复合主轴端板连接的结构特性

设计中,节点最关键的结构性能是其抗弯能力、刚度和转动能力。这些都取决于连接梁的截面高度。对梁截面高度达530mm的复合端板连接进行一系列试验。试验结果显示楼板的断裂通常导致结构失效,但是这一般都发生在该处的转动变形已经充分发展之后。     

无支撑钢框架在高温下的楼层稳定性

受高温影响的钢框架的侧向稳定性与其在环境温度下的稳定性不同。这是由于高温下钢框架的弹性模量的降级将导致柱的横向刚度也跟着减少。在楼层屈曲理论的基础上调查无支撑钢框架在高温下的横向稳定性。首先,模拟一个钢柱暴露于高温之下,提出一个分析模型来测试无支撑框架中的轴向载荷、高温以及受热边界限制的钢管的横向刚度所带来的影响。之后,提出一个评估无支撑钢框架在高温下的稳定承载力的方法。数值实例用于展示所提出的方法的评估过程,并且研究不同场景中框架构架暴露于高温时框架结构的稳定性。使用有限元分析对所提出方法进行了数值分析,验证了该方法的有效性。     

Storey-based stability of unbraced steel frames at elevated temperature

The evaluation of the lateral stability of steel frames subject to elevated temperatures is different from that at ambient temperature. This is because the degradation of the Young's Modulus of steel associated with elevated temperature will lead to the loss of column lateral stiffness. In this study, the lateral stability of unbraced steel frames subjected to elevated temperature is investigated based on the concept of storey based buckling. First, to simulate a steel column exposed to the elevated temperature, an analytical model was proposed to examine the effects of axial loading, elevated temperature, and thermal boundary restraints on the lateral stiffness of steel columns in unbraced frames. Then, a method of evaluating the stability capacity of unbraced steel frames at elevated temperature is proposed. Numerical examples are presented to demonstrate the evaluation procedure of the proposed method and investigate the frame stability subjected to different scenarios of frame members exposed to the elevated temperature. The validity of the proposed method is verified by the numerical analysis with the use of finite element analysis.     

The effect of elastic foundations on the nonlinear buckling behavior of axially compressed heterogeneous orthotropic truncated conical shells

The buckling problem of a heterogeneous orthotropic truncated conical shell subjected to an axial load and surrounded by elastic media is analyzed based on the finite deformation theory. Using von-Karman nonlinearity, the governing equations of elastic buckling of heterogeneous orthotropic truncated conical shells surrounded by elastic media are derived. The governing equations are solved using superposition and Galerkin methods and obtained expressions for upper and lower critical axial loads. The influences of elastic foundations, heterogeneity, orthotropy and geometric characteristics on the upper and lower critical loads of conical shells with and without elastic foundations are studied in detail.     

Elastic lateral buckling of cantilever Litesteel Beams under transverse loading

The LiteSteel Beam (LSB) is a new hollow flange channel section developed by OneSteel Australian Tube Mills using its patented dual electric resistance welding and automated continuous roll-forming technologies. The LSB has a unique geometry consisting of torsionally rigid rectangular hollow flanges and a relatively slender web. Its flexural strength for intermediate spans is governed by lateral distortional buckling characterised by simultaneous lateral deflection, twist and web distortion. Recent research on LSBs has mainly focussed on their lateral distortional buckling behaviour under uniform moment conditions. However, in practice, LSB flexural members are subjected to non-uniform moment distributions and load height effects as they are often under transverse loads applied above or below their shear centre. These loading conditions are known to have significant effects on the lateral buckling strength of beams. Many steel design codes have adopted equivalent uniform moment distribution and load height factors based on data for conventional hot-rolled, doubly symmetric I-beams subject to lateral torsional buckling. The non-uniform moment distribution and load height effects of transverse loading on cantilever LSBs, and the suitability of the current design modification factors to include such effects are not known. This paper presents a numerical study based on finite element analyses of the elastic lateral buckling strength of cantilever LSBs subject to transverse loading, and the results. The applicability of the design modification factors from various steel design codes was reviewed, and suitable recommendations are presented for cantilever LSBs subject to transverse loading.     

偏心荷载下非弹性工字型钢梁的侧向屈曲抗力

承受梯度弯矩的工字型钢梁的侧向—扭转屈曲(LTB)强度取决于弯矩梯度系数Cb。而Cb取决于弯矩图的非均匀性、无支撑长度内所施加的横向荷载的大小和钢梁支座类型。一般地,Cb由规范根据弹性LTB分析理论给出。然而,同样的Cb被用在梁的非弹性屈曲分析中。提出1个三维有限元ANSYS模型用于工字型钢梁的非线弹性弯-扭分析,并用此模型研究了无支撑长度和偏剪心荷载(分别位于中心、上翼缘和下翼缘)对非弹性性能区域弯矩梯度的影响。研究发现AISC-LRFD的钢结构规范(AISC360-05)和结构稳定研究委员会导则给出的Cb对加载点在非弹性屈曲工字型钢梁的中心和下翼缘的情况并不准确。AISC-LRFD的抗弯公式过高评估了梯度弯矩作用下非弹性工字型钢梁的实际抗弯承载力。因此,提出了一个适用于该工况下非弹性区域的简单公式,用于替代规范给出的公式。     

Elastic distortional buckling of tee-section cantilevers

The paper presents the results of a finite element study of the elastic distortional buckling of tee-section cantilevers, which can be thought of as beams fully braced at one end and unbraced at the other. The finite element procedure is described briefly, and then three loading cases, viz., a tip moment, a tip load and a uniformly distributed load are considered. All of these loading cases place the unstiffened or free edge of the stem or web into compression. The effects of distortion are quantified for the three loading cases, as are the effects of fully restraining the top flange against lateral deflection and twist by a discrete brace positioned anywhere along the cantilever. It is shown that the effects of distortion during buckling cannot be ignored in a tee-section cantilever with even a moderately slender web.     

