考虑横向惯性效应的非饱和土中单桩的竖向动力响应

基于非饱和土的动力控制方程,考虑横向惯性效应,建立了三相非饱和介质中嵌岩桩的竖向动力响应连续介质模型,对桩侧非饱和土的动力控制方程进行Laplace变换,在频域内,通过引入势函数、算子分解等手段对控制方程进行解析,得到了桩侧土体剪应力及竖向振动位移的表达式.结合桩基的竖向振动方程及桩–土接触面的连续性条件,使桩土耦合振动系统得以解答,最终在频域内得到了桩顶复刚度、导纳、桩–土系统振动位移及应力的解析解,借助Laplace逆变换得到了半正弦激励载荷下桩顶的速度时程曲线.最后,通过算例分析验证了计算结果的准确性,分析了横向惯性、泊松比、饱和度、长径比、桩土模量比等因素对桩基动力响应的影响.结果表明:(1)单桩动刚度、阻尼、导纳等变量随频率变化发生周期性振荡,在桩基各阶固有频率处发生共振;(2)泊松比、饱和度、长径比、桩土模量比等因素对桩基的动力响应有较大影响,且频率越大,影响越明显;(3)泊松比越大,单桩动刚度、阻尼、导纳的波动幅值及对应的频率越小,桩顶时程曲线中的桩底反射信号越弱;(4)饱和度越大,对应各动力响应的波动幅值越大,且桩底反射信号的波峰越大.     

饱和粘弹性土层中悬浮桩的纵向振动

基于不可压饱和粘弹性多孔介质理论和一维杆件模型,利用虚拟土桩的概念,研究了不可缮压饱和粘弹性土层中悬浮桩纵向振动的动力刚度.通过势函数解耦和三角级数解法,得到了悬浮桩纵向谐调振动的半解析解和桩头动力刚度.在此基础上通过数值计算,研究了物性参数、几何参数等对刚度的影响,结果表明:悬浮桩纵向振动的动力刚度受桩长、土的软硬程度、激振频率等的影响,对比了细长桩和短粗桩的响应差异,这些结果可为悬浮桩-土相互作用的数值分析和工程设计提供参考.     

黏弹性土层中单桩纵向振动的对比分析

在频率域内研究了黏弹性土层中端承桩纵向振动的动力特性.将土骨架视为具有分数阶导数本构关系的黏弹性体,基于黏弹性理论,采用平面应变模型给出了分数阶导数黏弹性土层的动力阻抗.考虑桩纵向振动时的横向惯性效应,将桩等效为Rayleigh-Love杆,得到了桩头动力复刚度和导纳的解析表达式.通过数值计算,分析了不同模型土条件下桩头动刚度因子和阻尼随激励频率的动力响应.同时,研究了Rayleigh-Love和Euler-Bernoulli两种模型桩动力特性的差异.分析了桩-土界面连续性模型和相对滑移模型对黏弹性土层中桩纵向振动的影响.结果表明:1随着阶数和材料参数比的增加,桩头刚度因子和阻尼明显减小;2对于大直径桩,随着外荷载激励频率的增加,桩横向效应对刚度因子和阻尼有显著影响.3连续性模型条件下桩头的刚度因子和阻尼在共振时的振幅小于相对滑移模型条件.     

梁-桩-土竖向耦合振动特性分析

针对桩与连续梁组合这一基础形式,研究了梁—桩—土竖向耦合振动特性. 首先,将桩假设为一维黏弹性杆件,采用平面应变模型模拟桩侧成层土对桩的动力作用. 同时,桩顶对梁的支撑简化为竖向点支撑. 然后,分别根据一维杆件纵向振动理论和Timoshenko梁理论给出桩和梁的竖向振动控制方程并求解,进而借助离散傅里叶逆变换获得在梁上瞬时激振下梁和土层以上桩段的时域响应半解析解. 通过与有限元模拟结果对比,验证了解的合理性. 在此基础上,讨论了梁几何参数和桩身缺陷对梁—桩—土竖向耦合振动特性的影响.      

分数导数粘弹性模型描述的土中管桩竖向振动的频域解

在分数导数粘弹性本构模型的基础上综合考虑桩周土和桩芯土的平衡方程和几何方程建立了桩周土和桩芯土的竖向运动的控制方程．在频率域内利用分离变量法和分数导数的性质求解了桩周土和桩芯土竖向振动控制方程．考虑管桩与桩周土、管桩与桩芯土的边界连续性条件以及三角函数的正交性得到了分数导数粘弹性模型描述的土中管桩的竖向振动,通过数值分析研究了管桩和土体模型参数和几何参数对管桩的桩顶复刚度的影响规律．结果显示：桩芯土本构模型的分数导数的阶数对管桩竖向振动的影响较桩周土本构模型的阶数要小,且与频率有一定关系;桩芯土与桩周土的模型参数比τ1 和τ2 对等效阻尼的影响较对刚度因子的影响要大;管桩桩周和桩芯的直径比d 越小,管桩复刚度的实部和虚部就越大;土体力学性能对管桩竖向振动的影响要比管桩桩身力学性能的影响小．     

