Limit analysis of local failure in shallow spherical concrete caps subjected to uniform radial pressure

Many thin-walled shallow concrete shells (or caps or domes) have experienced structural collapse during or subsequent to their erection. Very few experimental investigations have been reported of shallow concrete spherical caps that allow for the effects of geometric and material non-linearities and imperfections to be identified, despite this information being essential for the validation of sophisticated numerical treatments. Classical thin-shell theories for axisymmetric domes predict a global buckling mode, but observations from experimental tests show that failure in concrete spherical caps is usually localised within a relatively small region and at a load significantly less than the classical buckling load. An investigation of the non-linear behaviour of thin-walled concrete spherical caps is currently being carried out both experimentally and theoretically at The University of New South Wales. As part of this study, an approach based on limit analysis has been developed on the basis of a local failure model and it is used for analysis of concrete/mortar spherical caps described in the published literature. The ultimate bending strength of a unit width of a spherical shell section is derived from a typical non-linear concrete stress-strain relationship and the in-plane thrust from shell membrane theory. The analytical results based on a local failure yield line model are compared with the available experimental results in the literature as well as with classical theoretical buckling results.     

高架式钢筒仓中转折连接的试验研究

用于散料储存的大型高架式钢筒仓结构通常由仓筒、锥形漏斗以及支承裙筒组成。在漏斗壁拉力水平分力的作用下 ,漏斗 -仓筒 -裙筒连接处产生相当大的周向压力 ,通常通过设置环梁以增加其强度。转折连接的主要破坏模式包括塑性破坏及环梁的弹性或塑性屈曲。国际上对这类连接的屈曲及破坏强度已有了大量的理论研究 ,并建立了基于理论的设计建议 ,但其试验研究尚属空白。本文总结了作者针对钢筒仓转折连接的稳定与强度问题近期在香港理工大学进行的一项大型试验研究项目。首先简单介绍该项目所建立的一套用于薄壳结构屈曲试验的试验装置 ,然后给出三个系列试验 (内压下的圆锥 -圆柱筒连接、散料荷载下的漏斗 -仓筒 -裙筒连接及散料荷载下的漏斗 -仓筒 -裙筒 -环梁连接 )的主要试验结果     

Short-term behaviour of shallow thin-walled concrete dome under uniform external pressure

Thin-walled spherical concrete shells or domes find widespread use in many applications, including in many iconic engineering structures of historical and religious significance. Despite this, very few experimental investigations have been reported in the open literature of shallow spherical concrete domes which allow for the effects of geometric and material non-linearities and of imperfections to be identified. This information is essential, however, in order to validate sophisticated numerical treatments, as well as to calibrate practical design and construction guidelines and is therefore much-needed. This paper reports an experimental study of a shallow thin-walled concrete dome under short-term loading, without the use of reinforcement in the concrete. The dome is 30 mm thick and has a base diameter of 3 m, being supported on a steel ring beam. The testing of the dome to failure under a uniform external pressure is described in the paper, and it is shown that it failed in a non-axisymmetric buckling mode well before the concrete reached its compressive strength. The failure pressure is compared with the ‘theoretical’ buckling results and the analytical results based on finite element analyses. In particular, this paper presents a comprehensive set of experimental data for the load–displacement and load–strain relationships and their distributions across the spherical dome throughout the loading regime.     

加劲薄壁钢管混凝土柱的试验性能

对加劲薄壁钢管混凝土柱的结构性能进行试验研究。通过在钢管内表面焊接纵向加劲肋实现加强的作用。同时对12个不带加劲肋的钢管混凝土柱进行试验,部分柱的填充混凝土中加有钢纤维。试验结果表明,加劲肋可以有效延迟钢管局部屈曲现象的出现。当加劲构件达到最大承载量时钢板才出现屈曲,所以相比普通钢管混凝土,加劲钢管混凝土具有更高的适用性。     

Experimental behaviour of concrete-filled stiffened thin-walled steel tubular columns

An experimental study on the structural behaviour of concrete-filled stiffened thin-walled steel tubular columns is presented in this paper. The stiffening was achieved by welding longitudinal stiffeners on the inner surfaces of the steel tubes. Companion tests were also undertaken on 12 unstiffened concrete-filled steel tubular (CFST) columns, with or without steel fibres in the infill concrete. The test results showed that the local buckling of the tubes was effectively delayed by the stiffeners. The plate buckling initially occurred when the maximum load had almost reached for stiffened specimens, thus they had higher serviceability benefits compared to those of unstiffened ones. Some of the existing design codes were used to predict the load-carrying capacities of the tested composite columns.     

