foundation. Static behaviors of the models were studied first. The assessments on seismic performance for case 2 and case 3 were made using one-dimensional wave-equation program EQWEAP under different levels of earthquake. The moment capacities of the piles were calculated and used to verify the damages of the pile. Acceleration records at seismic stations near to the foundation sites were obtained and calibrated to moderate, design and maximum consideration earthquake levels in correspondence with the seismic design code in Taiwan (2011) by PGA calibration method. The study reveals hat: seismic data calibrated by the design spectrum method were not that rational, the PGA calibration method would provide feasible ground vibrations. According to the result of physical modeling, the material model parameters and the interface element between piles and soils will affect much the simulations. The load and displacement distributions will be influenced by the load directions with respect to the pile orientation. Internal stresses of the foundation can be computed by the post process based on FEM analysis. Lower ground stiffness and horizontal seismic forces will increase the loads carried by the pile foundation. Using the solid elements and hybrid solid-beam elements to model the pile will yield similar results. From the results of foundation studies, the piles are the keys to the safety design of the foundation. The influence of the pile location is typically important for the ring-shaped pile group foundation rather than the mega-size piled raft foundation. The displacements of the piles parallel to the direction of the seismic force were found larger than those of the piles perpendicular to the force. According to the seismic performance assessments, the mega-size foundation would be safe at the levels of moderate and design earthquakes, however it would be vulnerable to the maximum consideration earthquake. The ring-shaped foundation would be safe in different levels of the earthquake. The rigid connection presumed at the interface of the raft and the piles will provide larger internal bending moment at the pile head, which would result in conservative assessment.
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