Behaviour of welded aluminium T-stub joints under monotonic loading

This paper deals with the behaviour of aluminium alloy T-stub joints subjected to monotonic tensile loads. In particular, a numerical model developed by means of the non linear code ABAQUS has been accurately calibrated on the basis of previous experimental test results related to 26 specimens, characterised by different geometry and connection type. In particular, four different geometries (by varying geometrical dimensions, plate thickness, number and location of the bolts), three aluminium alloys as base material and three types of bolts (including both aluminium and steel bolts) have been taken into consideration. Such parameters determine the modification of the connection response in terms of initial stiffness, ultimate strength and deformation capacity. The comparison with the experimental results shows that the proposed model is able to accurately reproduce the behaviour of the joint in all examined cases. Test results have been also compared with the ones derived from the application of the calculation method provided in the final version of Eurocode 9 (EN-1999-1-1), showing the reliability of the codified procedure, also in relation to a direct evaluation of the connection effective width (b_eff), as it is evidenced by the developed numerical simulations.     

Buckling of prismatic structures under biaxial loading

This work presents an analysis and numerical results for the buckling of longitudinally stiffened prismatic structures that consist of an assemblage of flat plate and cylindrical panel components. The applied loading is assumed to induce uniform in-plane biaxial stresses, and all types of buckling modes (general, local, and coupled) are automatically accounted for in the analysis. Previous analyses of such problems by the exact (Wittrick) and approximate (Cheung) finite strip methods are based on the assumption that the structure is simply supported on the transverse ends by diaphragms (SS3 classical simple supports). The present work extends these analyses to include the case of completely clamped transverse ends (CC4), as well as simply supported ends. A one term Galerkin method is used to replace the governing partial differential equations of equilibrium for each component by ordinary differential equations in terms of the transverse coordinate, and the ordinary equations are solved exactly. The principle of virtual work is employed to obtain the stiffness matrix for each component. Numerial results are presented for a wide variety of CC4 and SS3 prismatic structures. These results compare favorably with available solutions.     

Topological design of modular structures under arbitrary loading

A numerical method for the topological design of periodic continuous domains under general loading is presented. Both the analysis and the design are defined over a single cell. Confining the analysis to the repetitive unit is obtained by the representative cell method which by means of the discrete Fourier transform reduces the original problem to a boundary value problem defined over one module, the representative cell. The repeating module is then meshed into a dense grid of finite elements and solved by finite element analysis. The technique is combined with topology optimization of infinite spatially periodic structures under arbitrary static loading. Minimum compliance structures under a constant volume of material are obtained by using the densities of material as design variables and by satisfying a classical optimality criterion which is generalized to encompass periodic structures. The method is illustrated with the design of an infinite strip possessing 1D translational symmetry and a cyclic structure under a tangential point force. A parametric study presents the evolution of the solution as a function of the aspect ratio of the representative cell.     

Optimal design of layered structures under dynamic loading

A direct search procedure is presented to treat problems of optimization of layered structures subjected to dynamic loading conditions. The procedure combines the basic idea of the complex method of Box to locate the next trial point, with a local direct pattern search of Hooke and Jeeves to insure the continuous accelerative movement toward the optimum. The method is applied to two problems of optimal design for minimum tensile stress at the interfaces in layered structures under time-harmonic loads and transient loads, respectively.     

Optimal design of rigid-plastic structures under dynamic loading

The optimal design of rigid-plastic beams with piecewise constant thickness under impulsive or dynamic pressure loading is considered. Such dimensions of the structure for which the beam of constant volume attains a minimum of local or mean residual deflection are sought. The equations of motion are integrated (i) exactly and (ii) approximately by making use of the method of mode form solutions. The method of solution is applied also to reinforced beams.The stiffness of the beam can be increased if we provide it with additional supports. The locations of such supports must be chosen so as to minimize the global compliance of the beam. Optimality conditions for this problem are discussed.     

Nonlinear analysis of concrete cylinder structures under hydrostatic loading

The paper reports the recent results of four aspects of nonlinear analysis of concrete cylinder structures under hydrostatic loading—(1) further development of concrete constitutive relationships based on a plasticity formulation; (2) kinematics of failure mode for crushing and cracking concrete; (3) implementing the results in the form of a subroutine suitable for incorporation in a large finite-element analysis computer code (NONSAP program); and (4) studying the behaviour and strength of concrete cylindrical shell structures under hydrostatic pressure.The computer model developed includes the nonlinear displacement and material behaviour which is capable of performing parametric studies on the influence of geometric imperfections, variable restrained end supports, nonlinear nature of stress-strain-fracture response, and the non-conservative nature of hydrostatic loading. Using this computer model, comparisons have been made with the results of tests on actual cylinders, providing the needed confirmation of the validity of the model.     

