深厚场地上特大桥墩-群桩-土相互作用体系地震反应特性的二维和三维分析
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摘要
本文以某特大型跨江大桥主墩为研究对象,对相应于抗震设防水准为1000a和2500a地震重现期的6条人工地震波,首先采用一维波动模型分析了主桥墩场地的地震动效应,进而,采用二维整体有限元法和三维子结构有限元法对特大桥墩-群桩-土相互作用体系的地震反应进行了数值计算,分析了桩体不同深度处的地震加速度反应峰值、反应谱特征,探讨了深厚软弱场地上特大桥墩-群桩-土动力相互作用效应对群桩地震反应的影响。结果表明:桥墩场地的深厚软弱覆盖层对输入地震波具有明显的滤波和放大效应;由于特大桥墩-群桩-土体动力相互作用的影响,二维和三维计算得到的桩体加速度反应峰值较同高程处的自由场加速度反应大,且两者的频谱特性有显著的差别,群桩加速度反应峰值的空间分布与输入地震波特性有很大关系,不同桩在同一高程处的加速度反应峰值可能相差20%以上;二维和三维地震反应分析结果之间存在一定的差距,群桩中桩体的加速度反应峰值平均相差10%～25%左右,但两种方法得到的加速度反应峰值沿高程的分布具有相似的规律性。
In this study, the seismic response of the site where the bridge pier is located is conducted firstly. In analysis, the soil layer is simplified to a semi-infinite horizontally layered soil deposits overlying a uniform half-space. The input earthquake wave is regarded as vertically propagating shear wave. Site seismic response analysis is performed by using SHAKE91 program. The filtration and amplification of the seismic waves on the site is analyzed. To investigate more precisely the seismic response of pier-group piles-soil interaction (PPSI) system, a two-dimensional integral finite element model and a three-dimensional substructure finite element model are respectively adopted to simulate the pier-group piles-soil interaction system, and the results obtained from 2-D and 3-D analysis methods are compared. Seismic response of the PPSI system due to six earthquake inputs according to two earthquake design levels of 1000 years and 2500 years earthquake return period is analyzed. In 2-D finite element model, 4-node plane isoparametric element is used to simulate the soil. The piles are simulated by beam element and a rigid block element represents the bridge pier. For soil elements, every node has two DOFs and every node of beam elements has three DOFs. 3-D analysis is performed by substructure method supplied in SASSI2000 program. In this method, piles are simulated by 3-D beam elements with three translational DOFs and three rotational DOFs per node. The pier is simulated by 3-D solid element with three translational DOFs per node. Compared with free field response analysis results, on the same elevation, the peak values of acceleration responses obtained from 2-D analysis and 3-D analysis are larger. The response spectrum characteristics obtained from free filed analysis and PPSI system analysis are different significantly. Although the peak values of acceleration responses obtained from 2-D analysis have a deviation of about 20 per cent from those obtained from 3-D analysis.The distribution of peak accelerations along the elevation is coincident. The 3-D analysis results indicating the space distribution of peak accelerations is related to input earthquake waves and at the same elevation, the peak accelerations of piles may have a deviation of 10～25 per cent.
In this study, the seismic response of the site where the bridge pier is located is conducted firstly. In analysis, the soil layer is simplified to a semi-infinite horizontally layered soil deposits overlying a uniform half-space. The input earthquake wave is regarded as vertically propagating shear wave. Site seismic response analysis is performed by using SHAKE91 program. The filtration and amplification of the seismic waves on the site is analyzed. To investigate more precisely the seismic response of pier-group piles-soil interaction (PPSI) system, a two-dimensional integral finite element model and a three-dimensional substructure finite element model are respectively adopted to simulate the pier-group piles-soil interaction system, and the results obtained from 2-D and 3-D analysis methods are compared. Seismic response of the PPSI system due to six earthquake inputs according to two earthquake design levels of 1000 years and 2500 years earthquake return period is analyzed. In 2-D finite element model, 4-node plane isoparametric element is used to simulate the soil. The piles are simulated by beam element and a rigid block element represents the bridge pier. For soil elements, every node has two DOFs and every node of beam elements has three DOFs. 3-D analysis is performed by substructure method supplied in SASSI2000 program. In this method, piles are simulated by 3-D beam elements with three translational DOFs and three rotational DOFs per node. The pier is simulated by 3-D solid element with three translational DOFs per node. Compared with free field response analysis results, on the same elevation, the peak values of acceleration responses obtained from 2-D analysis and 3-D analysis are larger. The response spectrum characteristics obtained from free filed analysis and PPSI system analysis are different significantly. Although the peak values of acceleration responses obtained from 2-D analysis have a deviation of about 20 per cent from those obtained from 3-D analysis.The distribution of peak accelerations along the elevation is coincident. The 3-D analysis results indicating the space distribution of peak accelerations is related to input earthquake waves and at the same elevation, the peak accelerations of piles may have a deviation of 10～25 per cent.
引文
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[2] 范立础.桥梁抗震[M].上海:同济大学出版社,1997.
[3] TakeshiK.CorrelationbetweennonlinearresponseofBridgePiersandNatural Period DependentspectrumIntensity[A]12WCEE[C],NewZealand,2000,ReferenceNO,0806.
[4] SmithW.Non reflectingPlaneBoundaryforWavePropagationProblems[J\〗.J.Comp.Phys,1973,15.
[5] 韦晓,范立础,王君杰.考虑桩-土-桥梁结构相互作用振动台试验研究[J].土木工程学报,2002,35(4):92-97.
[6] 屈爱平,高淑英.梁-墩-桩基的动力特性研究.西南交通大学学报[J].2001,36(6):641-644.
[7] 陈国兴.土体-结构体系地震性能整体分析方法与程序开发[J].南京建筑工程学院学报[J],1997(2):1-7.
[8] JohnLysmer,FarhangOstadan,ChihChengChin.SASSI2000:ASystemforAnalysisofSoil-StructureInteraction[R].DepartmentofCivilandEnvironmentalEngineering,UniversityofCalifornia,Berkeley,1999.
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