DYNAMIC ANIMATION OF STRESS MODES VIA THE INTEGRATED FORCE METHOD OF STRUCTURAL ANALYSIS

Dynamic animation of stresses and displacements, which complement each other, can be a useful tool in the analysis and design of structural components. At the present time only displacement-mode animation is available through the popular stiffness formulation. This paper attempts to complete this valuable visualization tool by augmenting stress-mode animation to the existing art. The reformulated method of forces, which in the literature is known as the Integrated Force Method (IFM), became the analyser of choice for the development of stress-mode animation because stresses are the primary unknowns of its dynamic analysis. Animation of stresses and displacements, which have been developed successfully through the IFM analyzers, is illustrated in several examples along with a brief introduction to IFM dynamic analysis. The usefulness of animation in design optimization is illustrated considering the spacer structure component of the International Space Station as an example. An overview of the integrated force method analysis code (IFM/ANALYSERS) is provided in the appendix. © 1997 John Wiley & Sons, Ltd.     

基于Kresling折纸构型的空间结构可控失稳模式研究

基于Kresling折纸构型的空间结构受压时会发生多种失稳模式。为了实现破坏模式可控的智能结构,该文采用有限质点法对基于Kresling折纸构型的空间结构的失稳模式进行研究,提出通过调节该结构材料、预应力和结构布置的方法对结构刚度和失稳模式进行控制。该文建立了采用有限质点法进行索、杆、膜结构计算的基本流程,通过对Kresling双稳态失稳模式的分析验证了分析方法的有效性。对Kresling索杆模型、索杆膜模型、嵌套模型的失稳模式进行了分析,研究了弹性模量、预应力水平和结构布置对结构失稳模式和结构应变能的影响。实现了双层Kresling模型的失稳模式控制,为设计破坏可控的智能结构提供了一种新的方法。     

Optimal design of joints in elastic structures

Summary The problem of optimal design of linear elastic beams and frames with unspecified location, stiffness and prestress of joints is considered. It is assumed that the structure is subjected to loads and initial distortions. Using the adjoint problems and their respective fields, sensitivity operators and optimality conditions are derived for an arbitrary integral objective functional expressed in terms of displacements, stresses or strains. Illustrative examples are presented.With 6 Figures     

Sizing design sensitivity analysis of non-linear structural systems. Part II: Numerical method

A numerical method is presented to carry out sizing design sensitivity calculations outside established finite element analysis codes, using postprocessing data only. Geometric as well as material non-linearities are treated. To demonstrate the accuracy of the proposed method, numerical results are presented for structural systems with linear elastic material, large displacements, large rotations and small strains. A distributed parameter approach to structural design sensitivity analysis is used to retain the continuum elasticity formulation throughout the derivation of design sensitivity results. Using this approach and an adjoint variable method, design sensitivity computations are carried out. For structural performance functionals stress and displacement are considered. It is shown that computations can be performed with the same computational effort as for sizing design sensitivity analysis of linear structural systems. Accurate design sensitivity results are obtained for both linear and non-linear structural systems without the uncertainty of numerical accuracy and high cost associated with the selection of finite difference perturbations. Also, the method does not require differentiation of element stiffness and mass matrices in conventional finite element models.     

连续曲线箱梁预应力效应分析

本文通过预应力曲梁微元体的平衡方程,将作用于微元体上的预应力以一组等效荷载来代替。然后根据薄壁曲梁空间翘曲单元的刚度矩阵和各类荷载作用下固端力计算公式［１］,应用矩阵位移法分析曲梁中的预应力效应。计算预应力损失时,考虑了反向摩擦影响。结合某工程实例计算了由预应力引起的结构内力和变形。     

