Analysis of the elastic buckling of a plywood column

Buckling tests were conducted on specimens of 5-ply lauan plywood for a range of slenderness ratios to measure its buckling stress. Three-dimensional finite element calculations of buckling stress were performed and their validity examined by comparison with experimental results. Both experimental and calculated results revealed that buckling stress is influenced by Young’s modulus values (a measure of stiffness) obtained not only under flexural loading but also under axial loading. When the axial Young’s modulus is larger than the flexural Young’s modulus, the buckling stress is measured as larger than that obtained using the flexural Young’s modulus alone. Inversely, when the axial Young’s modulus is smaller than the flexural Young’s modulus, the buckling stress is measured as smaller than that obtained using the flexural Young’s modulus alone. Therefore, both the Young’s modulus values should be taken into account for determining the buckling stress of a plywood column.     

驶入式钢货架立柱计算长度系数分析

驶入式货架立柱在垂直货物通道方向上的计算长度系数受多种因素影响。首先,对于上端受水平位移约束而下端受扭转约束的单柱模型,推导了弹性屈曲荷载计算公式。计算结果表明,当模型中约束刚度较小时,约束刚度的变化对立柱计算长度系数产生明显影响。此后,采用有限元分析方法分析了单列立柱模型的弹性屈曲荷载,得到立柱的计算长度系数。柱列模型中的牛腿梁和荷载分布不均现象均可以提高模型的弹性屈曲荷载。基于数值计算结果,得到了立柱计算长度系数的简化计算公式。若已知立柱柱脚转动刚度和支撑体系侧向刚度,可由此公式计算立柱的计算长度系数。     

轴向时变冲击载荷作用下直杆弹性动力屈曲研究

基于应力波理论,用半解析半数值方法对轴向时变冲击载荷作用下的直杆进行研究,给出了一种利用压应力波前附加约束条件求解轴向时变载荷作用下直杆弹性动力屈曲问题的方法。以三角脉冲载荷作用下的直杆为例,对其临界屈曲长度、初始屈曲模态和动力特征参数进行了求解,探讨了脉冲载荷峰值和载荷持续时间对临界屈曲长度和屈曲模态的影响。总结了三角脉冲载荷作用下直杆弹性动力屈曲的规律,并与阶跃载荷作用下的情况进行对比分析,结果与之前文献研究结果吻合良好。     

功能梯度压杆在均布载荷作用下的后屈曲形态

对受均布载荷作用功能梯度材料(FGM)压杆的屈曲及后屈曲行为进行了分析。基于杆的大变形理论, 考虑杆的轴线伸长, 建立了受均布载荷作用下细长FGM压杆的几何非线性平衡方程, 其中假设FGM杆的性质沿厚度方向按照幂函数连续变化。采用打靶法和解析延拓法数值求解非线性两点边值问题, 获得了一端自由一端固定FGM杆的后屈曲数值解。给出了不同梯度指标下FGM杆的后屈曲特征曲线, 并与金属和陶瓷两种单相材料杆的相应特性进行了比较, 分析和讨论了材料的梯度性质参数对杆变形的影响。结果表明: FGM杆后屈曲行为与各向同性均质杆有很大区别, 梯度指数对杆的屈曲载荷以及后屈曲形态有明显的影响。      

结构胶侧扭约束玻璃柱轴压承载力设计方法研究

玻璃柱在工程应用中的一种形式是采用结构胶将其与玻璃面板连接,结构胶对玻璃柱的约束效应能改善玻璃柱稳定性。考虑结构胶对玻璃柱的侧向弹簧约束及扭转弹簧约束作用,基于平衡方程推导了结构胶约束玻璃柱的轴压临界荷载公式,讨论了结构胶弹性模量及玻璃柱的柱高、截面高度、截面厚度对临界荷载的影响,发现结构胶弹性模量、玻璃截面厚度影响最为显著。基于等效刚度法将临界荷载公式推广至夹层玻璃柱。针对工程实践中常用的硅酮结构胶,提出了结构胶约束玻璃柱在轴压荷载作用下考虑玻璃开裂及结构胶破坏的承载力设计准则,并通过有限元模型计算验证了理论研究的合理性。     

