预应力混凝土钢桁架叠合板受弯性能试验与理论研究

通过在预应力混凝土平板上增设钢桁架方式,提出一种预应力混凝土钢桁架叠合板,解决了现阶段其他类型叠合板用预制底板存在开裂荷载较低、临时支撑密集等问题。为研究不同桁架类型对预制底板及叠合板受弯性能的影响,对6块预制底板试件和4块叠合板试件开展了静力加载试验,得到其破坏特征、开裂荷载、挠度曲线及应变分布等。试验结果表明：钢桁架能够显著提高底板的开裂荷载,其中钢管桁架底板试件的开裂荷载最高,达到平板试件的194%以上,适用跨度最大达到9m;不同桁架类型底板试件的破坏模式存在一定差异,钢板桁架、钢管桁架和钢筋桁架预制底板的破坏特征分别为钢板屈曲、焊缝断裂及钢筋弯曲;钢桁架能显著增强底板及叠合层混凝土的整体受力及协同工作性能;桁架类型对叠合板的受弯性能影响较小,不同桁架类型叠合板的开裂荷载均相同,开裂挠度差异在10%以内。基于试验结果,建立了底板上弦失稳弯矩计算公式,并提出预制底板失效弯矩应取开裂弯矩与上弦失稳弯矩的较小值,计算值与试验值吻合较好。     

Study on the influence of the initial deflection and load combination on the collapse behaviour of continuous stiffened panels

Using the finite element analysis, a series stiffened panels under combined normal loads and biaxial compressions are conducted to investigate the effect of several influential factors on the ultimate limit states. Two spans/bays FE model with periodical boundary condition is adopted to consider the interaction between adjacent structural members. The initial deflections assumed as Fourier components including symmetric and asymmetric modes are used to identify the half-wave number of collapse of the local plate, which is compared with half-wave number of buckling calculated by formula. Based on the numerical results, the influences of half-wave number assumed in the equivalent initial imperfection and loads combination on the collapse behaviours of stiffened panels are discussed. It is found that lateral pressure might increase the ultimate strength of stiffened panels for the stiffener-induced failure modes. The one half-wave region of local plate influences significantly the load carrying capacity of stiffened panels.     

Interaction of buckling modes in castellated steel beams

This paper investigates the behaviour of normal and high strength castellated steel beams under combined lateral torsional and distortional buckling modes. An efficient nonlinear 3D finite element model has been developed for the analysis of the beams. The initial geometric imperfection and material nonlinearities were carefully considered in the analysis. The nonlinear finite element model was verified against tests on castellated beams having different lengths and different cross-sections. Failure loads and interaction of buckling modes as well as load-lateral deflection curves of castellated steel beams were investigated in this study. An extensive parametric study was carried out using the finite element model to study the effects of the change in cross-section geometries, beam length and steel strength on the strength and buckling behaviour of castellated steel beams. The parametric study has shown that the presence of web distortional buckling causes a considerable decrease in the failure load of slender castellated steel beams. It is also shown that the use of high strength steel offers a considerable increase in the failure loads of less slender castellated steel beams. The failure loads predicted from the finite element model were compared with that predicted from Australian Standards for steel beams under lateral torsional buckling. It is shown that the Specification predictions are generally conservative for normal strength castellated steel beams failing by lateral torsional buckling, unconservative for castellated steel beams failing by web distortional buckling and quite conservative for high strength castellated steel beams failing by lateral torsional buckling.     

An experimental study of ultimate compressive strength of transversely stiffened aluminium panels

An experimental investigation was carried out to determine the ultimate strength of welded stiffened aluminium panels in alloy 6082 T6 subjected to in-plane compressive loads normal to the directions of the stiffeners. This load case is not treated in the European standard for aluminium structures, Eurocode 9. A total of 21 panel specimens with various side aspect ratios and both open and closed stiffener sections were tested in a purpose made test rig. Great care was taken to ensure the rig gave very precise boundary conditions. The panels were manufactured by metal inert gas arc welding and friction stir welding. An extensive measurement program was carried out to determine the distribution of material strength and initial geometric imperfections. Small imperfection amplitudes were found. Tensile tests revealed variation in material properties, but the strength values were on average higher than the values stated in Eurocode 9. The panels failed by two different deformation modes; global flexural buckling and local buckling of the plate elements between the stiffeners.     

