梯度材料薄壁钢梁的热弹性动力稳定性研究

研究剪力作用下梯度材料薄壁钢梁的动力稳定性,分析基于小变形和二阶非线性位移引起的中等旋转。钢梁承受外部轴向动力荷载。模型考虑了热弹性影响,推导了热量传导公式,用于计算钢梁横截面域的温度。采用Galerkin和Bolotin方法,详细地推导出控制方程和动力稳定的区域,以比例的形式评估并阐述了稳定区域,研究非稳定区的纵向振动影响。结果表明,荷载频率接近纵向自振频率的影响最大,并有更广的稳定区域。在分析中,也考虑了温度梯度、剪力弹性、轴向惯量、不同截面和不同梯度材料的影响。     

Dynamic instability of three-layered cylindrical shells containing an FGM interlayer

In this study, the dynamic instability of three-layered cylindrical shells containing a functionally graded (FG) interlayer subjected to static and time dependent periodic axial compressive loads are investigated. The governing relations, modified Donnell type dynamic stability and deformation compatibility equations are derived. The governing equations are reduced Mathieu–Hill equation by using Galerkin׳s method and the expressions for boundaries of unstable regions of three-layered cylindrical shell with an FG interlayer are found. Finally, the effects of variations of volume fractions of FG interlayer and shell characteristics on the magnitudes of boundaries of unstable regions are studied numerically.     

Nonlinear dynamic stability of composite plates with piezoelectric layers subjected to periodic in-plane load

Nonlinear dynamic stability characteristics of composite plates subjected to periodic in-plane load are investigated via the finite element method with dynamic response analysis. Piezoelectric actuator layers are embedded at the top and bottom of the laminated composite plate. The theoretical formulation is based on the modified first order shear deformation theory (MFSDT) incorporating the von Kármán type nonlinear strains. The structural system is considered to be undamped. The nonlinear governing equations obtained are solved using the Newmark's direct integration method coupled with the direct iteration method. The boundaries of dynamic instability regions are obtained using Bolotin's approach. Effects of in-plane forcing frequency and applied voltage on the characteristic features of dynamic stability behaviour are investigated using both linear and nonlinear dynamic response analyses.     

The effects of axial tension on the sagging-moment regions of composite beams

This paper studies the effects of axial tension on the sagging moment regions of steel–concrete composite beams. The study comprised an extensive experimental programme and nonlinear finite element analyses. Six composite beams were designed and tested under the combined effects of axial tension and positive bending moment. The beams were loaded to their ultimate capacity and the experimental moment-axial tension interaction diagram was constructed. Following the tests, a finite element model was used to simulate the nonlinear response of the composite beams. The validity of the model was thoroughly assessed against the available experimental data and a parametric study was conducted to study different beam sizes and the effect of partial shear connection on the interaction diagram. It was found that the moment capacity of a composite beam is reduced under the presence of an axial tensile force acting in the steel beam section. In addition, the use of partial shear connection does not affect significantly the shape of the interaction diagram. The tensile capacity of the composite section, however, is limited by the axial capacity of the steel beam alone. Based on the experimental results and the finite element analyses, a simplified equation is proposed for the design of composite beams subjected to positive bending and axial tension.     

钢筋砼框架考虑斜裂缝影响的非线性全过程分析

本文通过对一些约束梁、约束柱及连续梁抗剪试验资料的分析研究，采用斜压场模型模拟有反弯点的剪跨区段当斜裂缝出现后剪力作用下的受力状态，并考虑剪跨比的影响及纵筋的暗销作用，导出了剪力、剪切变形、轴力之间的关系。用此模型对不同条件的５８根剪切破坏的约束梁、柱进行了计算比较。同时，将模型引入框架非线性分析中的单元刚度矩阵及固端力列阵，编写了框架非线性全过程分析程序，对一定数量发生弯曲剪切破坏的两跨连续梁进行了计算比较，计算结果与实测结果符合较好。本文进行了三榀门式框架及二根两跨连续梁的试验，用上述程序进行了框架、连续梁考虑斜裂缝对内力重分布及构件变形影响的比较分析。     

