关于圆柱壳几何缺陷分析中傅立叶级数选取的探讨

圆柱壳屈曲一般对壳壁上微小几何缺陷的型式和幅值均十分敏感.为了能将缺陷的不同分量和圆柱壳的结构特征联系起来以及研究缺陷各分量对壳屈曲强度的影响,缺陷通常采用傅立叶级数分解.然而,大多数先前的研究选取不适当的傅立叶级数得到不正确的结果.本文首先考察傅立叶级数的数学描述基础,进而讨论不同傅立叶级数在描述不同型式几何缺陷的表现,从而得出如何选取适当的傅立叶级数用来描述圆柱壳几何缺陷的结论.采用这些适当的傅立叶级数,能更好地了解圆柱壳几何缺陷的特征分量以及这些分量对壳体屈曲强度的影响.     

Effect of imperfections on wind-loaded cylindrical shells

The wind-induced buckling of thin-walled steel cylinders, such as silos and tanks, was investigated by wind-tunnel testing for a limited range of the parameters. While wind-buckling of short, stocky shells can be well represented by an equivalent uniform external pressure, long shells show quite a different behaviour, which is strongly influenced by the axial compression forces. This paper presents a study of this behaviour, based on numerical analyses and considering various types of imperfection shape. The effect of geometrical nonlinearity, as well as material plasticity, has also been included. A comparison with test results shows good agreement with the numerical results of imperfect shells, provided that special eigenmode-shaped modes of imperfections are excluded.     

Stresses in thin spherical shells with imperfections. Part II: Influence of local imperfections

The changes in stress resultants in thin spherical shells, associated with a local imperfection introducing curvature errors in all directions, are investigated. An axisymmetric finite element model of the shell and imperfection is employed to carry out the linear elastic analysis. Parametric studies have been performed, to identify the main parameters controlling the response, for the case of internal pressure. The results are compared with those obtained in Part I for axisymmetric imperfections, and bounds for maximum elastic stress resultants are established to cover the possibility of both local and axisymmetric imperfections.     

Stresses in thin spherical shells with imperfections. Part I: Influence of axisymmetric imperfections

A finite element model of axisymmetric geometry is used to obtain stress and moment fields in the region of an imperfection in thin, spherical shells. In Part I the studies are restricted to axisymmetric imperfections with a cosine variation along the meridian. Parametric studies are carried out to identify the main parameters controlling the response for internal pressure and gravity load. The results show that the behaviour of the shell is similar to the imperfect cylindrical shell with the same radius as in the spherical shell.     

Effects of imperfections of the buckling response of composite shells

The results of an experimental and analytical study of the effects of initial imperfections on the buckling response and failure of unstiffened thin-walled compression-loaded graphite-epoxy cylindrical shells are presented. The shells considered in the study have six different shell-wall laminates two different shell-radius-to-thickness ratios. The shell-wall laminates include four different orthotropic laminates and two different quasi-isotropic laminates. The shell-radius-to-thickness ratios includes shell-radius-to-thickness ratios equal to 100 and 200. The numerical results include the effects of traditional and nontraditional initial imperfections and selected shell parameter uncertainties. The traditional imperfections include the geometric shell-wall mid-surface imperfections that are commonly discussed in the literature on thin shell buckling. The nontraditional imperfections include shell-wall thickness variations, local shell-wall ply-gaps associated with the fabrication process, shell-end geometric imperfections, nonuniform applied end loads, and variations in the boundary conditions including the effects of elastic boundary conditions. The cylinder parameter uncertainties considered include uncertainties in geometric imperfection measurements, lamina fiber volume fraction, fiber and matrix properties, boundary conditions, and applied end load distribution. Results that include the effects of these traditional and nontraditional imperfections and uncertainties on the nonlinear response characteristics, buckling loads and failure of the shells are presented. The analysis procedure includes a nonlinear static analysis that predicts the stable response characteristics of the shells, and a nonlinear transient analysis that predicts the unstable response characteristics. In addition, a common failure analysis is used to predict material failures in the shells.     

