基于耦合模式下初始后屈曲分析的复合柱形壳的有限元分析

运用多模式预测方法,基于Koiter初始后屈曲理论提出了一个有限元公式。初始后屈曲理论提供了压力作用下结构缺陷敏感性的直接信息,同时也是非线性降阶模型的理论基础。研究的目的在于说明包含了模态作用在内的壳结构的屈曲承载力分析。利用一部分有代表性的模态模型,对包含非线性前屈曲效应的轴压作用的复合柱形壳进行了耦合模式下的初始后屈曲分析。当结构缺陷较小时,从降阶模型中得到的极限屈曲荷载与从全模型非线性分析中得到的结果具有良好的一致性,这说明可以运用本文提议的方法对带有缺陷的壳结构的耦合模态响应进行快速的预测。     

天津某贝壳形单层网壳整体稳定分析

对天津于家堡站房贝壳形单层网壳进行了全面的整体稳定分析。首先进行线性稳定性分析,预测结构屈曲荷载的上限;然后,运用非线性有限元方法,考虑几何非线性以及初始缺陷的影响,全面分析网壳在各个工况荷载下的稳定性能,得到较准确的结果。     

对称双圆弧柱壳的非线性屈曲

对于潜艇艇体耐压壳结构,屈曲特性在设计中被广泛关注。针对一种新型潜艇耐压艇体结构-对称双圆弧环肋柱壳,推导了相应的弹塑性失稳系数。采用非线性大挠度理论,给出了静水压力作用下含初始缺陷的对称双圆弧环肋柱壳大挠度弹塑性屈曲临界压力计算式。讨论了开口角、周向相当波数和初始几何缺陷对临界压力的影响。计算结果表明,开口角对结构弹塑性屈曲的临界压力影响很小,而周向相当波数是影响临界压力的主要因素。     

基于改进一致缺陷模态法的轴压圆柱薄壳极限承载力分析

对结构进行缺陷稳定分析的主要方法是一致缺陷模态法和随机缺陷模态法,一致缺陷模态法对薄壁圆柱壳结构进行非线性分析得到的极限承载力与其实际承载能力有一定差距,随机缺陷模态法则工作量很大。基于圆柱薄壳轴压失稳呈现出多模态屈曲的特点,本文提出改进一致缺陷模态法,通过对圆柱壳分别施加不同屈曲模态找到最不利缺陷分布形式。文中通过有限元法验证了改进一致缺陷模态法的可靠性,同时指出按照某一类高阶屈曲模态施加初始缺陷能得到薄壳的最不利极限承载力。     

Post-buckling analysis of corrugated panels in the presence of multiple interacting modes

In this paper, the effect of the interaction between one overall buckling mode and many local modes on the post-buckling behaviour of uniformly compressed corrugated sheets is analysed by means of the general theory of elastic stability. The analysis is restricted to third order terms of the energy expansion and, therefore, it can be fruitfully applied to the investigation of structures with asymmetric post-buckling behaviour only. Initial imperfection effect is taken into account. By using the finite strip method, an extensive parametric analysis is performed.     

初始缺陷对矩形钢管混凝土柱壁板屈曲后强度的影响

为了研究焊接残余应力和几何初始缺陷对矩形钢管混凝土柱壁板屈曲后强度的影响,采用有限元软件ABAQUS进行参数分析,参数包括壁板的屈服强度、宽厚比、几何初始缺陷取值大小、是否施加几何初始缺陷和焊接残余应力,并考虑两者耦合作用,给出考虑焊接残余应力和几何初始缺陷后的矩形钢管混凝土柱壁板有效宽度计算公式,并与试验数据进行比较。结果表明:建立的有限元模型能较好地模拟壁板的局部屈曲和屈曲后行为,焊接残余应力和几何初始缺陷都会降低壁板屈曲后强度,且焊接残余应力的影响较大; 不同强度等级的钢材都需要考虑焊接残余应力和几何初始缺陷的影响,对于屈服强度大于460 MPa的高强钢材,当宽厚比大于65时,可以忽略几何初始缺陷的影响; 给出的有效宽度计算公式可以较为准确且偏保守地预测矩形钢管混凝土柱壁板屈曲后强度。     