On free vibration analysis of thin-walled beams axially loaded

In this paper, a numerical–experimental study about natural frequencies of thin-walled beams axially loaded is presented. Moreover, the influence of axial load in the frequencies is studied. The equations of motion are based on Vlasov’s theory of thin-walled beams, which were modified previously to include the effects of shear flexibility, rotatory inertia in the stress resultants. Moreover, a constant axial load is incorporated to the formulation, both in the time and frequency domain. The differential equations are shown to be particularly suitable for analysis in the frequency domain using a state variables approach. A numerical investigation is carried out to reveal the influence of the axial load in several boundary conditions. Finally, free vibration experimental tests are presented, which allow verify the theory presented in this paper and provide good quality data that can be used for checking the accuracy and reliability of different theories.     

Dynamic stability analysis of composite skew plates subjected to periodic in-plane load

Here, the dynamic stability characteristics of simply supported laminated composite skew plates subjected to a periodic in-plane load are investigated using the finite element approach. The formulation includes the effects of transverse shear deformation, in-plane and rotary inertia. The boundaries of the instability regions are obtained using the Bolotin's method and are represented in the non-dimensional load amplitude-excitation frequency plane. The principal and second instability regions are identified for different parameters such as skew angle, thickness-to-span ratio, fiber orientation and static in-plane load.     

Curvilinear fiber optimization tools for design thin walled beams

An investigation of the possible performance improvements of thin walled composite beams through the use of the variable stiffness concept with curvilinear fiber is presented. The beams are constructed from a single-cell closed cross section and a number of non-classical effects such as material anisotropy, transverse shear, warping inhibition and nonuniform torsional model are considered in the beam model. The governing equations were derived by means of the extended Hamilton's principle. Also the extended Galerkin's method is used to solve governing equations. Composite beams subjected to different loading with given geometry and material properties are optimized for maximum failure load. Improvements of variable-stiffness beams over conventional, constant-stiffness beams are also demonstrated.     

伸出地面上基桩的动力稳定性研究

基于Hamilton原理,导出了桩在轴向动力荷载下的激振频率计算式,分析了伸出地面上基桩的动力稳定性问题,探讨了桩端不同约束、桩长比、桩径、地基土水平抗力和激振频率等参数对桩动力稳定性的影响,结合实例,分析了桩在不同桩端约束条件下桩的动力不稳定区域的差异。     

A study of the dynamic stability of anisotropic thin-walled beams

The dynamic stability of an anisotropic thin-walled beam subjected to harmonically varying axial compression is studied in this paper. The analysis is based on the assumption that the shape of the cross-section of the beam does not change during deformation. The shear of the middle surface of the beam is taken into account. The dynamic equations are reduced to a set of coupled Mathieu equations by using the Galerkin method. Then Hill's infinite-determinant method and a shooting-type numerical method are used to investigate the Mathieu equations to determine the stability regions of the beam in the parameter space. Two examples, one of which is a complete circular tube and the other is a circular tube with a longitudinal seam, are given.     

基于软化拉-压杆模型的钢筋混凝土框架节点受剪分析

软化拉.压杆模型是基于经典桁架模型理论而发展起来的一种模型,主要用于混凝土结构中应力紊乱区的受剪设计.以包含钢筋混凝土变梁中节点在内的各类框架节点为研究对象,建立框架节点等效核心区在剪、压复合荷载作用下的计算模型,应用软化拉.压杆模型对节点等效核心区进行受力分析,推导框架节点的受剪承载力计算公式,并通过国内外110个钢...     

