Simplified analysis of dynamic response of pile-supported bridge under local scour and verification by centrifugal shaking tests
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摘要: 冲刷导致桩周土体流失,改变了场地的地震动特性,同时影响了桥梁的抗震性能。为研究局部冲刷下桩基桥梁动力特性及潜在破坏风险,探讨了冲刷场地的地震动特性,给出局部冲刷场地的地震动快速计算方法；并在此基础上提出局部冲刷后桩基桥梁地震分析简化模型,通过振型叠加法计算桥梁的墩底、桩身剪力和最大位移,用以快速评估局部冲刷后桩基桥梁的破坏风险,进而确定临界破坏冲刷深度；为验证模型的正确性,开展了50g重力下桩基桥梁离心振动台试验,桥梁上部结构简化为质量块,基础采用3×3群桩。按照冲刷深度分为3个工况,每个工况下输入白噪声和El-Centro波。研究表明,一般冲刷使得场地周期减小,但局部冲刷对场地周期影响不明显,局部冲刷坑坡角对场地的地震动几乎没有影响；局部冲刷场地与一般冲刷场地的地震动差别较大,但与原场地的地震响应基本相似；桥梁响应最大时的冲刷深度取决于桥梁频率与地震波主要波频的关系。该简化方法计算的桥梁反应与试验结果相近,因此,可用于局部冲刷条件下桥梁的地震反应快速计算、震害风险评估和临界破坏冲刷深度确定。Abstract: Scour causes the erosion of soil, changes the seismic response of site, and also influences the seismic properties of bridge. To investigate the dynamic behavior and damage potential of pile-supported bridges at local scour site, the effects of scour on the ground motion of site are firstly discussed, and a fast calculation method for calculating ground motion is presented. Next, a simplified model describing the bridge response is put forward, which considers the ground motion changes caused by local scour. Then the shear force at the pier bottom and pile head, and the maximum displacement of bridge are calculated by the modal superposition method so as to quickly evaluate the damage potential of bridge after scour and further determine the critical damage scour depth. Finally, the centrifugal shaking tests on the pile-supported bridge are conducted under 50g to verify the accuracy of this model. The superstructure of the bridge supported by 3×3 pile-group foundation is simplified as a mass block. There are 3 scour depth conditions. It is found that the site period obviously decreases under general scour, and hardly changes under local scour. The slope angle of local scour hole has virtually no impact on the ground motions of site. The motions at local scour sites are quite different from those at general scour sites, but similar to those at the original free site at the same depth. The scour depth, where the bridge reaches the greatest response, depends on the interrelation between bridge period and dominant frequency of the earthquake. The centrifuge tests further prove that the proposed model can be applied to the fast calculation of seismic response, the assessment of potential of scoured bridge and the critical scour depth under earthquakes.
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图 3 冲刷坑坡角对坑底加速度反应谱的影响
Figure 3. Influences of slope angle on acceleration spectrum at bottom of scour hole
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图 5 基岩到冲刷面（或地面）加速度传递函数
Figure 5. Transfer function for acceleration from site bottom to erosional basal surface
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图 6 0.1g El-Centro波作用下场地加速度反应谱
Figure 6. Acceleration spectra of site under El-Centro waves
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图 7 考虑冲刷导致地震动改变的简化模型
Figure 7. A simplified model for bridge considering scour induced ground motion changes
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图 16 0.2g El-Centro作用上部结构的位移
Figure 16. Displacements of superstructure under 0.2g El Centro waves
表 1 模型设计相似比例
Table 1 Scale of design model
部件 参数 模型 原型 比例 桩 刚度/(N·m2) 71.9 44937500 1∶504 外径/m 0.014 0.7 1∶50 弹模/GPa 70 — — 承台 质量/kg 0.78 97500 1∶503 上部 质量/kg 2.31 288500 1∶503 结构 周期/s 0.009 0.51 1∶50 
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表 2 试验工况
Table 2 Testing programs
冲刷深度/m 峰值加速度/g 白噪声 El-Centro波 0 0.05 0.1, 0.2, 0.3 2.5 0.05 0.1, 0.2, 0.3 5.0 0.05 0.1, 0.2, 0.3
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