考虑杆件失稳的单层球面网壳稳定性分析与试验研究

为研究杆件初始缺陷和杆件失稳对网壳结构稳定性的影响,采用ANSYS有限元分析软件,引入杆件的初始缺陷,考虑杆件自身的屈曲破坏,确定其最不利整体缺陷模态和杆件缺陷模态。采用传统梁单元模型和精细化壳单元模型分别对采用圆钢管、矩形钢管、工字钢三种不同截面形式杆件的网壳结构进行稳定性分析,结果表明,考虑杆件缺陷的网壳结构稳定承载力大幅度降低。进行杆件轴力跟踪分析和杆件失稳传导分析发现,单层网壳的整体稳定破坏源于单根杆件的率先屈曲,杆件失稳瞬间迅速向相邻杆件传导,导致结构整体破坏。进行了K6（2）工字钢截面网壳的加载破坏试验,验证了预设缺陷的单根杆件失稳并传递诱发了整体破坏,且承载力受工字钢弱轴缺陷的影响很大。     

On lateral buckling of subsea pipe-in-pipe systems

Lateral buckling is a common phenomenon in unburied high temperature and high pressure (HT/HP) subsea pipe-in-pipe systems. An effective finite element model based on beam element and tube-to-tube contact element is proposed to study lateral buckling in pipe-in-pipe systems, in which the initial imperfection (out-of-straightness), nonlinear pipe-soil interaction and nonlinear material properties are considered. The results show that it is the resultant axil force of the inner and outer pipe that governs the lateral buckling phenomenon in pipe-in-pipe systems. The initial imperfection and pipe-soil interaction are the most important factors which influence evolution of the displacement and stress in pipe-in-pipe systems. Nonlinear material properties are necessary when plastic strain may occur in post-buckling stage. At last, a simplified model is proposed which can easily calculate the critical buckling temperature of the corresponding pipe-in-pipe system.     

Finite element technique for design of stub columns

This paper presents a finite element analysis on the post-buckling behaviour of thin-walled cold-formed lipped channel and hat-section stub columns under axial compression. The specimens were modelled exactly as that used in the carefully controlled stub column tests conducted by Zaras and Rhodes. Numerical predictions on the load versus end-shortening characteristics and ultimate load capacity of the structures were obtained using a non-linear finite element analysis. The effect of the input parameters such as the degree of prescribed initial imperfection and the size of the element mesh, on the convergence of the solution is examined. The initial imperfection for post-buckling analysis is achieved by making use of the exact linear buckling wave form. Standard design procedures are developed for post-buckling analysis for thin-walled stub columns using finite element method. Results from the design procedure correlate well with experimental data and BS5950 predictions.     

某车站跨层重载桁架稳定承载力缺陷敏感性研究

为了研究大庆西站跨层重载平面桁架对几何缺陷的敏感性,采用大位移控制法(LDC),提出确定跨层桁架初始几何缺陷的静力变形模态法。分析6种不同幅值初始缺陷条件下跨层桁架结构的非线性稳定承载力,并与一致缺陷模态法计算结果对比,对该平面桁架结构几何缺陷敏感性进行研究。部分结果与相关缩尺模型试验进行比对。研究表明:几何缺陷对结构的承载力影响较小,对结构变形失稳影响较大。经对比分析,静力变形模态法可以作为计算结构稳定承载力的有效方法之一。     

顺层边坡岩体结构的不稳定性态研究

针对顺层边坡岩体结构的初始缺陷特性和非完善屈曲特征,应用初始后屈曲理论和突变理论探讨了缺陷结构的后屈曲性态,分别以载荷和位移的形式确定了边坡结构分岔点附近的邻域,建立了边坡结构溃屈破坏和剪切滑动的判据,并以雅砻江下游的霸王山边坡为例给出了工程实例。研究结果表明：位能增量泛函中表征结构初始缺陷的零阶余项与尖点突变模型中的剖分因子之间具有确定的关系;顺层边坡岩体结构在其分岔点处的载荷临界值是由其突变模型的分岔集确定的;只有考虑了岩体边坡的实际缺陷,才可得出其溃屈破坏和剪切滑动的极限值。     

