上拔荷载作用下等截面桩的承载性状数值模拟

利用轴对称模型对等截面桩进行承受上拔载荷作用的有限元模拟.通过数值模拟研究桩的承载性状,包括桩的上拔位移、载荷传递特性和桩侧摩阻力的发展,并分析桩底部土体的局部应变以及土体位移场的分布.研究发现,在达到极限上拔载荷时,桩端周围土体出现剪应变方向相反的两个区域并形成剪切带,桩周土体的位移场逐渐呈倒锥形分布.     

离散元-有限差分跨尺度耦合下桩与土体稳定性细观数值分析

为准确分析桩体宏观变形与周围土颗粒细观力学行为,采用离散元-有限差分的跨尺度耦合进行桩和土体接触过程中的稳定性分析,研究桩下沉过程中周围土体的细观变形、应力分布和桩体自身变化情况,通过FLAC3D建立桩和外部土体有限差分网格单元,对桩周围侧土体应用PFC3D离散元建立土颗粒微观结构模型.研究结果表明:离散元与有限差分耦合方案能够模拟桩周围土体细观力学行为;外部区域土体位移场呈包裹式C形对称分布,应力场以竖直应力为主导,离散元土颗粒接触力链网格以树根状向周围递减削弱分布;桩弹性单元体下沉时,应力场和位移场均有分层现象,应力场以压应力为主,但存在局部拉应力即反弹现象.     

沿海围垦区建筑荷载下邻近超长桩三维模型

沿海地区软弱土层深厚，工程事故率高，因此对该地区桩土相互作用研究意义重大。采用有限元软件ABAQUS按实际工程的参数和设计工况要求对建筑荷载作用对邻近超长桩受力影响进行了三维弹塑性数值分析。得出了建筑荷载作用下桩身侧向位移、桩项间距、桩顶沉降和桩侧摩阻力的规律，为该工程提供了参考。结果表明，在三维空间内桩身侧向位移可分解为X向和Y向两个分量（设桩长方向为Z分量），据此可以确定建筑荷载作用下桩身侧向位移大小和方向的改变并得到桩项问距的变化情况，对研究桩基上部结构受力性状有一定帮助。由于是超长桩，桩身与土体接触面积大，在实际工程中应考虑负摩阻力的影响。结合计算工况，建筑荷载引起的桩侧负摩阻力将达到单桩设计承载力的20％甚至更多。     

桩承加筋路堤中褥垫层作用分析

桩承加筋路堤中褥垫层是重要组成部分,对桩土应力以及沉降等的调整起了十分关键的作用。利用有限元对路堤荷载下桩承加筋路堤中的单桩以及加固范围内的土体进行了数值模拟。模拟结果表明,在桩承加筋路堤中的桩体受力状态有别于一般刚性基础下的桩体,受正摩阻力和负摩阻力共同作用,并不断调整。通过分析认为,褥垫层的模量和厚度对桩土应力调整以及路堤沉降等均会产生影响,随着褥垫层模量、厚度,会导致更多的荷载转移到桩体上,桩土应力比增加,桩土差异沉降略有减小,最终使得路堤沉降也是减小的。通过总结认为,改善褥垫层性质,加强其对桩顶局部压入作用的限制是保证桩体承担较多荷载的关键。     

往复载荷作用下超大型载荷板现场试验研究

往复荷载作用下土体既产生弹性变形,同时又产生永久变形,这种累计的永久变形无疑是研究的重点,而关于往复荷载作用下土体变形特性的研究较少,鉴于此,笔者开展了往复载荷作用下超大型基础板沉降现场试验研究,通过对不同位置、不同深度土体中孔隙水压力、分层沉降和侧向位移的连续监测,研究土体中孔隙水压力、分层沉降和侧向位移对往复荷载的响应特征,得出了土体中孔隙水压力的增长和消散,以及分层沉降和侧向位移随载荷变化的规律,为工程设计的应用提供了依据。     

