Shaking Table Study on Bridge Model with Scoured Piled Foundation

本研究探討橋梁受複合式災害，因樁基礎沖刷裸露對耐震性能之影響，使用大型雙軸向剪力試驗盒，進行縮尺橋梁樁基礎沖刷裸露之振動台實驗。實驗試體採用單自由度單柱橋梁、單樁基礎與乾性石英砂質土壤。實驗規劃數組不同基樁裸露長度之組別，以觀察地震作用下樁基礎於裸露前、後對橋體、基樁與土壤間之互制行為。實驗結果發現，上部結構位移反應隨基樁裸露深度增加而增加，加速度則成相反之趨勢；基礎板與土壤之接觸行為明顯影響上部結構之動態反應；樁身最大彎矩發生位置隨基礎裸露深度而降低，但均鄰近土壤表面，且反曲點位置也隨之改變。為能有效模擬土壤與基樁之互制行為，本研究將土壤簡化為單一彈簧，根據土體之慣性力與變形量計算等值勁度，經比較實驗結果，可有效掌握上部結構在中小度地震下之加速度與位移反應。此外，本研究亦比較國內外橋梁設計規範及經驗公式有關土壤彈簧之適用性。結果顯示，根據日本道路橋示方書.同解說之基樁彈簧所得之結構反應較符合於本實驗觀察，未來仍需進一步檢討強震下土壤彈簧之模擬方式，以更準確掌握橋梁結構安全。In this study, the impact of scouring on the seismic performance of bridge pile foundation was investigated under multi-hazard circumstances. Large biaxial laminar shear box was tested on shaking table to simulate the scouring effect on bridge pile foundation. Test specimen is comprised of a bridge girder rested on a single column with a single degree of freedom, a single pile foundation and dry quartz sand. Different pile exposure lengths were the focus in parametric study to explore the effect of pile foundation on the seismic soil-structure interaction before and after scouring. Experimental results showed that the superstructure displacement increases as the pile exposure length increases, while the case is reversed for acceleration. The contact behavior of the footing and soil significantly influences the dynamic response of the superstructure. The maximum bending moment in the pile, constantly in the vicinity of the soil surface, lowers down as the exposure length increases, and the inflection point of the pile changes accordingly as well. Soil was simplified to be a single spring to efficiently simulate the soil-structure interaction. By comparing the test results and calculating the equivalent stiffness through the force of inertia and the deformation of soil, the seismic acceleration and displacement response of the superstructure can be effectively controlled for small ground motion. In addition, the applicability of the related empirical formula on soil spring in different bridge design codes was also studied. The experimental observation in this study reasonably coincides with the structural response defined by the pile spring theory according to the ‘Seismic Design Specifications for Highway’. Simulation methods of soil spring under strong ground motions need further investigation in the future to enhance the structural safety of bridges