Wind pressures and buckling of cylindrical steel tanks with a dome roof

An experimental/computational strategy is used in this paper to evaluate the buckling behavior of steel tanks with a dome roof under exposure to wind. First, wind tunnel experiments using small scale rigid models were carried out, from which pressure distributions due to wind on the cylindrical part and on the roof were obtained. Second, a computational model of the structure (using the pressures obtained in the experiments) was used to evaluate buckling loads and modes and to study the imperfection sensitivity of the tanks. The computational tools used were bifurcation buckling analysis (eigenvalue analysis) and geometrical nonlinear analysis (step-by-step incremental analysis). Geometric imperfections and changes in the buckling results due to reductions in the thickness were also included in the study to investigate reductions in the buckling strength of the shell. For the geometries considered, the results show low imperfection sensitivity of the tanks and buckling loads associated with wind speeds 45% higher than those specified by the ASCE 7-02 standard.     

半离散法分析薄壁柱的畸变屈曲

采用广义梁理论(GBT)的耦合差分方程解决了半离散法分析薄壁柱的畸变屈曲问题。作者近期发表的两篇文章对类似GBT的新型半离散分析方法进行了阐述。对横截面进行离散分析,寻找沿梁变化的解析解。采用新方法,利用齐次和非齐次方程建立确定梁全部变形的一般GBT方程并求解,从而使GBT的(复杂)变形方程变形为可降阶的微分方程。提出的半离散方法在广义梁理论(GBT)基础上增加了用于柱的失稳分析和失稳形态识别的几何刚度因素。通过势能的变化并在梁理论中引入约束条件,对初始应力下建立的GBT齐次微分方程进行修正,以分析其变形特性。对简支梁梁端施加横向位移和轴力,建立GBT初始应力方程,通过该方程寻求失稳的解决方法。根据已知的边界条件,利用三角函数关系式和求解特征值的方法求解这些可降阶的微分方程,使得屈曲形态和相关特征值与分叉荷载因素相符。因此,无需通过模态分解,可由耦合的GBT方程直接求得屈曲形态的解析解。通过实例分析了柱的整体失稳、屈曲变形和局部纵弯失稳,以及如何将新方法用于描述特征曲线和弹性屈曲曲线。将该方法的分析结果与ABAQUS、GBTUL和CUFSM软件的分析结果进行对比,验证了该方法的正确性。     

Wind pressures and buckling of cylindrical steel tanks with a conical roof

Tanks with a conical roof are studied in this paper under wind load, for a roof which is supported by rafters and columns. Buckling occurs in the form of deflections in the cylindrical shell and the buckling mode is localized in the windward region. Both bifurcation analysis and geometrically nonlinear analysis have been performed using finite element discretizations of the structure. The wind pressures have been obtained from wind tunnel experiments performed as part of the research, and have been obtained for tank geometries for which information was not previously available. The results show high imperfection sensitivity of tanks with a conical roof, and buckling loads for wind velocities in the same order as those expected to occur in the Caribbean region.     

Distortional buckling modes of semi-discretized thin-walled columns

This paper presents distorting buckling solutions for semi-discretized thin-walled columns using the coupled differential equations of a generalized beam theory (GBT). In two related papers recently published by the authors a novel semi-discretization approach to GBT has been presented. The cross section is discretized and analytical solutions are sought for the variation along the beam. With this new approach the general GBT equations for identification of a full set of deformation modes corresponding to both homogeneous and non-homogenous equations are formulated and solved. Thereby giving the (complex) deformation modes of GBT which decouple the state space equations corresponding to the reduced order differential equations.In this paper the developed semi-discretization approach to generalized beam theory (GBT) is extended to include the geometrical stiffness terms, which are needed for column buckling analysis and identification of buckling modes. The extension is based on an initial stress approach by addition of the related potential energy terms. The potential energy of a single deformation mode is formulated based on a discretization of the cross section. Through variations in the potential energy and the introduction of the constraints related to beam theory this leads to a modified set of coupled homogeneous differential equations of GBT with initial stress for identification of distortional displacement modes. In this paper we seek instability solutions using these GBT initial stress equations for simply supported columns with constrained transverse displacements at the end sections and a constant axial initial stress. Based on the known boundary conditions the reduced order differential equations are solved by using the trigonometric solution functions and solving the related eigenvalue problem. This gives the buckling mode shapes and the associated eigenvalues corresponding to the bifurcation load factors. Thus the buckling modes are found directly by the analytical solution of the coupled GBT-equations without modal decomposition. Illustrative examples showing global column buckling, distortional buckling and local buckling are given and it is shown how the novel approach may be used to develop signature curves and elastic buckling curves. In order to assess the accuracy of the method some of the results are compared to results found using the commercial FE program Abaqus as well as the conventional GBT and FSM methods using the software packages GBTUL and CUFSM.     