新型卷边钢板组合截面PEC柱(强轴)滞回性能试验研究

为进一步改善现行薄壁钢板组合截面PEC柱构件双向不等刚度和翼缘局部屈曲过于集中等力学性能缺陷,该文提出了采用翼缘钢板卷边的新型卷边钢板组合截面PEC柱类型。以拉结板条间距作为基本参数,对4个按新型卷边钢板组合截面强轴布置的足尺试件进行了恒定竖向荷载下的水平低周反复荷载试验,观察了试验过程中PEC柱卷边钢板组合截面翼缘钢板局部屈曲和混凝土部分裂缝开展与压溃发展过程,得到了试件的荷载-位移滞回曲线。根据试验结果分析了PEC柱的承载力、抗侧刚度、耗能与延性、变形模式与破坏模态等力学性能。结果表明:试验卷边PEC柱试件充分发挥了薄板局部屈曲后性能,具有优越的变形与耗能能力;构件的破坏模式为柱脚部位卷边钢板组合截面翼缘局部屈曲部位不断扩展和伴随混凝土大面积压溃。     

C‐H‐O Systems in the presence of iron oxide and iron carbide

Abstract

In this paper, we apply the powerful Koiter‐Budiansky's general theory of buckling of elastic structure through the language of functional analysis and perturbation technique to find the analytical expressions for buckling load, post‐buckling behavior, and peak load versus imperfection for 3‐connection built‐up columns, an imperfection sensitive structure. The asymptotic expressions between the load‐shortening relationship for a perfect column and the load‐shortening‐imperfection relationship for an imperfect column are derived. The results are consistent with the existing numerical solutions.     

海洋结构CFRP环向约束钢管混凝土柱在压弯扭荷载下的力学性能研究

为了研究CFRP环向约束钢管混凝土柱在压弯扭复合荷载下的力学性能,该文开展了4个CFRP环向约束钢管混凝土柱和2个钢管混凝土柱的拟静力试验,主要参数为CFRP层数和轴压比,得到了在弯扭、压弯扭两种荷载作用下柱的破坏模式和荷载-变形曲线。试验结果表明, CFRP环向约束钢管混凝土柱的破坏模式为弯型破坏,其破坏过程为:塑性铰区域钢管局部屈曲、CFRP形成环向裂纹,随后CFRP断裂、与钢管剥离,最后钢管局部屈曲部位开裂。轴力的存在会使得钢管更容易出现“象腿”破坏模式。在弯扭荷载下, CFRP环向约束对钢管混凝土的延性以及承载力提高不明显;在压弯扭荷载下, CFRP环向约束能有效提高钢管混凝土的延性及其耗能能力,减缓刚度退化,但对承载力的提高不明显。此外,增加CFRP层数能有效抑制钢管的局部屈曲,增强耗能能力。     

Buckling length determination of concrete filled steel tubular column under axial compression in standard fire test

The structural behavior of the column under fire becomes essential since it concerns the safety of buildings and people inside in case of fire event occurs. The standard fire tests were usually used to evaluate the resistance of the members under fire. This paper presents a method to determine the buckling length that was used to predict the resistance of the concrete filled steel tubular (CFST) column under axial compression in standard fire test. In this method, the fourth-order differential equation was used to develop the analytical model. Various boundary conditions of the column at working condition, e.g. fixed–fixed, fixed–pinned, and pinned–pinned, were considered in this analytical model. Design formulae and charts were developed to determine the buckling lengths that considered varying flexural stiffness along the column height in fire. Temperature-independent buckling lengths were also proposed to further simplify the fire resistance design procedure. To establish the validity, the determined fire resistance of CFST columns based on the analytical and proposed buckling lengths were compared with test results in literature and predictions based on empirical buckling lengths.     