适用瞬态响应计算的火箭结构动力学模型修正方法

大型火箭结构十分复杂,其连接型式多,存在多分支结构。分析火箭结构中存在弱刚度的内-外翻边连接、螺栓点连接等典型连接型式,基于元素型法,以这些部位刚度为修正参数,开展动力学模型修正方法研究。结果表明,采用的方法能保证修正后模型的物理意义和真实性,能够适用于瞬态外激励作用下的结构响应计算,特别是内力响应计算。修正后模型的各阶模态结果和试验吻合较好,且与实际试验中瞬态响应的实测结果相比,计算具有较高精度,能够满足工程需求。     

索穹顶结构局部断索分析

索穹顶结构是由索和杆为基本构成单元,通过预应力提供刚度的一类新型索杆空间结构体系。为考察结构中局部索(杆)破坏对整体结构的影响,以某体育馆索穹顶结构方案为背景,采用动力学方法模拟了各类杆件破断后其他杆件内力的变化和结构最终到达的平衡位置。通过观察局部索(杆)破坏是否引起其它索松弛从而造成较大节点变位来判断结构体系是否安全,通过观察局部索(杆)破坏是否引起其它索杆的强度破坏来判断结构是否连续倒塌,同时区分了不同位置杆件的安全等级。该研究对于该类结构的正常使用和维护有一定的借鉴意义。     

基于CFD/CSD耦合的结构几何非线性静气动弹性数值方法研究

柔性飞行器在气动力作用下会发生大变形,产生结构几何非线性,线性小变形方法难以获得准确的气动弹性分析结果。基于RANs的三维N-S流场控制方程耦合非线性结构静力学方程时域分析方法,用于考虑结构几何非线性的静气动弹性分析。该方法在结构静力学方程求解上采用非线性增量有限元方法进行迭代求解,考虑结构刚度矩阵随结构位形的变化,采用径向基函数方法实现气动/结构界面的数据交换和动网格变形。在建立某型宽体客机复材机翼三维有限元模型的基础上,对其静气动弹性进行了数值仿真,分析了线性结构和考虑结构几何非线性的结构在静气动弹性作用下翼面扭转、展向位移、垂向位移以及升力系数等物理量。算例结果表明,与线性结果相比,非线性结构由于结构几何非线性的影响,在展向和垂向变形上两者存在显著差异。为准确进行柔性结构的气动弹性分析,必须考虑结构几何非线性的影响。     

基于频率响应和统计理论的结构损伤识别研究

为了解决测量噪声下的损伤定位和定量识别问题,提出了基于测量位移的两阶段识别方法和基于统计理论的损伤敏感度分析方法。首先利用损伤刚度的改变特性和测量位移来确定损伤指标,再通过损伤指标的相关自由度变化来确定损伤单元,然后利用自由度缩聚的方法推导出损伤程度的定量计算公式。并利用统计理论来分析噪声对识别结果的影响,推导出损伤指标和损伤程度的噪声敏感性计算公式。数值仿真结果表明,基于测量位移的两阶段损伤识别方法不仅可以精确的识别出损伤位置和程度,而且识别效果受测量噪声的影响较小,该方法明显优于直接广义逆法。     

Simplified finite element mesh for dynamic analysis of a nuclear reactor building

The design of nuclear facilities requires dynamic analysis to evaluate their performance during an earthquake. In modal analysis, a method recommended for dynamic analysis by the Nuclear Regulatory Commission, the natural frequencies and mode shapes of the structure are commonly calculated using a stiffness matrix computed using the finite element method. A primary goal of this study was to design a finite element mesh, capable of modelling a reactor structure, with a minimum number of nodes and elements. This is necessary since analysis using a mesh fine enough to include all of the structural elements is not feasible because of exorbitant computer time and storage requirements. It was found that for lateral motion, a representative reactor structure could be accurately modelled using a simplified mesh with only one isoparametric quadratic quadrilateral element per floor. In this mesh essentially infinite springs must be placed between the masses at each floor level to prevent internal movements of the lumped masses within the quadratic quadrilateral elements. It was also found that a structure with rigid heavy bottom floors and a light top floor can be subjected to large displacements in the top floor during earthquakes. It was concluded that the properties of that light top floor (especially shear area) controls the value of the fundamental frequency of the building.     