水平单调载荷作用下的桩基桥墩滞回特性

为研究桩基桥墩-地基系统的非线性性能,根据模型与原型的物理相似关系制作1∶5比例的桩基桥墩模型. 采用力-位移混合控制加载的拟静力试验方法,通过在墩顶施加水平单调增加载荷,得到墩顶水平载荷下桩基桥墩的载荷-位移滞回曲线、骨架曲线和滞回特性. 用非线性弹簧单元模拟土体、用梁单元模拟桩和桥墩,建立模型桥墩的计算模型. 计算模型的骨架曲线与试验模型的骨架曲线吻合较好,表明采用非线性弹簧单元和梁单元分别模拟土体和桩是可行的,可以为考虑土-结构相互作用时的桥梁抗震分析提供参考依据.     

基于交通荷载作用的道路地下塌陷模型研究

随着社会的不断发展,因交通作用导致的道路地下塌陷问题已经屡见不鲜.为此,分别从交通荷载特性和路面动力特性两方面进行分析研究,通过路面弹性模量与路面应变的关系模拟道路地面结构层中的路面效应.然后把路面结构划分成有限个单元,并将其进行离散化处理,通过对位移、速度和加速度三个矢量进行不断的迭代处理,直到达到收敛状态.再次通过有限元模型的计算,分析研究交通工具行驶过程荷载的不平稳增长对路面结构疲劳损伤的影响.最后选取5种常见的不同轴载车辆,分别从位移和应力两方面进行分析,研究其对道路路面上施加的静荷载稳定性以及路面结构稳定性的影响.通过对道路路面上监测节点的测试,证明了上述方法可以准确预测出车辆的非平稳增长对路面所造成的疲劳损伤,同时说明了对交通荷载的非平稳增长加以考虑是非常必要的.     

城市轨道爆破引发邻近建筑振动效应模拟研究

爆破技术凭借经济和高效的特点被应用在城市轨道建设中,但其所产生的振动效应会引起邻近建筑物质点位移、速度和频率等变化,从而产生安全隐患.针对城市轨道爆破过程中引发的邻近建筑物振动效应进行模拟研究.首先对邻近建筑物和城区轨道构建有限元模型,通过气体压力与体积变化的状态方程Jone-Wikin-Lee对炸药进行描述.通过数值模拟的计算结果,了解建筑物因爆破导致的振动效应,依据爆破理论和爆破荷载的经验,求解荷载峰值,通过微分方程的稳态解,结合折算荷载的幅值求出广义坐标向量,进而求解出建筑物在振动过程中的实际振动位移.根据爆破振动波形施加荷载,对质点振动速度进行观察,由微分方程得到质点的振动位移和振动速度曲线.通过回归研究,对爆破点随机监测5次记录,分析了爆破所产生的振动波在邻近建筑物周围的传播规律,结合振动波衰减方程,证明了通过质点的振动速度来控制建筑物的安全性较为合理,上述研究结果对城市轨道建设过程中的爆破施工技术具有一定的参考价值.     

城市轨道爆破引发邻近建筑振动效应模拟研究

爆破技术凭借经济和高效的特点被应用在城市轨道建设中,但其所产生的振动效应会引起邻近建筑物质点位移、速度和频率等变化,从而产生安全隐患.针对城市轨道爆破过程中引发的邻近建筑物振动效应进行模拟研究.首先对邻近建筑物和城区轨道构建有限元模型,通过气体压力与体积变化的状态方程Jone-Wikin-Lee对炸药进行描述.通过数值模拟的计算结果,了解建筑物因爆破导致的振动效应,依据爆破理论和爆破荷载的经验,求解荷载峰值,通过微分方程的稳态解,结合折算荷载的幅值求出广义坐标向量,进而求解出建筑物在振动过程中的实际振动位移.根据爆破振动波形施加荷载,对质点振动速度进行观察,由微分方程得到质点的振动位移和振动速度曲线.通过回归研究,对爆破点随机监测5次记录,分析了爆破所产生的振动波在邻近建筑物周围的传播规律,结合振动波衰减方程,证明了通过质点的振动速度来控制建筑物的安全性较为合理,上述研究结果对城市轨道建设过程中的爆破施工技术具有一定的参考价值.     