Safety of glass panels against wind loads

Safety of glass panels under wind loads is investigated. Noting that strength of glass displays significant size dependent scatter, the theory of brittle fracture statistics is applied to find the probability of failure of brittle rectangular plates under uniform lateral loads. The effects of plate size, plate thickness, intensity of lateral load and strength of glass on probability of failure are studied. The results are presented graphically and a specific example is studied in detail.     

The buckling characteristics of some integrally formed bead stiffened composite panels

Composite panel stability can easily be improved by using vertical male beads. In this paper, new methods of stabilizing techniques used for the panels, webs and ribs of composite structures are studied. A parametric study is performed to assess the effects of important design considerations such as, bead length, number of beads, bead radius, bead depth and bead spacing on the initial buckling load of the panels. The results show that, there is an optimum bead spacing for each panel containing more than one bead which can be estimated using a simple equation. Integration of vertical beads with a length of less than 0.5 times the panel's length has no significant effect on the buckling load. There are no significant changes on the buckling loads of the beaded panels with bead depths greater than 0.6 times the bead radius. In this investigation, the instability of the nose and main ribs of a light airplane wing structure made of woven E-glass material and stiffened by P.V.C foam core and vertical male beads are also studied using experimental methods. The experimental results show that we can easily improve the buckling capability of the panels and webs by using vertical male beads instead of sandwiched construction. It is estimated that this would cause a weight reduction of about 50% and a manufacturing time reduction of about 50%.     

Shear buckling in foam-filled web core sandwich panels using a Pasternak foundation model

Web core panels, foam-filled sandwich panels with interior webs, are a structurally efficient option for transverse load bearing applications. In web core panels, the interaction between the webs and core material can have a substantial impact on web shear buckling strength and is a key element of lightweight structural design. The present work is an investigation of web buckling behavior in web core panels under a distributed load. To solve this problem, web shear buckling was analyzed for the case of pure shear loading with foam support, and this analytic model was extended to the case of panels with a transverse distributed load. The webs are modeled as simply supported plates resting on a Pasternak elastic foundation. To that end, a buckling model for plates on a Pasternak foundation is presented, along with closed-form approximations of the solution for square and infinitely long plates. An accurate model for the foundation constants is developed using energy methods. Applicability of the plate buckling model to web core panels with transverse loads is presented via a finite element study. In panels, the slenderness and spacing of the webs have a slight effect on the boundary conditions between the webs and face sheets. The effect is relatively small, however, and the model presented in this work underpredicts buckling strength by less than 25%. The model in this work is thus a reasonable approach to the practical design of web core panels.     

Dynamic buckling of fiber composite shells under impulsive axial compression

The paper deals with dynamic buckling due to impulsive loading of thin-walled carbon fiber reinforced plastics (CFRP) shell structures under axial compression. The approach adopted is based on the equations of motion, which are numerically solved using a finite element code (ABAQUS/Explicit) and using numerical models validated by experimental static buckling tests. To study the influence of the load duration, the time history of impulsive loading is varied and the corresponding dynamic buckling loads are related to the quasi-static buckling loads. To analyse the sensitivity to geometric imperfections, the initial geometric imperfections, measured experimentally on the internal surface of real shells, are introduced in the numerical models. It is shown numerically that the initial geometric imperfections as well as the duration of the loading period have a great influence on the dynamic buckling of the shells. For short time duration, the dynamic buckling loads are larger than the static ones. By increasing the load duration, the dynamic buckling loads decrease quickly and get significantly smaller than the static loads. Since the common practice is to assume that dynamic bucking loads are higher than the static ones, which means that static design is safe, careful design is recommended. Indeed, taking the static buckling load as the design point for dynamic problems might be misleading.     

