台灣位於板塊交界處，地震頻繁，並造就許多崎嶇地形，使得橋梁結構廣泛運用。一般橋梁淺基礎主要承受垂直向之載重。然而，當上部結構受地震力或風力等側向力作用時，淺基礎將承受水平力、垂直力和彎矩聯合作用。在此載重組合作用下，產生相當複雜之土壤結構互制行為，為防範淺基礎結構發生破壞或產生過大之變形，值得針對此課題，進行深入之研究。 近年來耐震設計領域廣泛推動性能設計法(Performance Based Design)。即針對不同等級地震作用下，要求結構物須滿足規定之性能。要具體落實，需計算結構物承受地震力時，由小變形到大變形之完整受力行為，此計算過程稱為側推分析(Pushover Analysis)。目前橋梁淺基礎耐震分析較少談及側推曲線，相關數值分析均著重於翻轉(rocking)部分之行為，且多採用土壤非線性彈簧即溫克勒模型(Winkler modeling)進行分析，此模型無法模擬基礎側滑與翻轉耦合之行為。此外，還涉及基礎尺寸效應、彈簧參數和土壤承載力計算等影響因子，因此採用三維土壤與結構元素模擬，可更符合現地狀況。 本文採用FLAC3D有限差分程式建置三維橋梁淺基礎模型進行側推分析，並與SAP2000分析結果進行比較，結果顯示FLAC3D模型之側推剛度較小，於相同地震力下將產生較大之水平位移，但橋墩基礎於極限狀態所能承受之最大地震力相近。藉由參數影響分析迴歸出不同基礎尺寸與地盤N值所對應之垂直地盤反力係數，以供日後工程設計參數之參考。 ;Recently, Performance Based Design has been promoted in the field of seismic design. It is required that the structures should meet their given properties under different intensities of earthquakes. In Performance Based Design, calculating the structure’s deformation is needed while it is subjected to seismic force, and this process is called Pushover Analysis. Nowadays, pushover curve in the seismic analysis of shallow foundations is rarely mentioned, and the relative numerical analysis focuses on the rocking performance by non-linear elastic soil model. However, the model fails to simulate the coupling of horizontal sliding and rocking behavior in soil and should consider the dimensional effect, spring parameters and bearing capacity of soil. As a result, applying three-dimensional soil elements to simulate would be more suitable for situ condition. In this article, the model of a three-dimensional shallow foundation for bridge was set up by FLAC3D for pushover analysis. After comparing with the analysis results of SAP2000, the stiffness of the model produced by FLAC3D is smaller, which means the lateral displacement is more obvious. Nevertheless, in the ultimate limit state, the maximum seismic force that the bridge foundation can bear from these two analysis are almost the same.