Random imperfection method for stability analysis of a suspended dome

The traditional eigenvalue buckling mode method used for the stability analysis of suspended domes has been deemed unsafe and unreasonable. A random imperfection method is proposed in this paper that is suitable for the stability analysis of suspended domes. Using this method, the impact of initial imperfections, such as joint position deviation, eccentricity of members, variation in cable pretension and elastic modulus of materials, on the stability of nonlinear geometries as well as the stability of both nonlinear geometries and materials (i.e., ultimate bearing capacity) are studied. The analysis results using this method are compared to those obtained from the traditional eigenvalue buckling mode method. This study indicates that the random imperfection method can more reasonably demonstrate the impact of initial imperfections on the stability of suspended domes. Additionally, the shape and size of the initial imperfections applied by the random imperfection method are more reasonable, and the results are safer. The random imperfection method is successfully used in the stability analysis of a large-span suspended dome, and the results of this method are nearly equivalent to that of the model test, which verifies the method.     

单层组合加肋拱支网壳的稳定性

介绍了某相贯单层组合加肋拱支网壳结构在竖向荷载及风荷载作用下的整体稳定性。采用有限元软件MIDAS,在两种荷载工况下分别进行了此网壳结构的特征值屈曲、几何非线性屈曲分析,初始缺陷对网壳结构的整体稳定性影响分析。结果表明:网壳结构对初始缺陷较为敏感,初始缺陷会明显降低结构稳定性;不同的荷载工况下,失稳的情况亦不同且网壳结构稳定临界系数随着荷载的增大而降低。     

Selection of the initial geometrical imperfection in nonlinear FE analysis of cold-formed steel rack columns

This paper presents three methodologies to predict the load carrying capacity of cold formed steel rack columns via nonlinear finite element analysis (FEA). The column lengths are chosen in the range where the main failure is due to distortional buckling. It is demonstrated that for this range, to derive an accurate prediction of the ultimate load is more complex than for lengths where the main failure is local or global. The nonlinear analysis is carried out introducing an initial geometric imperfection on the upright. The three methodologies presented here allow for different imperfection shapes. The first one uses the critical mode shape (the first buckling mode). The second corresponds to an iterative methodology in which the shape that leads to the lowest ultimate load is used. These two first methodologies use exclusively the finite element method (FEM). The third one combines the finite element analysis with the generalised beam theory (GBT) in order to determine the modal participation of the FEM buckling mode and generate a particular combined geometric imperfection. To validate the predicted loads, the results of the three methodologies are compared with values obtained in experimental tests.     

青岛北站撑杆式预应力钢压杆的极限承载力分析

结构的屈曲分析可分为特征值屈曲分析和非线性屈曲分析。简要阐述结构特征值屈曲分析和非线性屈曲分析的的基本理论,利用ABAQUS对青岛北站中的预应力钢压杆进行特征值屈曲分析和考虑材料非线性和几何非线性的非线性屈曲分析,对比两种分析方法所得的极限承载力的差别,同时在非线性屈曲分析中,分析在不同的初始缺陷和不同的初始预应力的条件下预应力压杆极限承载力的变化,所得结论可为同类大跨空间结构预应力压杆的设计与计算提供参考。     

Imperfection sensitivity of plate girder webs subjected to patch loading

This paper is aimed at studying the influence of initial geometric imperfections on the postbuckling behavior of longitudinally stiffened plate girder webs subjected to patch loading. A sensitivity analysis is conducted herein using two approaches (deterministic and probabilistic) in order to investigate the effect of varying imperfection shape and amplitude on both, the postbuckling response and ultimate strength of plate girders under patch loading. This sensitivity analysis is performed by means of nonlinear finite element analysis. At first, the initial shape imperfections are modeled using the buckling mode shapes resulting from an eigenvalue buckling analysis. Thereafter, the amplitude of the buckling shapes for the various modes is varied, and then introduced in the nonlinear analysis. The results show the influence of these modes and amplitudes on the resistance to patch loading.