三向类网架结构弯曲和振动分析的分解刚度法

把三向类网架结构简化为夹层板,采用考虑剪切变形的具有三个广义位移的平板弯曲理论进行分析。基于分解刚度思想提出了三向类网架(三角锥网架、三向网架)静力分析和固有振动分析的方法,给出了简便实用的计算公式。通过有限元法的验证,表明了分解刚度法作为一种简化的计算方法,其精度是比较高的,绝大多数的误差都小于5%。可以应用于工程结构设计,便于快速得出网架结构挠度和内力的计算结果及分布规律。此外,与其它的计算方法相比其计算公式大大简化了。     

Robust topology optimization of compliant mechanisms with uncertainties in output stiffness

This work addresses the topology optimization approach to design robust compliant mechanisms with respect to uncertainties in the output stiffness, when compared to the traditional deterministic approach. To this end, two formulations are proposed: probabilistic and nonprobabilistic. The probabilistic formulation minimizes a joint objective function of expected output displacement plus a measure of its standard deviations, for given statistical distribution of the output stiffness. The nonprobabilistic formulation is written as minimization of a joint function of the median of output displacements, plus the width of the intervals that contains the extreme values of the output displacements, for a given interval of output stiffness. The Monte Carlo simulation method is used to evaluate expected values and standard deviations of output displacements in the probabilistic formulation and to assess results obtained with the deterministic approach. It is shown that both formulations lead to designs where output displacements are less sensitive to variations of output stiffness when compared to the traditional deterministic approach. Furthermore, as an additional benefit, it is observed that large variations of output stiffness can hinder the appearance of one-node connected hinges, usually found in the deterministic design of compliant mechanisms.     

Multi-objective design optimization of variable stiffness composite cylinders

The bending-induced buckling improvement in a variable stiffness (VS) composite cylinder (made by fiber steering) is studied. For such a cylinder, the effect of the variation of the direction of the load on its buckling performance of the cylinder is also examined. Compromise programming, as a multi-objective optimization method, is used to design for buckling of the VS cylinder subjected to bending load in either of the two opposite directions. Different combinations of weight factors for the structural performance in the two opposite directions were also applied to obtain the Pareto frontier as the main decision making tool for the designers in a multi-objective design problem.     

基于组合单元的混凝土结构徐变分析方法

对混凝土徐变的研究现状和数值计算方法做出评价,提出了混凝土结构徐变计算的空间有限元分析方法。以组合单元为基础,建立混凝土徐变的三维计算模型:将结构的变形、内力状态、结构形式及约束情况均沿时间轴进行划分,再根据结构在每个时间点上各参数的变化值,推导出了混凝土徐变计算的递推公式;又基于虚功原理建立混凝土徐变刚度矩阵,绕过应力与应变,直接通过位移增量计算自由徐变应变增量的等效荷载,提高了徐变数值计算精度;组合单元是对原有实体等参元的继承和拓展,能够分别考虑钢筋和混凝土各自对单元刚度矩阵的贡献,而两者又通过自由度变换的方法有机的结合在一起,构成一种可专门用于模拟钢筋混凝土结构的单元形式。实例分析结果表明:用组合单元做混凝土徐变分析,能够计入钢筋对徐变的阻碍作用,解决在钢筋和混凝土之间的应力重分布问题,更准确的计算出结构变形和内力变化。     

预应力内荷载的新计算方法-直接内载法

预应力内荷载是预应力束对结构作用产生的内荷载,是预应力结构设计和理论分析时需要计算的重要参数.依据预应力对结构作用的形态,得出预应力对结构的作用是由预应力束的法向反弹力、预应力束端部的锚固力和预应力束对构件的摩擦力组成.根据这个结论,提出直接内载法计算作用在结构上的预应力内荷载,推导了预应力摩擦应力损失方程,并证明了预应力内荷载的自平衡性,分析了它与等效荷载法的区别和一致性,得出了直接内载法能揭示预应力荷载的分布规律,说明了直接内载法是对目前普遍使用计算预应力内荷载的等效荷载法的修正和补充.同时,通过试验实测和有限元ANSYS结构分析程序进行数值模拟分析,得到的理论计算结果与实测结果相互吻合,验证了本文的结论.     