高层建筑风振时地基土材料阻尼比取值范围的初探

高层建筑风振响应分析中考虑土与结构相互作用后地基土的材料阻尼比幅值是多少,这是考虑土与结构相互作用对高层建筑结构风振响应有利还是不利的关键问题.通过对无桩基桩架-剪力墙结构的顺风向风振响应分析,发现考虑土-结构相互作用时结构基础底面土体的动剪应变幅值处于弹性应变范围,根据土动力学中土的动剪应变与土的阻尼比的关系,土体的材料阻尼比较小,其值不超过0.03,据此计算出相应的结构最大层间位移和结构顶层的最大加速度比将地基视为刚性的结构有较大增长;因此,在高层结构风振响应分析中应考虑土-结构相互作用的影响.文章通过实例分析给出无桩基高层建筑土体材料阻尼比的建议值,该研究为有桩基高层建筑的土体材料阻尼比的计算提供了借鉴.     

盾构机下穿桩基施工对单桩承载力影响的数值研究

在大型有限差分软件FLAC3D平台上进行二次开发,利用内嵌FISH语言编程,对盾构隧道动态施工过程中上部基桩承载力的影响进行数值仿真模拟,模型考虑盾构前方土仓压力、盾尾同步注浆、注浆凝结和未凝结两种状态以及衬砌管片施加等施工参数。从桩侧摩阻力、桩端阻力等方面对盾构开挖过程中上部桩基承载力进行分析,以及土仓压力变化对承载力影响。研究结果表明：随着隧道开挖,桩侧摩阻力、桩端阻力发生复杂变化,桩底部出现负摩擦力,桩端轴力为拉力,对基桩竖向极限承载力有一定的影响;并且开挖面距桩轴线不同位置,土仓压力对基桩竖向极限承载力影响不同。     

端承粘弹性桩纵向振动的轴对称解析解

将桩和土体视为粘弹性介质,在频率域求得了桩轴对称纵向振动时动力响应的解析解,进而得到桩头复刚度的解析表达式.通过数值计算,给出了桩头动刚度因子和等效阻尼随激励频率的响应,考察了土层粘性、桩长径比、桩土模量比、桩泊松比和粘性等参数对桩头刚度因子和等效阻尼的影响.研究表明:由于考虑了桩的径向变形效应以及土层对桩的径向力作用...     

液化土体中单桩的稳定性分析

根据地震场地液化特征,将土层分为表层非液化层,中部液化层以及底部基础层,基于桩-土相互作用的Winkler模型,得到考虑轴力作用的液化土层中桩-土系统的频率方程.利用此频率方程,研究了液化土层中轴压单桩的稳定性问题,分析了各种参数对受压桩临界载荷的影响.数值结果表明:桩临界荷载与抗弯刚度成正比；液化初期时,随着液化土层...     

圆柱形储罐考虑桩土相互作用地震响应的简化分析

以Penzien集中质量模型为基础建立了某圆柱形锚固储罐考虑桩-土-罐相互作用及罐壁柔性的简化分析模型,采用有限元软件ABAQUS建立了该桩-土-罐力学模型,分析了其自振特性及在水平基岩波激励下的动力响应.分析结果得知,相比于刚性地基,考虑桩土相互作用的储液罐对流频率几乎不变,第一阶柔性脉动频率略有减小;在动力响应方面,储液罐基底剪力和力矩最大值略有减小,但其对流分量和刚性脉动分量最大值均有增大,所以本文建议在储液罐进行设计时应充分考虑桩土相互作用后储罐动力响应的有利和不利变化,从整体上把握来确保储液罐的抗震安全性.     

单桩的动刚度测试和数据处理方法的研究

依据地基动力特性测试规范（GB/T50269-97）,提出了测试桩基竖向动刚度的稳态-敲击激励综合试验法，应用实验模态分析技术识别了桩基的模态参数，成功地完成了陕西某热电厂试桩的动刚度测试。     

Vibration analysis of periodic cellular solids based on an effective couple-stress continuum model

Cellular solids are usually treated as homogeneous continuums with effective properties. Nevertheless, these mechanical properties depend strongly on the ratio of the specimen size to the cell size. These size effects may be accounted for according to preliminary static analysis of effective continuums based on couple-stress theory. In this paper an effective dynamic continuum model, based on couple-stress theory, is proposed to analyze the behavior of free vibrations of periodic cellular solids. In this continuum model, the effective mechanical constants of the effective continuum are deduced by an equivalent energy method. The cellular solid structure is then replaced with the equivalent couple-stress continuum with same overall dimension and shape. Moreover, the finite element formulation of the couple-stress continuum for the generalized eigenvalue analysis is developed to implement the free vibration analysis. The eigenfrequencies of the effective continuum are then obtained via the shear beam theory or the finite element method. A conventional finite element analysis by discretizing each cell of the cellular solids is also carried out to serve as an exact solution. Several structural cases are calculated to demonstrate the accuracy and effectiveness of the proposed continuum model. Good agreement on structural eigenfrequencies between the effective continuum solutions and the exact solutions shows that the proposed continuum model can accurately simulate the dynamic behavior of the cellular solids.     