Collapse of axially compressed cylindrical shells with random imperfections

The establishment of an International Imperfection Data Bank is discussed. Characteristic initial imperfection distributions associated with different fabrication techniques are shown.

Using a first-order, second-moment analysis, a stochastic method is presented, whereby the stability of isotropic, orthotropic and anisotropic nominally circular cylindrical shells under axial compression, external pressure and/or torsion possessing general nonsymmetric random initial imperfections can be evaluated. Results of measurements of initial imperfections are represented in Fourier series and the Fourier coefficients are used to construct the second-order statistical properties needed. The computation of the buckling loads is done with standard computer codes and includes a rigorous satisfaction of the specified boundary conditions.

It is shown that the proposed stochastic approach provides a means to combine the latest theoretical findings with the practical experiences spanning about 75 years in an optimal manner via the advanced computational facilities currently available.     

Investigation of the buckling behavior of conical shells under weld-induced imperfections

The initial depression of shell skins is usually created through various panel processes such as rolling or welding. It is important to create some basic design regulations associated with the existing codes. A longitudinal imperfection caused by the continuous welding of a panel's edge to form a cone is the most important case in this context. The present paper discusses 14 laboratory specimens in 2 groups, labeled Shallow Conical Caps (SCC) and Deep Conical Caps (DCC), loaded under uniform hydrostatic pressure. The samples were modified to include either 1 or 2 line imperfections with amplitudes of 1t, 2t and 3t in depth (t the thickness of conical shell). The results presented here are in general agreement with international codes as well as theories concerning initial and overall buckling and collapse.     

大直径筒仓仓顶结构施工技术

介绍了25 m直径圆形筒仓仓顶结构施工方案,采用刚性滑模平台支撑施工锥壳、桁架吊挂施工大梁,减少了大量周转材料的投入,缩短了工期,保证了安全和质量。     

The effect of geometric imperfections on the vibrations of anisotropic cylindrical shells

Analytical–numerical models to analyse the flexural vibration behaviour of anisotropic cylindrical shells are presented. The two models (denoted as Level-1 and Level-2 Analysis) have different levels of complexity and can be used to study the influence of important parameters, such as geometric imperfections, static loading, and boundary conditions. A specific anisotropic shell is used in the calculations in this paper. The influence of the imperfection shape and amplitude on the natural frequency is investigated for this shell via both the Level-1 and the Level-2 Analysis. Imperfections with the shape of the “lowest vibration mode” give a decrease of the natural frequency with increasing imperfection amplitude. The results of the Level-2 Analysis for the effect of imperfections on the natural frequency are in reasonable agreement with Finite Element calculations.     

Buckling of elastic cylindrical shells considering the effect of localized axisymmetric imperfections

The effect of localized axisymmetric initial imperfections on the critical load of elastic cylindrical shells subjected to axial compression is studied through analytical modeling. Some classical results regarding sensitivity of shell buckling strength with respect to distributed defects having axisymmetric or asymmetric forms are recalled. Special emphasis is placed after that on the more severe case of localized defects satisfying axial symmetry by displaying an analytical solution to the Von Kármán–Donnell shell equations under specific boundary conditions. The obtained results show that the critical load varies very much with the geometrical parameters of the localized defect. These variations are not monotonic in general. They indicate, however, a clear reduction of the shell critical load for some defects recognized as the most hazardous isolated ones. Reduction of the critical load is found to reach a level which is up to two times lower than that predicted by general distributed defects.     

Finite element techniques for the analysis of cooling tower shells with geometric imperfections

Two finite element methods for analysing geometrically imperfect cooling tower shells are presented. In the first the geometry of the imperfection is modelled by the elements; in the second the imperfection is represented by an equivalent load on the shell. Axisymmetric and general shell elements have been considered.Results are given which show that the first approximation to the equivalent load is sufficiently accurate and that it is possible to represent local imperfections by axisymmetric imperfections which require less computation. It is also shown that axisymmetric elements should be used wherever possible, because of their greater efficiency, following the geometry of an axisymmetric imperfection but representing local imperfections by equivalent loads.     