三套管自复位屈曲约束支撑滞回性能研究

在结构中合理安装自复位屈曲约束支撑可以有效地减少结构损伤和结构残余变形。该文提出了一种新型三套管自复位屈曲约束支撑,对其滞回性能进行实验研究。通过拟静力试验考察了初始预应力,芯材管有效横截面积对支撑耗能能力以及控制残余变形效果的影响。采用有限元方法对支撑的滞回性能进行了参数分析,并通过理论推导给出了支撑的刚度和承载力计算公式。研究结果表明合理设计的三套管自复位屈曲约束支撑具有良好的耗能能力和自复位性能,有限元分析结果与试验结果吻合较好,理论推导得到的刚度和承载力计算公式能准确给出支撑滞回曲线上的特征点。     

预应力内荷载的新计算方法—直接内载法

预应力内荷载是预应力束对结构作用产生的内荷载,是预应力结构设计和理论分析时需要计算的重要参数。依据预应力对结构作用的形态,得出预应力对结构的作用是由预应力束的法向反弹力、预应力束端部的锚固力和预应力束对构件的摩擦力组成。根据这个结论,提出直接内载法计算作用在结构上的预应力内荷载,推导了预应力摩擦应力损失方程,并证明了预应力内荷载的自平衡性,分析了它与等效荷载法的区别和一致性,得出了直接内载法能揭示预应力荷载的分布规律,说明了直接内载法是对目前普遍使用计算预应力内荷载的等效荷载法的修正和补充。同时,通过试验实测和有限元ANSYS结构分析程序进行数值模拟分析,得到的理论计算结果与实测结果相互吻合,验证了本文的结论。     

An efficient analysis of repetitive structures generated by graph products

In this paper, an efficient method is developed for the analysis of regular structures. A structure is called regular if its model can be formed by a graph product. Here, instead of direct solution of the equations corresponding to a regular structure or finding the inverse of the stiffness matrix directly, modal analysis is used, and eigenvectors are employed for calculating the displacements and then internal forces of the structures. For this purpose, first an efficient method is developed for calculating the eigenvectors of the product graphs, and then a method is presented for using these eigenvectors for evaluating the displacements of a structure. Copyright © 2010 John Wiley & Sons, Ltd.     

Pre-buckling imperfection sensitivity of pultruded FRP profiles

This paper presents a geometrically non-linear one-dimensional model suitable for analyzing thin-walled fiber-reinforced polymer profiles, which accounts for the effect of manufacturing imperfections. The kinematic model is developed under the hypotheses of small strains and moderately large rotations of the cross-sections, and is able to take into consideration the contribution of shear strains and the effects related to warping displacements.The aim of the study is to develop a proper tool to analyze the pre-buckling behavior of such beams, since current approaches based on two-dimensional finite element method analysis demand significant computational efforts to be applied to real structures. The numerical results underline the effectiveness of the proposed mechanical model in analyzing case studies of technical interest in Civil Engineering, and the relevant influence of geometrical imperfections on the structural performance of FRP components with regard to serviceability design requirements.     

Simultaneous nonlinear structural analysis and design

Optimization techniques are increasingly being used for performing nonlinear structural analysis. Under these circumstances the structural design problem can be viewed as a nested optimization problem. The present paper suggests that there are computational benefits to treating this nested problem as a large single optimization problem. That is, the response variables (such as displacements) and the structural parameters are all treated as design variables in a unified formulation which performs simultaneously the design and analysis. Three truss examples are used for the demonstration comparing two nested optimization procedures with two computational procedures for the simultaneous solution. The examples show that the simultaneous approach is competitive with the more traditional nested approach.