An analytical approach for local buckling analysis of initially delaminated composite thin-walled columns with open and closed sections

Local buckling of intact thin-walled columns is generally performed by modeling the wall segments as long plates and by assuming that edges common to two or more plates remain straight. Thus, the buckling load can be determined by considering the wall segments as individual plates rotationally restrained by the adjacent wall segments. This technique is combined with plate theories as a new analytical method to predict the buckling load of an initially delaminated column with any arbitrary sections (open or closed). First, moments at the rotationally restrained edges of delaminated segment (web or flange) are obtained from the curvature and stiffness of the adjacent laminates. Then, the strain energy of this delaminated segment with distributed moment at edges is calculated based on the first-order shear deformation theory. Using the principal of minimum potential energy, the governing equations are obtained and solved by the Rayleigh–Ritz approximation technique. Results of the present approach are compared with three-dimensional finite-element results obtained from eigenvalue buckling analysis in ANSYS software for both box- and channel-section columns with cross-ply and angle-ply stacking sequences. Finally, the effects of delamination size and location are investigated on the buckling loads.     

压弯剪作用下钢筋混凝土柱荷载-变形分析

为研究不同破坏模式下钢筋混凝土柱的受力机理及性能,该文提出能够考虑压弯剪相互作用的钢筋混凝土柱荷载-变形分析模型。以修正压力场理论及传统纤维截面分析法为基础,将柱受力过程分为弯曲控制及剪切控制两个阶段,分别对控制截面受拉区和受压区进行分析,同时考虑了纵筋受压屈曲及P-Δ效应的影响,进而得到柱水平受剪承载力及其变形。最后,为验证模型的有效性,对所收集的拟静力试验柱进行了模拟。结果表明,压弯剪作用下钢筋混凝柱会表现出弯曲、弯剪及剪切三种不同的破坏模式,其荷载-变形性能差异较大,采用该文所提模型所得计算曲线与试验结果吻合较好,该模型能够被用于钢筋混凝土柱抗震性能分析。     

Optimization of clamped columns under distributed axial load and subject to stress constraints

The optimum designs are given for clamped-clamped columns under concentrated and distributed axial loads. The design objective is the maximization of the buckling load subject to volume and maximum stress constraints. The results for a minimum area constraint are also obtained for comparison. In the case of a stress constraint, the minimum thickness of an optimal column is not known a priori, since it depends on the maximum buckling load, which in turn depends on the minimum thickness necessitating an iterative solution. An iterative solution method is developed based on finite elements, and the results are obtained for n=1, 2, 3 defined as I=α _n A ⁿ , with I being the moment of inertia, and A the cross-sectional area. The iterations start using the unimodal optimality condition and continue with the bimodal optimality condition if the second buckling load becomes less than or equal to the first one. Numerical results show that the optimal columns become larger in the direction of the distributed load due to the increase in the stress in this direction. Even though the optimal columns are symmetrical with respect to their mid-points when the compressive load is concentrated at the end-points, in the case of the columns subject to distributed axial loads the optimal shapes are unsymmetrical.     

Cyclic responses of reinforced concrete composite columns strengthened in the plastic hinge region by HPFRC mortar

The brittleness of concrete raises several concerns due to the lack of strength and ductility in the plastic hinge region of reinforced concrete columns. In this study, in order to improve the seismic strength and performance of reinforced concrete columns, a new method of seismic strengthened reinforced concrete composite columns was attempted by applying High Performance Fiber Reinforced Cementitious composites (HPFRCs) instead of concrete locally in the plastic hinge region of the column. HPFRC has high-ductile tensile strains about 2–5% with sustaining the tensile stress after cracking and develops multiple micro-cracking behaviors. A series of column tests under cyclic lateral load combined with a constant axial load was carried out. Three specimens of reinforced concrete composite cantilever columns by applying the HPFRC instead of concrete locally in the column plastic hinge zone and one of a conventional reinforced concrete column were designed and manufactured. From the experiments, it was known that the developed HPFRC applied reinforced concrete columns not only improved cyclic lateral load and deformation capacities but also minimized bending and shear cracks in the flexural critical region of the reinforced concrete columns.     