Flow over spherical inflated buildings

The pressure distributions on three domes of different heights ($\text{h/c = 0.5, 0.37}\text{and}\text{0.25}$) have been measured in a boundary-layer wind tunnel in which the wind flow over a sparsely wooded area was simulated. The tests were made in the wall-law region of the boundary layer and the results are therfore first normalized using the undisturbed skin friction $\text{τ}{}_{\mathrm{<mtext>W</mtext>}}$.The tensions in inflated membranes of the same geometry were determined using the SAP IV finite-element program assuming that the effects of membrane weight and air friction are unimportant. The principal tensions then indicate where buckling would first take place and what would be the minimum internal pressure to just prevent it.The minimum internal gauge pressure to prevent buckling, which is shown to occur on the upwind side of the building, is 180 ($\text{h/c) τ}{}_{\mathrm{<mtext>W</mtext>}}$. This conclusion is compatible with the recommendations of the National Standards of Canada (1981) [1]. The location, orientation and magnitude of the largest principal tension indicate that rupture of the membrane would first occur near the top of the building.     

T-ring stiffened cone cylinder intersection under internal pressure

Cone–cylinder junctions are vastly used in the industries such as oil refineries and aeronautics. They can be seen in pressure vessels and piping such as tanks' roofs and pipes' reducers. When cone–cylinder junctions are subjected to the internal pressure, compression stresses are established near the joining point of the cone to cylinder and make the junction susceptible to non-symmetric buckling failure or axisymmetric failure. As it is practical to increase the shell wall thickness locally near the point of intersection, sometimes it is more convenient to attach a ring-beam exactly to the point of intersection. Only limited work has been done on the T-ring stiffened cone–cylinder junctions under internal pressure. In this study, experimental behavior along with numerical analysis of T-ring stiffened cone–cylinder intersection under internal pressure has been dealt and experimental results such as buckling mode and load are presented here and compared with numerical results. It can be seen that by wise consideration and manipulated use of material properties and geometric imperfections in nonlinear analysis, buckling mode and load resulted from non-linear analysis are compatible with that of experimental results. Two classes of non-linear analyses have been carried out and compared with each other, then it was inferred that even though pattern of geometrical imperfection is effective in determination of buckling modes, but in these kinds of structures it is not necessarily used for the analysis of buckling loads. Finally experimental results were compared with design proposals. It is shown that these proposals can conservatively predict the failure loads.     

可变振幅循环荷载作用下的纯弯曲冷弯圆形钢管混凝土梁

对钢管混凝土在循环荷载作用下的非弹性弯曲性能进行试验研究。对可变振幅循环荷载系列下,不同尺寸的钢管混凝土进行控制旋转、均匀循环弯曲试验,表明钢管混凝土梁具有局部弯曲稳定的滞后性能,可根据D／t比值说明强度和延展性的退化。根据强度、滞回曲线和破坏模式,提出抗震性能参数。在循环试验中获得的最大弯矩与采用钢和混凝土规定中所预测的弯矩进行比较,确定适合抗震结构系统设计和建造的新长细比限值。对新长细比限值与设计规范中的限值进行比较。     

Analysis based evaluation for buckling loads of two-way elliptic paraboloidal single layer lattice domes

The present paper proposes an efficient method for evaluating elasto-plastic buckling loads of two-way elliptic paraboloidal lattice domes of single layer under vertical loads. The procedure is composed of the following steps: linear buckling analysis to find the generalized slenderness ratio of the specific member that is the most relevant to their loading capacities; the generalized slenderness ratio is applied to obtain the axial strength for the specific member, based on which elasto-plastic buckling load is evaluated. The comparison of the evaluated buckling loads with those obtained from FEM elasto-plastic buckling analyses proves that the procedure is valid and efficient enough to be applied in design practice. Based on safety factors, given in some recommendations, the allowable column strength is also proposed for proportioning the members of elliptic single layer paraboloidal lattice domes.     

Degradation and collapse of square tubes under cyclic bending

This paper presents the results of an experimental investigation of the elastic–plastic degradation and collapse of steel tubes with square cross-section under cyclic pure bending in a curvature symmetric fashion. The results indicate that the structural performance of the tubes degrades due to the growth of periodic, transverse deflections in their flanges. The wavelength of these deflections is equal to the wavelength of the buckling mode of the tubes under monotonic pure bending. Persistent cycling induces localization of the amplitude of these deflections and leads to the formation of a kink in one of the flanges. This causes collapse of the tube.     

