Numerical simulation of shaking table tests on dynamic response of a bridge model with scoured piled foundation

本研究提出土壤結構互制之簡化分析流程，探討樁基礎橋梁受地震與沖刷之複合式災害作用下之受震反應。土壤結構互制行為係以土壤彈簧模擬，主要特點為採用實測之土壤剪力波速與動態剪力模數來推估線性土壤彈簧之初始值，並假設樁身近域土壤與樁體產生相同剪力變形，根據地震作用下之土層有效剪應變與最大剪力模數之關係對動態剪力模數進行修正，經少量迭代後獲致最終土壤彈簧參數。此外，本研究建議地震時之土壤分層方式，根據地震訊號作用下之剪力盒框架加速度與轉換函數，作為單層土及雙層土模型之依據。基於前述方法，本研究建立等效線性的數值模型來模擬樁土互制之行為，並比較先前之樁基礎沖刷橋梁模型之振動台實驗成果。 針對先前之振動台實驗模型進行系統識別，瞭解實驗模型之相關動力特性，掌握數值模型之精確度。在乾性砂土之實驗中，識別結果顯示橋梁模型隨著裸露深度增加，會延長結構系統週期及降低系統阻尼比；在飽和砂土之實驗中，識別結果顯示實驗模型之結構系統週期與系統阻尼比將比乾性砂土還大，當土壤未發生液化時，飽和砂土較容易受擾動而降低提供之勁度，若當土壤發生液化現象，結構週期將快速延長、系統阻尼比也有大幅提升的趨勢。 數值分析結果顯示，在土壤為乾性砂土之實驗中，無論於全覆土或基樁裸露的條件下，數值模型分析之結構週期均與實驗結果符合，且在各組實驗中，均能有效模擬上部結構之加速度與相對位移歷時反應、樁頂應變歷時反應，以及樁身剖面之最大應變，且雙層土模型之預測結果均較單層土模型更佳準確；在土壤為飽和砂土之實驗中，針對土壤未產生液化之組別，數值模型分析之結構週期均與實驗結果接近，並在El Centro 100gal地震歷時下均能有效模擬上部結構之加速度與相對位移歷時反應、樁頂應變歷時反應，以及樁身剖面之最大應變，同樣雙層土模型之預測結果均較單層土模型更佳準確。最後，根據本研究提出之簡化分析流程，建立等效線性的數值模型確實能夠有效預測沖刷橋梁模型的耐震行為，可解決目前有限元素軟體於分析上遇見的困難。The simplified analysis process of soil structure interaction is proposed in this re-search, which studies on the response of the bridge with scoured piled foundation under earthquakes. The simplified analysis applies soil springs to simulate soil structure interac-tion behavior. The real measurement of soil shear wave and dynamic shear modulus used to estimate initial value of linear soil spring is the special point in this research. The study supposes the soil in the near field effected zone and the pile occur the same shear de-formation and modify the shear modulus of soil according to the relationship between effective shear strain and maximum shear modulus under earthquakes, then obtain the final soil spring parameters after few amount of iterations. Furthermore, this research suggests simplifing soil according to significant frequency of transfer function between layers under seismic signal. Base on the method mentioned ahead, the study establishes equivalent linear numerical model to simulate behavior of soil-pile interaction, and com-pares to the previous results of shaking table tests of bridge model with scoured piled foundation. Conducting dynamic system identification to the previous shaking table model to realize dynamic characteristics and verifies the numerical model. In the dry soil, the ex-perimental identification results show that the period of the structure increase and the damping ratio of system decrease as exposure length of foundation increase. In the saturated soil, the experimental identification results show that the period and damping ratio of system will be larger than the results of the dry soil experiments. Before soil liquefaction occurs, the saturated soil will reduce the stiffness more easily due to the disturbances. When soil liquefaction phenomenon takes place, the damping ratio and the period of system will increase dramatically. The numerical analysis results show that whether in full cover or explosion of pile foundation in the dry soil experiment, the structure period of numerical mode will con-sistent with experiment result. It could simulate the responses of the acceleration and the relative displacement in the superstructure, and the strain responses of top pile and the maximum strain of pile effectively. In the prediction results, double layers model is more accurate than single layer model under several earthquake experiments. In saturated soil experiment under scoured piled foundation, the structure period of numerical model ap-proximate to the experiment result, and also can simulate the acceleration and relative displacement responses of superstructure, the strain response of top pile and the maxi-mum strain of pile under El Centro 100gal, and prediction results of double layers model is more exact than single one