Y形柱稳定性分析研究

Y形柱是机场航站楼屋盖支承体系中常采用的一种构件形式,作为主要承重构件,其稳定性决定了屋盖结构的整体承载能力。利用荷载-位移全过程跟踪技术,考虑几何非线性、材料非线性和初始几何缺陷,研究了在竖向和水平风、地震作用下薄壁Y形柱的稳定性。研究表明:竖向荷载作用下Y形柱以分叉段平面外屈曲为主要失稳模式;Y形柱对初始几何缺陷不敏感;水平风荷载对Y形柱稳定性不起控制作用;竖向和水平荷载作用下,发生面外失稳前柱子分叉处已经进入塑性变形,柱子强度问题先于整体失稳。     

Ultimate strength of stiffened plates with a square opening under axial and out-of-plane loads

The high strength to weight ratio and high stiffness to weight ratio of stiffened plates find wide application in aircraft structures, ship structures, offshore oil platforms and lock gates. The strength and stability of stiffened plates is highly influenced by openings and initial imperfections. The main objective is to study the behaviour of stiffened steel plates with openings up to collapse and to trace the post-peak behaviour under axial and out-of-plane loads. Four stiffened steel plates with a square opening were fabricated for testing. Angle sections were used as stiffeners. Imperfections in the plate, stiffener and overall imperfection of the whole panel were measured. All fabricated panels were tested to failure. A finite element (FE) model was developed for the analysis of stiffened plates with initial imperfections and validated with the test results. Parametric studies were conducted using the developed FE model, and interaction curves and equations were developed for the design of stiffened plates with initial imperfections and openings. The interactive effect for stiffened panels with a square opening was found to be linear, with proportional reduction of the ultimate axial load carrying capacity due to the constant out-of-plane load.     

大矢跨比单层球面网壳动力试验模型弹塑性稳定分析

为了解水平地震作用下具有不同失效机制的单层球面网壳结构在静力荷载作用下的弹塑性稳定性能,利用有限元软件ANSYS,对两个矢跨比为1/2的单层球面网壳结构试验模型进行双重非线性全过程分析,获得结构的弹塑性极限承载力,比较二者的失稳模态,初步了解二者之间的差异。考察结构杆件屈曲、初始缺陷等因素对结构稳定性能的影响,并分析各因素对结构极限承载力的影响规律。结果表明,地震作用下,具有强度破坏特征的网壳结构在静力下的失稳模式表现为结构的整体失稳,而发生动力失稳破坏的结构则表现为局部失稳破坏。杆件失稳和初始缺陷使结构的临界荷载大幅度降低,且地震作用下属于强度破坏的单层球面网壳结构在静力下对初始缺陷的敏感性大于动力失稳破坏结构。     

An investigation of lightly profiled sandwich panels subject to local buckling and flexural wrinkling effects

Sandwich panels exhibit various types of failure modes depending on the steel face used. For the flat and lightly profiled sandwich panels, flexural wrinkling is an extremely important design criterion as the behaviour of these panels is governed mainly by flexural wrinkling. However, in the lightly profiled panels, when the depth or spacing of the ribs increases, flat plate buckling between the ribs occurs leading to the failure of the entire panel due to the interaction between local buckling and flexural wrinkling modes. Current design formulae for sandwich panels do not consider such interactive buckling effects. To obtain a safe design solution, this interactive buckling behaviour should be taken into account in the design of lightly profiled sandwich panels. Therefore a research project was undertaken to investigate the interactive buckling behaviour of lightly profiled panels with varying depths and spacings of the ribs using a series of experiments and finite element analyses. A new improved design formula was developed for the safe and economical design of lightly profiled panels that takes into account the interaction between local buckling and flexural wrinkling. This paper presents the details of this investigation, the results and the new design formula.     