Explicit exact analysis of infinite periodic structures under general loading

This paper deals with the explicit analysis of infinite periodic structures under arbitrary loadings. In the context or structural stiffness optimization, with its inherent problem of multiple reanalysis, the purpose is to obtain expressions for the stress resultants anywhere in the infinite structure as an explicit function of the stiffnesses of the elements. Following the method of the representative cell the analysis of an infinite structures is reduced to the analysis of single module under transformed loading and boundary conditions by using the discrete Fourier transform. This produces the equilibrium, strain-displacements and constitutive equations in terms of complex-valued displacements, generalized strains and generalized stresses transforms. Next an existing formula is used to write the stress resultants transforms explicitly in terms of the stiffnesses. Finally one computes the stress resultants wherever needed in the real structure by means of the inverse Fourier transform. The exact formula for the stress resultants is usually impractical due to the large number of terms involved in the analytical expressions. What makes the approach practical herein is the very reduced size of the repeating module that is to be analysed, which renders the analytical formula more tractable in many cases. The technique is illustrated with the explicit analysis of an infinite truss with 1D translational symmetry and of an infinite grid of orthogonal beams on elastic supports with 2D translational symmetry. Received December 4, 2000     

Reliability analysis of spacecraft structures under static and dynamic loading

The aim of the present paper is to demonstrate that, thanks to recently proposed simulation-based methods, it is now possible to efficiently analyse the reliability of large-scale structures modelled with very large FE systems. This capability is of paramount importance for industrial applications, because the related FE models steadily grow in size and so does the number of model parameters associated with uncertainty. The analysis methods applied in this study belong to the class of advanced Monte-Carlo simulation methods, with which the computational costs induced by direct simulation can be drastically reduced. Based on the theoretical foundations of the methods, which are specifically geared towards problems featuring thousands of parameters affected by uncertainty, the present paper provides the first demonstration that these methods can be used to assess the reliability of complex structural assemblies, even for extremely high levels of reliability.The reliability analysis is performed using a refined finite element model (120,000 DOF) of a satellite, both under static and dynamic loading. For the static load case the reliability can be estimated with great efficiency, whereas for the dynamic load case the performance depends on the considered frequency range. The obtained results are very significant in that they show the feasibility of a full scale analysis of the structural reliability in a design context for large-scale structures.     

Topology optimization of nonlinear structures with damping under arbitrary dynamic loading

This study presents an extended unit load method in which the displacement of a chosen degree of freedom (DOF) in a nonlinear structure under arbitrary dynamic loading is expressed as an integration of mutual strain energy density over a continuum domain. This new integral formulation for the displacement of a chosen DOF is developed by using the virtual work principle and can be used for linear or nonlinear structural behaviours. The integral form of the displacement is then used to develop new formulations for structural topology optimization involving arbitrary dynamic loading using the moving iso-surface threshold (MIST) method. Presented are two specific topology optimization problems with two objective functions: (a) to minimize the peak of a chosen displacement; or (b) to minimize the average power spectral density (PSD) of the chosen displacement over a finite time interval. New MIST formulations and algorithms are developed for solving two damping topology optimization problems of a structure under arbitrary dynamic loading, with or without large displacements, and having cellular damping materials with multi-volume fractions. Several numerical examples are presented to demonstrate the validity and efficiency of the presented unit load method and the MIST formulations and algorithms.     

Constrained optimization of structures with displacement constraints under various loading conditions

Optimization problems often involve constraints and restrictions which must be considered in order to obtain an optimum result and the resultant solution should not deviate from any of the imposed constraints. These constraints and restrictions are imposed either on the design variables or on the algebraic relations between them. Constraints of allowable stress, minimum size and buckling of members in the absence of allowable displacement constraint are the most important factors in optimization of the cross-sectional area of structural elements. When the allowable displacement constraint is included in the problem as a determinant parameter, since the specifications of most of elements affect the displacement rate, the way of imposing and considering this constraint requires special care. In this research the way of simultaneous imposition of multi displacement constraints for optimum design of truss structures in several load cases is described. In this method various constraints for different load cases are divided into active and passive constraints. The mathematical formulation is based on the classical method of Lagrange Multipliers. Overall, this simple method can be employed along with other constraints such as buckling, allowable stress and minimum size of members for imposing the displacement constraint in various load cases.     