除尘器壳体双肢组合截面立柱轴压稳定性研究

除尘器壳体围护结构中的立柱因需要支承顶部较宽截面的箱形梁以及承担较大的轴力而设计成双肢H型钢组合截面。利用非线性有限元方法,研究此双肢组合截面柱在考虑初始缺陷情况下承受轴向压力时的稳定性。研究表明,立柱轧制、墙板与立柱焊接产生的残余应力会小幅降低立柱稳定承载力。立柱的失稳形式为柱顶跨段内前翼缘与前腹板组成T形截面整体弯扭失稳与腹板局部失稳同时发生的相关失稳。墙板与两肢间连接墙板作为蒙皮板,为立柱承担部分荷载并约束其侧向变形。连接墙板越靠近前翼缘,立柱的稳定性越好。连接墙板宽度、墙板宽度、墙板角钢加劲肋刚度和节点连接板厚度等因素对立柱稳定性影响较小。仅邻近立柱的墙板和连接墙板增厚可对立柱稳定性有增强作用,远离立柱的墙板壁厚影响较小。立柱稳定系数与H型钢柱翼缘宽厚比的关系不是单调的;立柱绕弱轴弯曲长细比、腹板高厚比的增大会降低立柱稳定性。基于大量非线性有限元计算结果,提出了除尘器壳体中双肢组合截面轴心受压柱稳定承载力的计算建议。     

Torsional and flexural buckling of composite FRP columns with cruciform sections considering local instabilities

In this paper, a semi-analytical finite strip method is developed for the prediction of torsional and flexural buckling stresses of composite FRP columns under pure compression. Numerical finite strip results will be compared with those obtained from closed-form equations for doubly symmetric open thin-walled FRP sections. The accuracy of the proposed finite strip method in determining critical flexural and torsional stresses of FRP columns will be assessed. Among the composite FRP columns with doubly symmetric open sections, buckling behavior of stiffened and unstiffened FRP cruciform sections will be evaluated and case studies performed. The effect of material properties and longitudinal stiffeners applied at the end of the web-plate and flange-plate on buckling modes of composite FRP cruciform sections is also reviewed.     

Buckling of functionally graded and elastically restrained non-uniform columns

Columns with non-uniform distribution of geometrical or material parameters i.e. functionally graded material distribution, varying cross-sectional area and flexural stiffness provide an economical solution to carry the desired higher compressive loads in engineering structures. In this paper, a low-dimensional mathematical model is presented, which is capable of computing the buckling loads of uniform and non-uniform functionally graded columns in the axial direction. The columns with spatial variation of flexural stiffness, due to material grading and/or non-uniform shape, are approximated by an equivalent column with piecewise constant geometrical and material properties. Such a formulation leads to certain transcendental eigenvalue problems where the matrix elements are transcendental functions. This model is further extended in analyzing some uniform and non-uniform elastically restrained or braced axially graded columns with equal or unequal spans. The mathematical modeling, closed-form transcendental functions and numerical solution technique are described and several examples of estimating buckling loads for various boundary configurations are presented. Some of the results are also validated against available solutions, representing the convergence, effectiveness, accuracy and versatility of the proposed modeling and numerical method. Formulation of such low-dimensional eigenvalue problems can also be extended for analyzing, designing and optimizing the static and dynamic behavior of structural components that are made of functionally graded materials.     