Nonlinear mechanical,thermal and thermo-mechanical postbuckling of imperfect eccentrically stiffened thin FGM cylindrical panels on elastic foundations

This paper presents an analytical approach to investigate the nonlinear stability analysis of eccentrically stiffened thin FGM cylindrical panels on elastic foundations subjected to mechanical loads, thermal loads and the combination of these loads. The material properties are assumed to be temperature-dependent and graded in the thickness direction according to a simple power law distribution. Governing equations are derived basing on the classical shell theory incorporating von Karman–Donnell type nonlinearity, initial geometrical imperfection, the Lekhnitsky smeared stiffeners technique and Pasternak type elastic foundations. Explicit relations of load–deflection curves for FGM cylindrical panels are determined by applying stress function and Galerkin method. The effects of material and geometrical properties, imperfection, elastic foundations and stiffeners on the buckling and postbuckling of the FGM panels are discussed in detail. The obtained results are validated by comparing with those in the literature..     

Instability of steel gusset plates in compression

As a result of the structural failure of the I-35W Bridge in Minneapolis in 2007, the Federal Highway Administration (FHWA) issued guidelines for the load rating of gusset plates and recommended that the capacity of these plates on non-redundant steel truss bridges be verified. The purpose of this study is to examine the buckling behaviour of steel gusset plates in greater detail, accounting for parameters that were not explicitly included in the guidelines, such as initial deformations of the gusset plate, stiffness of the framing members, load distribution from the framing members to the plate and load eccentricity. For this purpose, a finite-element model of a gusset plate was developed and verified against experimental measurements. Results show that, for in-plane compressive loads with no moment and no eccentricity, the FHWA guidelines for load rating are conservative and safe for initial out-of-plane deformations up to one plate thickness.     

The computational post buckling analysis of fuselage stiffened panels loaded in compression

Fuselage panels are commonly fabricated as skin–stringer constructions, which are permitted to locally buckle under normal flight loads. The current analysis methodologies used to determine the post buckling response behaviour of stiffened panels relies on applying simplifying assumptions with semi-empirical/empirical data. Using the finite element method and employing non-linear material and geometric analysis procedures, it is possible to model the post buckling behaviour of stiffened panels without having to place the same emphases on simplifying assumptions or empirical data. Investigation of element, mesh, idealisation, imperfection and solution procedure selection has been undertaken, with results validated against mechanical tests. The research undertaken has demonstrated that using a commercial implicit code, the finite element method can be used successfully to model the post buckling behaviour of flat riveted panels. The work has generated a series of guidelines for the non-linear computational analysis of flat riveted panels subjected to uniform axial compression.     

Structural behaviour of eccentrically loaded precast sandwich panels

Results of an experimental investigation to study the ultimate strength behaviour of precast concrete sandwich panels (PCSP) with steel truss shear connectors are reported. Six full-scale sandwich panels with variable slenderness ratio were cast and tested under eccentric loads. Deflection characteristics, variations of strains across the insulation layer, strains in shear connectors, crack appearance and propagation under increasing load were recorded and analysed. The role of the shear truss connectors in transferring load from the outer wythe (layer) to the inner and ensuring composite behaviour was also observed. Results obtained showed that all panels behaved in a fully composite manner under eccentric load till failure. The ultimate strength of the PCSPs was found to decrease non-linearly with the increase in the slenderness ratio. Because of the complex behaviour of PCSP due to its material non-linearity and the interaction between its various components, finite element analysis (FEA) was conducted. Comparison with test results indicated that the FEA closely estimate the wall strength and formulae based on reinforced concrete principles underestimate the wall strength.     

