Effect of Saturated Complex Site on the Seismic Response of Continuous Long-span Beam Bridges
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摘要: 我国沿海地区大量大跨桥梁位于饱和复杂场地，此类场地通常存在土饱和特性、起伏地形、上覆水层，其对地震波的散射效应将导致地震动空间变化，然而目前缺乏针对此类场地条件的实测地震记录。针对此问题，本文建立了适用于饱和复杂场地大跨桥梁抗震分析的多点地震动模拟方法。首先，基于边界元法求解饱和复杂场地地震动传递函数；然后采用谱表示法生成空间变化人工地震动加速度时程；最后，以五跨连续梁桥为例，开展大跨桥梁地震响应分析，研究局部场地效应、动水压力效应对大跨连续梁桥地震响应的影响规律。结果表明，饱和复杂场地地震动加速度峰值表现出明显的空间变化特性，多点输入可导致饱和场地中地震动峰值加速度放大2～3倍，进而引起墩顶位移较一致地震作用结果增大95%～110%；动水压力可引起墩顶位移、墩底内力增大34%、29%。饱和复杂场地大跨桥梁抗震分析时忽略场地条件对地震动、桥梁地震响应的影响偏于不安全。Abstract: In China, many long-span bridges are situated across saturated complex sites characterized by soil saturation, undulating topography and superimposed water, in which the propagation of seismic waves induces spatial variations in ground motions due to scattering effect. However, the seismic records under such specific site are lack. Addressing this concern, we establish a multi-point ground motion simulation approach to analyze the seismic response of long-span bridges in saturated sites. Initially, the boundary element method is employed to solve the ground motion transfer function of saturated sites. Subsequently, the spectral representation technique is used to synthesize spatially varying ground motion acceleration time-histories. Conclusively, a continuous five-span beam bridge is set as an example, and the seismic response is investigated to study the impact of local site effects and hydrodynamic pressures on the seismic behavior of the continuous beam bridge. The results indicate that the peak ground acceleration in the saturated regions has significantly spatial variances, and multipoint input can cause the peak acceleration of seismic motion in a saturated site to be amplified by 2~3 times. Consequently, situation leads to a 95%~110% increase in the displacement of the pier top compared to the results under uniform ground motions. Furthermore, dynamic hydrodynamic pressure is shown to precipitate pier displacement and internal forces by 34% and 29%, respectively. Disregarding the influence of saturated site conditions when evaluating ground motions and bridge responses for long-span bridges in saturated complex sites tends to engender hazardous outcomes.
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图 3 地表点1、2、3地震动加速度自功率谱密度
Figure 3. Auto-power spectral density of ground motion acceleration of surface points 1, 2, and 3
图 4 不同地表点间地震动加速度互功率谱密度
Figure 4. Cross-power spectral density of ground motion acceleration among different surface points
图 9 多点、一致地震动作用下连续梁桥墩顶位移
Figure 9. Top displacement of the piers of the continuous beam bridge under multi-support and uniform ground motions
图 10 多点、一致地震动作用下连续梁桥墩底剪力
Figure 10. Shear force of the pier bottom of the continuous beam bridge under multi-support and uniform ground motions
图 11 多点、一致地震动作用下连续梁桥墩底弯矩
Figure 11. Moments of the pier bottom of the continuous beam bridge under multi-support and uniform ground motions
图 12 多点地震动作用下动水压力对连续梁桥墩顶位移的影响
Figure 12. Effect of hydrodynamic pressures on the displacement at the pier top of the continuous beam bridge
图 13 多点地震动作用下动水压力对连续梁桥墩底剪力的影响
Figure 13. Effect of hydrodynamic pressures on the shear force at the pier bottom of the continuous beam bridge
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