悬浮隧道基于弹性边界的力学模型与移动荷载响应分析

悬浮隧道是一种创新型的水中交通结构,这种悬浮于水中的结构在考虑复杂边界条件下的力学建模及交通荷载引起的结构响应值得深入研究.本文将跨度范围内由多段锚索支撑的悬浮隧道视为弹性支撑梁,同时将两端的复杂边界条件考虑为具有不同约束刚度的竖向和转动弹性支撑,流体荷载由Morison方程考虑,建立了悬浮隧道在任意荷载作用下的动力学模型及考虑自振特性的特征方程.研究了跨内支撑刚度与边界约束参数多种组合条件下的频率、模态分布特征,得到了相应的敏感区间.同时,以某型号高铁列车为背景并将其考虑为一列移动集中力,研究了悬浮隧道结构关键位置的荷载响应与弹性支撑刚度之间的关系,结果表明跨内支撑刚度、边界约束刚度均对竖向位移存在显著影响,整体上约束刚度越大,相应的位移越小.     

Torsional actuator based on mechanically amplified shear piezoelectric response

A torsional actuator, based on the concept of mechanical amplification of piezoelectric shear strain and capable of generating large angular displacement, was proposed and studied experimentally. The actuator is a tube consisting of an even number of the segments poled along the length, which are adhesively bonded together, and the joints act as electrodes to apply the driving voltage. The experimental data measured on the prototype actuators (i) prove the proposed concept of mechanical amplification of small piezoelectric shear strain to generate large torsional motion, (ii) show that the actuator functions well both without load and under the torque load and (iii) demonstrate that the actuator can operate continuously for a long period of time without drop in its performance. Also, the results demonstrated that the proposed torsional actuator is capable of producing both large torque and large angular displacement in a compact package, sufficient to meet many smart structures requirements, and can be tailored for a variety of application requirements. Finally, one of the obvious advantages of the present design of the actuator is its simplicity: the piezoelectric shear strain is transformed directly into the angular displacement, whereas in the previously reported actuators, the conversion mechanism into the torsional motion was rather complicated which thus required a sophisticated design of the whole system.     

Postbuckling analysis of functionally graded plates subject to compressive and thermal loads

Postbuckling analysis of functionally graded ceramic–metal plates under edge compression and temperature field conditions is presented using the element-free kp-Ritz method. The first-order shear deformation plate theory is employed to account for the transverse shear strains, and the von Kármán-type nonlinear strain–displacement relationship is adopted. The effective material properties of the functionally graded plates are assumed to vary through their thickness direction according to the power-law distribution of the volume fractions of the constituents. The displacement fields are approximated in terms of a set of mesh-free kernel particle functions. Bending stiffness is estimated using a stabilised conforming nodal integration approach, and, to eliminate the membrane and shear locking effects for thin plates, the shear and membrane terms are evaluated using a direct nodal integration technique. The solutions are obtained using the arc–length iterative algorithm in combination with the modified Newton–Raphson method. The effects of the volume fraction exponent, boundary conditions and temperature distribution on postbuckling behaviour are examined.     

Plane strain sliding contact of multilayer magnetic storage thin-films using the finite element method

The sliding contact or scratch behavior of multi-layer thin-films such as those found in magnetic storage disks has been studied using the finite element method. A rigid cylinder sliding over a multilayered thin-film half-space was implemented to simulate the contact between a feature of the recording slider (such as the protrusion on the trailing edge of the slider, which is part of the thermal flying-height control, TFC) and the magnetic storage multilayer disk. The effects of different parameters such as normal load, friction coefficient and TFC radius on the von Mises, shear and principal stresses in the multilayer system were analyzed. Results showed that under sliding conditions, for a given normal load, the friction coefficient influences the location and magnitude of the plastic strain in the multilayer system. Repeated sliding contact was also performed to characterize its effect on the stress and strain behavior under various loading conditions and investigate shakedown behavior.     