Finite element buckling analysis of stiffened plates with filleted junctions

Modern aircraft wings are thin-walled structures composed of ribs, spars and stiffened panels, where the top skin is subject to compressive forces in flight that can cause buckling instability. If these panels are machined from a single billet of metal then the initial buckling performance can be significantly improved by increasing the fillet radius along the line junction between the stiffener webs and skin. Typically thin-walled structures are usually modelled with two dimensional elements. To model the stiffened panel with fillets three dimensional elements are required. For the stiffened panel selected for the analysis the paper shows that the three dimensional model shows a substantial increase in skin initiated buckling if the fillet is taken account of. A 5 mm radius leads to an increase of 34% increase in local buckling load performance for a skin portion of breath to thickness ratio of 100. The associated overall buckling load increases by 1.8%. The mass penalty for a 5 mm radius is 5.1%. To avoid local and overall buckling interaction an accurate measure of both buckling loads is very important and may have impact for designers. The three dimensional models with no fillets show very good agreement with the two dimensional models.     

Imperfections in steel girder webs with and without perforations under patch loading

In this paper, the problem of stability of web plates with imperfections, subjected to patch load, is studied. The aim is to give some insights about the best way to take into account real imperfections in non-linear stability analyses of plates with and without perforations. In this context, the study is developed on the basis of the measured imperfections or numerically deriving the deformed model (theoretical imperfections). The influence of the patch load length, out-of-plane imperfection amplitude, dimension and position of the hole on stability behaviour and buckling strength are studied comparing some theoretical deformed configurations corresponding to different modal shapes. The results obtained with a three-dimensional model of the whole real beam with stiffeners, with experimentally measured imperfections, and each corresponding single web panel are compared and discussed obtaining some insights about the accuracy of the simplified (and conservative) model of the single panel.The main insights of this work are as follows. The deformed shape, corresponding to the first buckling mode, can be assumed as the initial configuration of the panels with and without holes to study post-critical behaviour until ultimate condition. The shape of the imperfection does not severely change the critical buckling stress. A longer patch load reduces the ultimate stress in the panel. An initial imperfection amplitude of less than 1% of the height of the panel does not reduce the ultimate load by more than about 5%.     

不锈钢薄腹梁受剪性能试验研究

通过对7根不锈钢薄腹梁进行受剪性能试验研究,分析了梁腹板的剪切屈曲和屈曲后强度。结果表明：所有梁均发生剪切屈曲破坏,薄腹板中形成拉力带,上翼缘和横向加劲肋中出现塑性铰;根据腹板表面应变和侧向鼓曲变形测得的剪切屈曲应力均低于理论计算的弹性剪切屈曲应力;梁的受剪承载力显著高于腹板剪切屈曲时的荷载,具有较高的屈曲后强度;梁端设置封头肋板可以提高梁的受剪承载力。基于得出的试验结果及现有其他试验数据,对两种考虑腹板屈曲后强度的受剪承载力计算方法进行评估,我国GB 50017—2017《钢结构设计标准》中的公式仅考虑了腹板的受剪承载力,其计算结果总体偏于保守,但是对腹板高厚比较小(λs<1.5)的不锈钢薄腹梁,受剪承载力计算偏于不安全,且计算结果离散性较大;EN 1993-1-4中的计算公式中同时考虑了腹板和翼缘的受剪承载力,其计算结果偏于保守且离散性较小。     

Elastic buckling of plates containing eccentrically located circular holes

The finite elements method is used to obtain the elastic buckling loads for flat square panels containing eccentrically located circular holes. The applied inplane loads considered are uniaxial and biaxial compressions as well as pure shear. In all cases the load is distributed uniformly along the simply supported or clamped outer edges. The effect of the degree of eccentricity and the size of the hole on the buckling load is obtained.The triangular and rectangular finite elements used for calculating the inplane stresses prior to buckling are based on assumed strain, rather than displacement, fields and satisfy the exact representation of strain free rigid body modes of displacements. The combination of these elements with existing well-known bending elements is shown to produce results of acceptable accuracy without the use of an inordinately large number of elements.