Computational modelling of geometric imperfections and buckling strength of cold-formed steel

Computational modelling of the buckling strength of cold-formed steel members as influenced by initial geometric imperfections is studied. The geometric imperfections are represented by the member eigenmode shapes. Along with the classical measure — the amplitude of imperfections, an energy measure defined by the square root of the elastic strain energy hypothetically required to distort the originally perfect structural element into the considered imperfect shape is used. Based on the measures, two approaches for the choice of the most unfavourable imperfections are suggested. Normalising imperfections by the amplitude, the energy measure is calculated as indicative parameter of imperfection significance. Vice versa, when adopting normalisation by the energy measure, the amplitude is used as a supporting parameter. The suggestions are illustrated on calculating the strength of an axially compressed steel lipped channel column with eigenmodes exhibiting local-distortional interactions. For eigenvalue and geometrically and materially non-linear strength calculations, the FEM codes MSC.NASTRAN and COSMOS/M are employed.     

钢框架-混凝土芯筒结构竖向变形差分析解决

介绍了高层钢框架-混凝土芯筒(剪力墙)结构体系的优点和竖向变形差产生的原因,提出了竖向变形差的分析方法,并从结构材料选用、结构设计、施工下料和施工顺序方面进行了分析,总结了减小竖向变形差的解决方法.     

钢筋混凝土筒仓壁内外温差对仓壁设计的影响

针对水泥入库温度对钢筋混凝土筒仓仓壁产生的温差影响,分析了温差产生温度作用效应造成仓壁结构开裂的原因,提出结构构造设计、混凝土原料、配合比选择等方面的技术措施,从而控制温度应力裂缝的产生。     

大型落地式钢板仓地震作用分析

结合某落地式水泥钢板筒仓工程为例,利用有限元软件ABAQUS建立了三维筒仓模型,采用时程分析法,对钢板仓进行了地震作用分析,根据钢筒仓设计规范,探讨了钢筒仓的抗震性能,为落地式钢板筒仓的抗震设计提供了参考依据。     

联体筒仓滑模施工工艺

针对联体筒仓滑模施工工艺进行了阐述,分别对滑升模板的设计、模板滑升施工、滑模装置拆除进行了分析,并给出了一系列滑模施工质量控制措施,以确保施工的顺利进行。     

On the incorporation of equivalent member imperfections in the in-plane design of steel frames

This work deals with the incorporation of equivalent member imperfections in the global analysis of steel frames and, in particular, is intended to clarify the Eurocode 3 (EC3) provisions involved in such procedure. In fact, these provisions stem from the well-known “European column buckling curves”, which means that they are based on the behaviour of simply supported isolated members under uniform compression (columns). First, one addresses the geometrically non-linear behaviour of isolated columns displaying arbitrary support conditions and different initial geometrical configurations. Then, the results obtained are used to propose a systematic and rational method to evaluate the appropriate “equivalent initial imperfections” that need to be incorporated in the second-order global elastic analysis of a frame or isolated compressed member. This method (i) is fully consistent with the EC3 column buckling curves and (ii) adopts critical buckling mode shape initial imperfections with amplitudes determined by means of closed-form analytical expressions. In order to enable a better grasp of the concepts involved and also to illustrate the capabilities of the proposed methodology, several numerical examples are presented and discussed throughout the paper.     

浅谈索承网壳结构

介绍了索承网壳结构的特点，对其研究现状进行了分析，结合具体工程实例，阐述了其结构及总体施工方案，提出了索承网壳结构今后的发展方向。     

谈两种不同材质和不同型号管道接口的处理

结合管道工程施工经验,对不同材质和不同型号两种管道接口的处理方法作了介绍,具体就不锈钢卡箍连接、法兰连接、专用管件连接、丝扣连接的操作要点进行了叙述,为实际工程中管道连接提供参考。