轴向循环荷载作用下有矩形缺陷钢管的应变棘轮:数值模型

轴向循环荷载作用下,钢管易产生局部屈曲、起皱及塑性应变。由于反复的开启/关闭和温度变化,海底管道处于加载和卸载循环中。在工作中,由于受腐蚀,壁厚变薄,钢管通常发生材料损伤。建立数值模型,模拟循环荷载作用下有矩形缺陷钢管的棘轮性能。钢管首先受单轴轴向压缩,小幅起皱,随后施加持续轴向循环荷载。采用非线性各向同性/随动(混合)硬化模型,模型各参数通过小型试件的滞回试验获得。钢管棘轮性能的数值结果与试验数据基本吻合。相比于动力性能,表面缺陷对有缺陷钢管的棘轮性能影响更大。基于本模型,还可研究初始应变、应力振幅、加载制度、壁厚和材料硬化属性等因素对有矩形缺陷钢管的棘轮性能和连续塑性屈曲性能的影响。     

Behavior of square hollow steel tubes and steel tubes filled with concrete

This paper investigates the occurrence of local buckling in bare steel and concrete-filled tubes to study how different depth-to-thickness ratios affect the response of the steel component. The experimental set-up and results of 24 tests are presented in this paper. Specimens with values of depth-to-thickness ratios in the range of 50–125 have been considered. The presence of the concrete has been observed to affect the exhibited buckling mode and to significantly increase the buckling bearing capacity of the concrete-filled steel tubes. A numerical model has been developed using the commercial software ABAQUS and has been validated against the experimental results of this study. From a design viewpoint, it has been observed that local buckling needs to be included in the calculation of the contribution of the steel component to the bearing capacity of a concrete-filled tube when its depth-to-thickness ratio is over 50. For a slender plate, i.e., with a depth-to-thickness ratio over 120, its post-buckling behavior could be included in the calculation of the steel contribution as it evidently increases its bearing capacity. Finally, an equation for the calculation of the bearing capacity of composite sections with both stocky and slender steel elements has been proposed and validated against extensive experimental results available in the literature.     

方形钢管和钢管混凝土的性能研究

对方钢管和钢管混凝土的局部屈曲进行研究,主要分析不同的深厚比对钢构件的影响。通过对24个深厚比数值为50～125的构件进行试验发现,混凝土的存在会影响已有的屈曲模式,并显著增加钢管混凝土的屈曲承载力。采用ABAQUS软件建立数值模型,并与试验结果进行有效对比。从设计角度来看,当构件深厚比高于50时,计算钢构件对钢管混凝土承载力贡献的同时应包含局部屈曲效应。例如,对深厚比超过120的细长板,其后屈曲效应能明显增加其承载能力,所以在计算钢构件对承载能力的贡献时应考虑这一效应。最后,提出计算包括短粗和细长钢结构复合截面的承载力计算公式,并通过了大量试验结果的验证。     

Experimental and numerical study on the stability capacity of Q690 high-strength circular steel tubes under axial compression

This paper presents experimental investigation on the local and overall buckling capacity of Q690 high-strength circular steel tubes under axial compression. Coupon tests are undertaken to obtain the Young’s modulus, Poisson’s ratio, tensile yield stress and ultimate tensile strength of Q690 steel material. Forty-two specimens under axial compression are tested to evaluate the buckling behavior and the failure mode. Test results indicate that most of the existing design standards are conservative for the design of Q690 steel tubes. The distribution model of residual stress is measured by cutting ring method. Then, a finite element model based on modified column deflection curve method (CDC) is built to assess the effects of initial imperfection and residual stress on the stability capacity of the steel tubes. Finally, design methods and recommendations are provided for estimating the stability capacity of high-strength circular steel tubes.     

Strain ratcheting of steel tubulars with a rectangular defect under axial cycling: A numerical modeling

Cyclic axial loads in tubular steel sections might lead to local buckling, wrinkling and accumulation of plastic strains in the tube. For example, this can be caused by repetitive start-up/shutdown and temperature changes in an offshore pipeline which generates cycles of axial compression/relaxing in the line. During their life time steel tubes may also experience material loss due to corrosion or wall thinning.The current paper reports the result of a numerical modeling of ratcheting behavior of steel tubes with a rectangular defect under cyclic axial loadings. The tubes have been initially subjected to monotonic axial compression beyond initiation of small amplitude wrinkles and subsequently to persistent axial cyclic loads. A nonlinear isotropic/kinematic (combined) hardening model has been adopted for the material, which its parameters have been obtained from cyclic tests conducted on small coupon specimens. The results of the numerical simulation have been compared with experimental data. In general, a reasonable agreement has been noticed between the experimental and the numerical results for the ratcheting behavior of the tubes. It is shown that surface imperfections have a very pronounced effect on the ratcheting response of the defected tubes, as compared to the monotonic responses. The model has also been used to study effects of some key factors such as the initial strain level, the stress amplitude, the mean stress, the loading regime, wall thinning and the material hardening properties on the ratcheting response and on the progressive plastic buckling of steel tubes with a rectangular defect.     

