软黏土场地桩基桥墩系统地震损伤特性的三维有限元动力分析

Three-dimensional finite element analyses of clay-pile-pier systems subjected to seismic motions

大量研究表明软弱土与地下结构之间的相互作用效应明显,且会对地震波产生显著的放大效应,从而对建于软弱土地基之中或之上的结构物的抗震表现产生不利影响。以软黏土(以下简称软土)场地桩基桥墩系统为研究对象,采用了一种新型动力本构模型来模拟软土的刚度劣化及滞回阻尼特性,提出了基于等效原则的高效结构弹塑性模拟方法,开展了一系列的三维有限元显式动力分析,系统研究了基岩峰值加速度(PBA)、桩身抗弯刚度、桥梁质量等因素对软土-桩基-桥墩系统地震响应的影响。研究表明:对于所考虑的3组远场地震波,无论结构系统是否进入塑性变形阶段,软土场地桩基桥墩系统的弯矩响应与每一相应因素之间的变化趋势较为一致;即使结构处于弹性变形阶段,由于软土的非线性动力特性及软土-桩基相互动力作用关系的复杂性,桩基桥墩系统的弯矩和弯曲曲率响应随每一因素的变化趋势是非线性的;当结构进入塑性变形阶段,不同组地震波之间桩基桥墩系统的弯曲曲率响应开始产生较大的差异性,表明强震作用下桩基桥墩系统的塑性损伤状态与地震波的频谱分布特性紧密相关;基岩峰值加速度对桩基桥墩系统地震响应的影响最为显著,梁跨结构质量及桩身抗弯刚度依次对桥墩和桩基的地震响应具有相对较大的影响,表明地震过程中上部结构的惯性力和软土-桩基的相互动力作用分别对桥墩和桩基的地震响应起主导作用;此外,研究发现桩身抗弯刚度存在一个临界值,大于该值时,桩身的最大曲率随桩身抗弯刚度的变化而基本保持不变。相关研究发现对评估既有类似软土-桩基-桥墩系统的抗震安全性具有参考价值。

Numerous studies have shown that the interaction between soft clay and underground structure is significant,and soft clays have a significant amplification effect on seismic waves.As a result,the seismic performances of structures built in or on soft clay deposits are largely unassured.In this study,three-dimensional finite element analyses using an explicit integration algorithm were performed to investigate the seismic response of clay-pile-pier systems.The stiffness degradation and hysteretic damping of soft clays were taken into account by adopting a hyperbolic-hysteresis soil model,and the elasto-plastic behavior of the bridge structure elements was also considered.The effects of some important factors,such as peak base acceleration(PBA),flexural rigidity,and bridge gird mass,on the seismic response of soft clay-pile-pier systems were examined.For the three sets of far-field seismic motions considered,the bending moment responses of the clay-pile-pier systems generally had the comparable trends with each of the factors considered;when the bridge structure was still in the elastic deformation stage,because of the nonlinear dynamic behavior of the clay and the complexity of the dynamic clay-pile interaction,the seismic responses of the pile-pier system were generally nonlinear,regardless of the types of the earthquake and the factors considered.However,when plastic deformation of the bridge structure was induced,the bending curvatures of both pile and pier became evidently different among the three different sets of ground motions,indicating that the seismic damage evolution pattern of the pile-pier system was highly dependent on the earthquake type and the related frequency-spectrum characteristic.Among the three factors considered,PBA had the most significant influence on the seismic response of the whole system,and bridge gird mass and pile flexural rigidity respectively had relatively larger influences on the seismic responses of pier and pile,suggesting that the seismic responses of pier and pile were predominantly dependent on the inertial and kinematic forces during the seismic shakings,respectively.Furthermore,it was found that there was a critical pile flexural rigidity,beyond which the maximum pile curvature became relatively unchanged.The findings obtained from the present study may serve as a useful reference for evaluating the seismic performance of a similar clay-pile-pier system.