高压架空输电线路绝缘子串抗风偏性能自动测试方法

为了提高高压架空输电线路绝缘子串抗风偏性能,需要进行高压架空输电线路绝缘子串抗风偏性能自动测试,提出一种基于防风偏智能分析数学建模的高压架空输电线路绝缘子串抗风偏性能方法,根据线路防风偏设计标准,参考设计风偏系数,结合架空输电线路绝缘子类型、绝缘子结构高度、绝缘子串、导线距杆塔最小电气安全距离等参数构建性能测试的约束参量模型,根据线路运行经验,进行防风偏智能分析数学模型设计,结合特征分析和智能算法对高压架空输电线路绝缘子串的抗风偏能力进行计算分析。结果表明,采用该方法能有效实现对高压架空输电线路绝缘子串抗风偏性能自动测试,计算结果准确可靠,提高了高压架空输电线路绝缘子串抗风偏性能,对架空输电线路绝缘子串的抗风性能参数的准确估计能力较好。     

A generalized evolutionary method for numerical topology optimization of structures under static loading conditions

Generalized evolutionary methods, which successively construct and solve static equilibrium problems with progressive mesh adaptation, are useful tools for defining structures that utilize their construction material to greatest effect in the finite element sense. By basing the successive element erosion upon (i) the contribution of an element to the strain energy of a structure and (ii) a certain material efficiency indicator of a structure, several weaknesses associated with previous methods have been overcome. Under static loading conditions, the strain energy contribution of an element is determined solely by the related stiffness and displacement vector. Consequently, the method is effective, and efficient when applied to problems involving such loading conditions. The efficacy of the method is demonstrated through numerical applications to the problem of optimizing the topologies of two structures, a cantilever structure and a Michell structure     

Optimisation of tensile membrane structures under uncertain wind loads using PCE and kriging based metamodels

Tensile membrane structures (TMS) are light-weight flexible structures that are designed to span long distances with structural efficiency. The stability of a TMS is jeopardised under heavy wind forces due to its inherent flexibility and inability to carry out-of-plane moment and shear. A stable TMS under uncertain wind loads (without any tearing failure) can only be achieved by a proper choice of the initial prestress. In this work, a double-loop reliability-based design optimisation (RBDO) of TMS under uncertain wind load is proposed. Using a sequential polynomial chaos expansion (PCE) and kriging based metamodel, this RBDO reduces the cost of inner-loop reliability analysis involving an intensive finite element solver. The proposed general approach is applied to the RBDO of two benchmark TMS and its computational efficiency is demonstrated through these case studies. The method developed here is suggested for RBDO of large and complex engineering systems requiring costly numerical solution.     

Concurrent topology optimization of multiscale structures with multiple porous materials under random field loading uncertainty

Aim of this work is the synthesis of auxetic structures using a topology optimization approach for micropolar (or Cosserat) materials. A distributed compliant mechanism design problem is formulated, adopting a SIMP–like model to approximate the constitutive parameters of 2D micropolar bodies. The robustness of the proposed approach is assessed through numerical examples concerning the optimal design of structures that can expand perpendicularly to an applied tensile stress. The influence of the material characteristic length on the optimal layouts is investigated. Depending on the inherent flexural stiffness of micropolar solids, truss–like solutions typical of Cauchy solids are replaced by curved beam–like material distributions. No homogenization technique is implemented, since the proposed design approach applies to elements made of microstructured material with prescribed properties and not to the material itself.     

Analysis of steel transmission pole structures

The possible use of three different numerical methods of structural analysis is investigated for the analysis of guyed and unguyed steel transmission pole structures. These three numerical schemes are (1) method of finite differences, (2) displacement or stiffness matrix method and (3) Newmark's method of successive approximations. Their relative advantages in terms of ease in problem formulation, computational effort and accuracy of results is indicated. These methods are ued in an iterative form and are easily programmed. Only plane structures are considered and no local buckling is assumed.     

Analysis of cyclic loading of plane RC structures

A numerical procedure for cyclic loading response of planar reinforced concrete structures is presented. A nonlinear orthotropic stress strain law for biaxially loaded plain concrete is developed and compared with experimental results for monotonic biaxial loading and uniaxial cyclic loading. The stress-strain law recognizes strength and ductility changes due to biaxial stress, and strength and stiffness degradation with cycles of loading. The stress strain law is incorporated into a finite element computer program which utilizes isoparametric quadrilaterals with extra non-conforming deformation modes. Numerical and experimental results are presented for a monotonically loaded shear wall-frame system and a cyclically loaded shear wall.     

面向输电线故障信息传输的多播路由

针对无线传感器网络应用于输电线路故障传输时存在通信代价高、实时性差的问题,提出一种输电线路故障传输多播路由算法(MRFT)。抽象出输电线路故障信息传输网络模型;根据时延最短路径树(SPT)的最大端到端时延确定多播树时延上限,将时延上限边接入多播树;设计最小代价启发函数将剩余叶子节点接入多播树。仿真结果表明,与KPP算法相比,MRFT算法构造的多播树在多播树时延、端到端时延方差和多播树代价3个方面均有良好表现。该算法能够有效保证输电线路故障信息传输的实时性,降低通信代价。