Buckling of Sharp Knuckle Torispheres Under External Pressure

The effect of knuckle size and length of the cylindrical flange on the buckling strength of externally pressurised torispheres with a sharp knuckle is examined numerically. Both elastic and elastic-perfectly plastic modelling is used. It is illustrated how sensitivity of the load-carrying capacity to the boundary conditions can be removed by the adoption of a large enough knuckle or a cylindrical flange of appropriate length.

Experimental results available for the sharp knuckle domes are collected and 10 new tests are carried out on machined steel domes. Two tests on spun torispheres are also included. The tests demonstrate that the safety margin as used for externally pressurised hemispheres and deep torispheres is inadequate for sharp knuckle torispheres with a 6% knuckle.     

Behavior of thin-walled circular hollow section tubes subjected to bending

Thin-walled steel circular hollow sections (CHSs) are widely used in wind turbine towers. The tower tubes are mainly subjected to bending. There have been a few experimental studies on the bending behavior of thin-walled CHS steel tubes. This paper describes a series of bending tests to examine the influence of section slenderness on the inelastic and elastic bending properties of thin-walled CHS. In addition, the influence of stiffeners welded in the steel tube is considered. Sixteen bending tests were performed up to failure on different sizes of CHS with diameter-to-thickness ratio (D/t) varying from 75 to 300. The experimental results showed that the specimens with small diameter-to-thickness ratios failed by extensive plastification on the central part of the tube. With the increase of diameter-to-thickness ratio, the local buckling phenomena became more pronounced. The stiffeners in the steel tubes increased the load carrying capacity and improved the ductility of the specimens. The experimental results were compared with current design guidelines on thin-walled steel members in AISC-LRFD, AS4100 and European Specification. It was found that the test results agreed well with the results based on AS4100 design code.     

Experimental characterization and finite element analysis of inflated fabric beams

An airbeam is a high-strength fabric sleeve with a highly flexible internal bladder that can be used as a load-bearing beam or arch when inflated. Due to their fabric construction, airbeams are inherently thin-walled structures that are prone to local buckling. In this study, airbeams were tested in bending at different inflation pressures to quantify their load–deformation response and the effect of inflation pressure on response. Tension–torsion tests of the airbeam fabric were conducted to estimate the fabric shear modulus, and the bend test results were used in conjunction with Timoshenko beam theory to estimate the fabric elastic modulus. Three-dimensional membrane finite element (FE) models were then used to predict the beam load–deformation response given these moduli. The FE models successfully predicted localized fabric buckling and softening of load–deflection response. Comparison of FE model-predicted load–deflection response with beam theory shows that conventional beam theory is accurate prior to local buckling of the airbeam fabric. The FE model and test results indicate that the consideration of work done by pressure under deformation-induced volume changes may increase beam capacity beyond previously derived theoretical limiting values.     

Deflection and buckling behavior of thin,circular cylindrical shells under wind loads

Some wind-tunnel tests have been conducted on the buckling behavior of closed-ended, thin cylindrical shells such as silos. Detailed measurements of the prebuckling deflections as well as of the buckling pressures were made with a variety of elastic cylinders in both smooth and turbulent flows.The results indicate that the prebuckling deflection is extremely sensitive to the wind pressure distribution, while the buckling pressure is less sensitive to it. It was also found that the pressure—deflection relationship exhibits a marked nonlinearity as the wind pressure approaches the buckling pressure. The experimental results were compared with the results of a stability analysis based on Donnell's theory, and a relatively good agreement was derived with respect to the buckling pressure.Furthermore, on the basis of the experimental results, an empirical formula for the buckling pressure was proposed as function of the height/radius ratio and the radius/ thickness ratio of the cylindrical shell.     

Experimental investigation on the axial collapse of expanded metal tubes

Expanded metal sheeting is widely used around the world for catwalks, granting, and decorative purposes. In the literature, little information can be found regarding the behavior of structural elements made of expanded metal. This paper is aimed at studying the axial collapse of squared and round tubes made of expanded metal sheeting under compressive loading. When expanding the metal a diamond-like cell pattern is formed in the metal sheets, then these patterns are characterised by two geometrical axes. A set of experimental tests was conducted to investigate the influence of the angle formed between the major axis of the expanded metal and the compressive load. From the results, three types of collapse response were mainly observed depending on the orientation of the axes: (1) Mode 1 characterised by a plastic collapse mechanism; (2) Mode 2 local buckling of the individual cells; and (3) Mode 3 global buckling of the tube walls.