Discrete element visco-elastic modelling of a realistic graded asphalt mixture

The Discrete Element Model has been used here to simulate constant strain rate uniaxial compression tests for a realistic asphalt mixture comprising graded aggregates. A numerical sample preparation procedure has been developed to represent the physical specimen. A parallel bond model has been used in the elastic modelling to give moment resistance at the contacts. Uniaxial constant strain rate loading and unloading tests have been simulated. The effects of the normal to shear contact stiffness ratio on the bulk properties, the parallel bond radius, the number of particles and their positions, and the loading speed have been investigated. A modified Burger's model has been used to introduce time-dependent contact stiffness with the ability to transmit moment and torsion. Two-ball clumps have been used to investigate the effect of particle shape. The effect of Burger's model parameters, the ratio of normal to shear Burger's model parameters, the bond radius multiplier, the friction coefficient and the bond strength distribution in the viscoelastic simulations have been investigated. Constant strain rate uniaxial compression tests have been undertaken in the laboratory where the axial stress–strain response has been measured for comparison with the numerical modelling results. The modified Burger's model has proved to be useful and ready for simulating uniaxial constant strain rate and creep tests in the laboratory.     

轴压组合叠合梁自由振动的动态刚度分析

采用动态刚度分析方法,对自由振动和轴压组合叠合梁的屈曲性能进行研究。在公式中考虑了轴力、泊松效应、轴向变形、剪切变形和转动惯量的影响。通过直接求解轴压组合叠合梁自由振动的控制微分方程,建立严格的动态刚度矩阵,并用该矩阵对已有文献中其他组合梁进行计算比较,证明了动态刚度矩阵的适应性。同时,论证了轴力和边界条件对自然频率、屈曲荷载和叠合梁模式的影响。     

钢筋混凝土梁式构件抗爆分析的改进等效单自由度方法

爆炸荷载作用下钢筋混凝土梁式构件可能会出现多种破坏模式，主要有弯曲破坏、直剪破坏和弯剪联合破坏等。现有结构构件抗爆分析方法主要考虑爆炸荷载下结构构件的弯曲破坏，对直剪破坏和弯剪联合破坏研究较少，尤其缺少快速准确的抗爆分析方法。基于等效单自由度模型理论，改进了钢筋混凝土梁抗爆分析的直剪单自由度方法，提出了直剪、弯剪联合破坏的判定准则，给出了两种破坏模式下钢筋混凝土梁动态响应的计算方法。进一步考虑爆炸荷载作用下钢筋混凝土梁的弯曲破坏、直剪破坏和弯剪联合破坏，提出钢筋混凝土梁式构件抗爆分析的改进等效单自由度方法，并给出了其分析步骤。该方法可以直接评估爆炸荷载下钢筋混凝土梁式构件的破坏模式，并计算其动态响应。通过有限元软件LS-DYNA数值模拟了不同爆炸工况下钢筋混凝土梁式构件的动态响应和破坏模式，与改进的等效单自由度方法的计算结果进行了对比，验证了提出的改进等效单自由度分析方法的正确性。     

Linear and non-linear stability analyses of thin-walled beams with monosymmetric I sections

The paper investigates beam lateral buckling stability according to linear and non-linear models. First, the classical linear stability solutions are derived from the stability equation in the case of monosymmetric cross-sections. Bending distribution, load height parameter and Wagner's coefficient effects are taken into account. In the second step, they are extended to non-linear stability by considering pre-buckling deformation and improved solutions are then obtained. Based on a finite element model developed for large torsion of thin-walled beams with open sections, the stability of beams under gradient moments (M₀, ψM₀, ?1≤ψ≤1) is particularly investigated. It is then concluded that beam lateral buckling resistance depends not only on pre-buckling deformation but also on section shape and load distribution. For bisymmetric I beam, closed form solutions are possible and pre-buckling deformations have an incidence. In the case of beams with monosymmetric I and Tee sections, effects of pre-buckling deflections are important only when the largest flange is in compression under M₀ and positive gradient moment. Analytical solutions are possible. For negative gradient moments all available solutions fail and numerical solutions are more powerful. Effect of gradient moments on stability of redundant beams is investigated at the end. Under such boundary conditions, important axial forces are present due to non-linear beam deformation. These forces, omitted in literature, have an incidence on stability. The element is then concerned with beam-column behaviour rather than beam stability.     