A numerical model for the isolation of moving-load induced vibrations by pile rows embedded in layered porous media

A numerical model is developed to analyse the isolation of moving-load induced vibrations using pile rows embedded in a layered poroelastic half-space. Based on Biot’s theory and the transmission and reflection matrices (TRM) method, the free wave field solution for a moving load applied on the surface of a layered poroelastic half-space and the fundamental solution for an harmonic circular patch load are determined. Using Muki and Sternberg’s method, the second kind of frequency domain Fredholm integral equations for the dynamic interaction between pile rows and the layered poroelastic half-space are derived. The time domain solution is recovered via inverse Fourier transform in order to obtain the amplitude reduction ratio and thus assess the vibration isolation efficiency of pile rows. A special case of the present model shows good agreement with an existing solution. Numerical results of this study show that the speed of moving loads has an important influence on the isolation of vibrations by pile rows: the same pile rows can achieve better isolation efficiencies for higher speed loads than for lower speed loads. Pile rows embedded in a two-layered poroelastic half-space with a softer overlying layer usually generate better vibration isolation effects than those with a stiffer overlying layer. Finally, better isolation vibration may be realized by increasing the pile length and decreasing the net spacing between neighboring piles in a pile row.     

桩侧扰动分区饱和土中桩竖向振动特性研究

考虑沉桩过程中桩周土受到扰动产生软化或硬化的现象,建立了饱和土中径向分区桩竖向振动简化模型,通过分离变量方法,得到了桩竖向振动频域解析解和时域半解析解。分析了分区土对桩竖向振动特性的影响,数值计算结果表明,土层模量比、分区半径和桩底支承系数对桩竖向振动有较明显的影响。研究结果还表明,在较强的振动作用下,由于分区软化或强...     

夹层圆柱壳振动的谱有限元分析

从哈密顿变分原理获得夹层圆柱壳的运动微分方程和边界条件,将谱有限元法用于夹层圆柱壳结构,推导出不同周向模态下夹层圆柱壳单元的动力刚度矩阵和隐式动力形状函数,分析长径比、径厚比、芯表厚度比、芯表模量比对固有频率和模态损耗因子的影响.研究表明:小径厚比、大长径比及大芯表厚度比有利于抑制夹层圆柱壳振动.     

简化平面应变土体模型的精度研究及适用性分析

以严格平面应变解为基础,对简化平面应变土体模型即三个单元Voigt体串联模型的精度进行了研究.在各有关影响因素(如土体剪切模量、密度、剪切波速及结构物截面尺寸等)变化条件下,就两种模型土体作用于结构物交界面上的复刚度的实部和虚部分别进行了对比研究,全面地分析了两种模型复刚度的差异性,并进一步对桩顶受纵向激振力作用下的频域及时域动力响应进行了求解,研究了两种模型下桩顶动力响应的差别.结果表明,对土体复刚度而言,在大多数情况下,简化平面应变模型具有足够的精度,可以满足一般工程应用的需要;对桩顶频域响应和时域响应而言,简化平面应变模型已具有较高的精度,其误差可以忽略,具有很强的工程适用性,为该模型在相关领域的应用提供了坚实的理论依据.     

Numerical simulation and experimental study of Parallel Seismic test for piles

The theoretical capabilities of the Parallel Seismic (PS) test for determining the length of piles and characterizing possible defects are investigated in this paper. In deriving the theory, a correction factor is proposed in this paper to enhance the accuracy of the prediction. An axisymmetric finite element (FE) model was developed to carry out a series of parametric studies which included the effects of the pile length, the pile-to-borehole distance and the stiffness of surrounding soil. A miniature experiment using an aluminum bar embedded in epoxy prism was also designed and tested in the lab to verify the theory and to demonstrate the use of the correction factor. The results from the experiment and the numerical FE model were also compared to expose the potential of this nondestructive method to in situ application. The model was further modified to simulate the PS test on defective piles with axisymmetric necks and bulges. It can be concluded that the geometric configuration of a pile and the possible significant defect can be characterized with this nondestructive test.     

The dynamic behaviour of slender structures with cross-sectional cracks

A dynamic model for beams with cross-sectional cracks is discussed. It is shown that a crack can be represented by a consistent, static flexibility matrix. Two different methods for the determination of the flexibility matrix are discussed. If the static stress intensity factors are known, the flexibility matrix can be determined from an integration of these stress intensity factors. Alternatively, static finite element calculations can be used for the determination of the flexibility matrix. Both methods are demonstrated in the present paper. The mathematical model was applied to an edge-cracked cantilevered beam and the eigenfrequencies were determined for different crack lengths and crack positions. These results were compared to experimentally obtained eigenfrequencies. In the experiments, the cracks were modelled by sawing cuts. The theoretical results were, for all crack lengths, in excellent agreement with the experimental data. The dynamic stress intensity factor for a longitudinally vibrating, centrally cracked bar was determined as well. The results compared very well with dynamic finite element calculations. The crack closure effect was experimentally investigated for an edge-cracked beam with a fatigue crack. It was found that the eigenfrequencies decreased, as functions of crack length, at a much slower rate than in the case of an open crack.