The active buckling control of some composite column strips using piezoceramic actuators

The overall flexural buckling control of composite column strips using piezoceramic actuators is examined in this paper. The buckling control is investigated through the use of induced strain actuation associated with the piezoelectric effect and in conjunction with a closed-loop control system. Three column strip specimens have been fabricated from commercially available carbon-epoxy pre-impregnated sheets. The layup configurations are [90₂/0]_s, [± 45/0]_s, and [90₂/± 45]_s. The length and width of each test specimen is 280 mm and 35 mm respectively. After manufacture of the composite column strips, piezoceramic actuators were surface bonded at their mid-heights on both sides of the column. Due to imperfections in the material, and of a geometrical nature, the composite column strips, with inactivated piezoceramic actuators, will deflect from the onset of loading and reach an ultimate load capability at high deflection levels. As a result of the presence of imperfections, this ultimate load will be less than the critical buckling load of the ideal structure. By applying a controlled voltage to the actuators a reactive moment will be induced at the column centre thereby removing the lateral deflections and enforcing the column to behave in a perfectly straight manner. An exact theoretical buckling analyses is outlined. This is used to evaluate the critical buckling loads of the individual composite test specimens. The test procedure is outlined and load-deflection plots, obtained with and without active control, are presented. The composite column strips with active control are shown to clearly demonstrate an increase in axial compressive load capacity compared to those without control. For the layup configurations considered, increases in load carrying capability are of the order of 19.8%–37.1%.     

RC圆柱受弯承载力极限值计算方法研究

运用数值计算方法对钢筋混凝土(RC)圆柱破坏时截面受弯承载力极限值进行计算,计算时考虑了箍筋对混凝土的约束效应和纵筋应力强化效应。计算结果显示截面受弯承载力极限值比规范规定的理论值有较大增加,特别是在轴压较高、箍筋约束效果明显时。截面受压区高度计算是RC圆柱截面受弯承载力计算的关键。圆柱破坏时截面受压区高度主要受轴压比、配箍特征值和纵筋配筋特征值三个参数影响。根据数值计算的结果进行回归,提出了受压区对应圆心角απ计算方法。在此基础上通过理论分析并结合95个大比例试件试验结果,提出截面受弯承载力极限值计算方法,计算结果与试验结果吻合很好。参数分析的结果表明在轴压比为0.6时截面受弯承载力极限值比规范公式计算值能提高60%左右,这一显著的弯矩强化效应值得在设计中引起重视。     

Mechanical performance of hybrid bismaleimide composites reinforced with three-dimensional braided carbon and Kevlar fabrics

Short, unidirectional and laminated hybrid composites have been extensively investigated. However, very limited work has been conducted on three-dimensional (3-D) braided hybrid composites. In this work, 3-D braided carbon and Kevlar fibres were hybridized to reinforce a bismaleimide (BMI) resin. The purpose of this paper was to investigate the effect of carbon to Kevlar ratio on such mechanical properties as load–displacement behaviour, flexural strength and modulus, shear strength, and impact properties. The effect of surface treatment of hybrid fabrics on the flexural properties was also determined. Experimental results showed that the flexural strength and modulus of the 3-D braided carbon/Kevlar/BMI composites increased with relative carbon fibre loading up to a carbon to Kevlar ratio of 3:2 and then dropped. Positive hybrid effects were observed for both flexural strength and modulus. The results presented in this work proved that hybridization with certain amount of ductile Kevlar fibre markedly promoted the shear strength, impact energy absorption characteristics and damage tolerance of the all-carbon composite, which is of importance for the 3-D braided composites to be used in bone fixations. Fracture surfaces and microstructures of various 3-D braided hybrid composites were analyzed to interpret the experimental findings.     

Application of linear buckling sensitivity analysis to economic design of cylindrical steel silos composed of corrugated sheets and columns

The paper deals with global stability of steel cylindrical silos composed of corrugated walls and vertical columns with loads imposed by a bulk solid following Eurocode 1. The optimum silo design with respect to the steel weight was based on a sensitivity analysis method. The changes of silo column profiles at each design step were performed by means of influence lines for the buckling load factor due to the unit column bending stiffness variation. The corrugated walls were simulated as an equivalent orthotropic shell and vertical thin-walled columns as beam elements. The results were compared with the Eurocode 3 approach and verified by FE results of linear buckling and non-linear analyses with geometric non-linearity carried out with the commercial finite element code ABAQUS for a 3D shell model of a perfect silo. The proposed procedure allowed for a rational silo design with an economic material use. Some recommendations for the silo optimum design were elaborated.