Nonlinear analysis of composite beams with concrete-encased steel truss

Composite beams constituted by a concrete-encased steel truss welded to a continuous steel plate are analyzed using a nonlinear finite element formulation based on Newmark's classical model. The web member of the steel truss is made by deformed or structural steel rebars and behaves like a deformable shear connection. In order to avoid slip locking, finite elements based on second-order interpolation of longitudinal displacements and flexural rotations are employed. Simply supported composite beams subjected to a uniformly distributed transverse load are considered. The bending capacity is evaluated for short up to long spans, taking the nonlinear behavior of concrete, steel and shear connection into account. The effects of the shear connection ductility are put in evidence, showing that, for short spans, the interfacial stress transfer resulting from the yielding of connection may be penalizing. In fact, the high slip gradient arising in sections near the supports may lead to a premature concrete failure. In this case, the exact solution to the linear elastic problem for steel–concrete composite beams can be used for design purposes.     

Shear deformations in inflated cylindrical beams: An old model revisited

The paper considers the effects of shear deformations on the load-displacement response of pressurized thin-walled circular-cylindrical beams subjected to lateral loads. A classical structural model, able to account for the inability of the wall's constituent material to withstand compressive stresses, is used to model the nonlinear behavior of partially wrinkled beams under simultaneous bending and shear. Particular attention is devoted to accurate determination of the shear stiffness within the wrinkled zones of the beam. The system of nonlinear equations governing the equilibrium of the inflated beams after the onset of the post-critical stage is amenable to numerical solution via standard incremental-iterative algorithms, as long as the wrinkling of the cross sections is small or moderate. Application of the proposed model to some well-documented problems drawn from the literature serves to illustrate the proposed algorithm's potential.     

Dynamic response of axially loaded monosymmetrical thin-walled Bernoulli–Euler beams

The dynamic bending–torsion coupled vibrations of elastic axially loaded slender thin-walled beams with monosymmetrical cross-sections are investigated by using normal mode method. The Bernoulli–Euler beam theory is employed and the effects of warping stiffness and axial force are included in the present formulations. The theoretical expressions for the displacement response of axially loaded slender thin-walled beams subjected to concentrated or distributed loads are presented. The method is illustrated by its application to two test examples to describe the effects of warping stiffness and axial force on the dynamic behavior of thin-walled beams. The numerical results for the dynamic bending displacements and torsional displacements are given. The proposed theory is fairly general and can be used for thin-walled beam assemblage of arbitrary boundary conditions subjected to various kinds of loads.     

Strength deterioration of reinforced concrete beam–column joints subjected to cyclic loading

This paper proposes a method to predict the ductile capacity of reinforced concrete beam–column joints failing in shear after the development of plastic hinges at both ends of the adjacent beams. After the plastic hinges occur at both ends of the beams, the longitudinal axial strain at the center of the beam section in the plastic hinge region is expected to increase abruptly because the neutral axis continues to move toward the extreme compressive fiber and the residual strains of the longitudinal bars continue to increase with each cycle of additional inelastic loading cycles. An increase in the axial strain of the beam section after flexural yielding contributes to a widening of the cracks in the beam–column joints, thus leading to a reduction in the shear strength of the beam–column joints. The proposed method includes the effect of longitudinal axial strain of a beam in the plastic hinge region of the beam on the joint longitudinal strain and the strength deterioration of the joint. In order to verify the shear strength and the corresponding deformability of the proposed method, test results of RC beam–column assembly were compared. Comparisons between the observed and calculated shear strengths and their corresponding deformability of the tested assemblies showed reasonable agreement.     

钢箱梁非弹性剪滞建模

对钢箱梁中非弹性剪滞性能进行研究。带翼缘受弯构件中的剪滞效应经常被认为是不均匀的纵向变形和沿翼缘的正应力。弹性剪滞性能被广泛研究并在结构设计中给予考虑,然而对非弹性剪滞涉及较少。带翼缘受弯构件在其极限状态下可能会发生塑性变形,故采用基于最小功的方法来模拟非弹性剪滞性能。在非弹性剪滞模型中,有效模量采用公式表达,泊松比按塑性理论取值。通过两个钢箱梁的试验结果对该分析方法进行验证。经与试验数据对比后表明,所给出的变分法可以精确地得到钢箱梁的塑性正应变分布和变形。