Pattern optimization of eccentrically loaded multi-fastener joints

For structural joints subject to dynamic loading, the fatigue strength is controlled by local stresses. For steel plate structures joined with fasteners, fatigue is governed by local friction and slip near the fasteners. As a working hypothesis for this study, a “weakest link” approach has been adopted. An optimum fastener pattern is attained by reducing the shear load in the most severely loaded fastener in the group. In this study, a typical eccentric multi-fastener bracket-to-beam joint was studied using constrained geometric optimization. Alternate assumptions concerning the distribution of the direct shear force between fasteners in a group were assessed. An analytical expression for non-uniform direct shear force resulted in an optimized fastener pattern with approximately 20% lower von Mises equivalent strain. The comparative finite element method based topology optimization analysis resulted in a contour, which was in close agreement with the fastener pattern attained analytically by exploiting geometry optimization.     

Degenerated plate-shell elements with refined transverse shear strains

Although traditional three-dimensional plate-shell elements relax the constraint so that normal cross-sections remain normal to the neutral plane during transverse shear deformation, the section is still constrained to remain plane. The work reported here relaxes this constraint by introducing shape functions across the thickness to approximate the transverse shear strain field in the thickness direction. These shape functions are treated in the manner of generalized angles undergoing small deformations. They are added as new degrees of freedom to the ordinary displacement field of the degenerated shell elements. The field is still able to simulate large deformation behavior of the element. Each shape function yields two independent variables, one in each direction. In this work, two types of shape functions are proposed allowing a parabolic transverse shear strain, as well as an unsymmetric transverse shear strain distribution in the thickness direction. These two modes of deformation are particularly important in the case of diffused material failure in shell structures. Displacement field representation and finite element formulation based on a total Lagrangian approach are given. Examples are presented demonstrating the applicability of this element in a variety of problems.     

填埋场典型复合衬垫系统单剪试验数值模拟

为研究填埋场典型复合衬垫系统接触面间的滑动特性,建立典型复合衬垫系统单剪试验的有限元模型,用ANSYS对其进行数值仿真分析.研究复合衬垫不同界面在不同法向力下的滑动特征;绘制衬垫系统不同界面节点的应力 位移关系曲线,得到不同法向力作用下各个界面不同位置点沿剪切位移方向和垂直剪切位移方向的切应力变化规律;分析接触面内节点的剪切位移随载荷步的变化特征.结果表明：随法向力的增加,滑动界面将发生转移;在低法向力（56 kPa）条件下,土工网 土工膜界面发生滑动;在高法向压力条件下（压力超过560 kPa）,土工膜 黏土界面发生滑动.     

Analysis of plates using isoparametric quadratic element—Shear,reaction, patch loading and some convergence studies

The derivation of reactions at the supported edge of a plate using isoparametric quadratic element has been presented. Straight and curved plates have been analysed to study the influence of shear rigidity, boundary conditions and support geometry on the value of shears and reactions at the edges. It has been found that convergence is slower for shear than for moments. The explicit value of the element loading matrix due to a patch load has been indicated, and plates having different aspect ratios due to a central patch load have been studied.     

Monomial test: Testing the flexural behavior of the degenerated shell element

Shear locking of the degenerated shell element is a result of the development of spurious shear strain by the element, when subjected to high order Kirchhoff displacement fields. The shear locking phenomenon is analyzed in this paper using an analytical test. The effects of the integration scheme, order of the element, order of the modeled Kirchhoff field and the element distortion are analyzed explicitly and quantitatively for the four-node, eight-node and nine-node degenerated shell elements.