Buckling optimization of variable thickness prismatic folded plates

This paper deals with the structural shape optimization of prismatic folded plates under buckling load consideration. Buckling loads are determined using linear, quadratic and cubic, variable thickness, C(0) continuity, Mindlin-Reissner finite strips. The whole structural optimization process is carried out by integrating finite strip analysis, cubic spline shape and thickness definition, semi analytical sensitivity analysis and mathematical programming algorithm. The objective is either the maximization of the critical buckling load or minimization of the cross-section of the prismatic folded plate with constraints on the volume and buckling loads. Several examples are included to illustrate various features of the optimization algorithm, including plates and stiffened panels.     

Seismic performance of floor-by-floor assembled steel braced structures with stiffened connections

This paper presents an overview of a full-scale testing on a tension-only concentrically braced beam-through frame (TCBBF). The implementation of TCBBF facilitates the construction of low-rise industrialised residential steel houses by means of floor-by-floor assembling. This type of TCBBF system features cold-formed hollow structural section columns connected to H-section through beams by end plate with bearing-type high-strength bolts. A two-storey, four-span by one-span TCBBF subjected to vertical loads was cyclically loaded horizontally to examine the seismic behaviour. Stable behaviour was observed up to a storey drift angle of 1/10. The cyclic behaviour was characterised by a linear response, a slip range and a significant hardening response. Deteriorating pinched hysteretic behaviour was notable for cyclic loading primarily because of cyclic brace compression buckling and tension yielding. TCBBF incorporates very slender bracing members that are unable to bear much axial load when subjected to compression. Alternating brace compression buckling and tension yielding induce unrecoverable plastic deformation, which results in a sharp decrease in the lateral system stiffness of TCBBF when lateral displacement becomes zero or around zero. Additionally, bracing members and frame members share different proportions of horizontal force although the dual systems bear the lateral forces collaboratively. The variation philosophy of distribution proportion of bracing and frame members is evaluated. Pushover analysis is undertaken to duplicate the test results and develop an analytical model, which is able to predict the elastic stiffness and the strength reasonably.     

Column effective lengths considering inter-story and inter-column interactions in sway-permitted frames

The physical meaning of the traditional effective length coefficients (ELCs) in a sway permitted frame is first explained. It is found through an example that the critical loads of columns in a story have the same percentage increase or decrease after considering the inter-story interaction. It is also pointed out that the ELCs obtained by an overall buckling analysis are applicable only in the weakest story, for the other stories their ELCs simply implied the proportional loading assumption.With three assumptions of the conventional effective length approach discarded, this paper presents a mergence approach for buckling analysis of frames in which the column linear rigidities and axial loads are summed up, and the rotational restraints provided to columns by beams are summed at the merged beam-to-column joints. Effective lengths considering both inter-story and inter-column interactions are determined in unbraced two-story and three-story frames by simple algebraic operations. For frames with more than three stories, a three-story sub-assemblage, which includes beams at far ends of the columns above and below the story under consideration, is used to find the effective lengths of columns in the middle story.For frames with different bases, a equivalence procedure transforms the frames into those with the same type of bases before the mergence. For multi-span frames, the mergence approach overestimate the story lateral stiffness, a modification is proposed to eliminate this overestimation.Although overall buckling analysis of frames is easily carried out today, the findings and the proposed method of this paper will help to understand the inter-column and the inter-story interactions in frames.     

The use of single diagonal bracing in improvement of the lateral response of tall buildings

The improvement of the lateral response of tall buildings is a subject of great concern both in high wind areas and seismic regions. The lateral deflection of a tall building subjected to lateral loads can be decomposed into shear and bending components. Properly oriented single diagonal bracings are introduced in order to bring advantageous interactions between these two modes of deflection resulting in a reduction of the overall lateral deflection of the frame. The deflection of a panel with a single diagonal, when subjected to vertical downward load, has a lateral component caused by the axial force developed in the single brace, which, due to asymmetry, results in the lateral deflection. In order to restrict the panel from lateral deflection a compensating lateral force is required. By locating the single diagonal bracings in an improved manner, the vertical tension and compression on the opposite sides of the frame caused by the bending effect automatically generate the above mentioned lateral compensating force which opposes the inter story drift. A simple truss model is introduced to study the relationship between the vertical loading and the lateral load required to retain the vertical alignment of a panel with a single diagonal. This relation is then studied in detail and the optimal values of the height-to-width and brace-to-column stiffness ratios, needed to produce the maximum equilibrating lateral force are computed. Further, the single diagonal bracings are applied to typical building frames. Results from elastic analyses are compared with that of the conventional X-braced frame. Pilot analysis shows a reduction of about 2% in the lateral deflection of a typical 24 story braced frame building with rigid connections and five bays using the proposed bracing scheme.     

On the application of the extrapolation techniques in elastic buckling

The critical buckling load of a structure undergoing lateral buckling can be determined experimentally by using the extrapolation or plotting techniques, without having to subject the structure to loading in the vicinity of critical. To achieve this, it is only necessary to have data relating load to a deformation characteristic. A search of the literature reveals that only certain and just a few deformation characteristics such as lateral displacement and/or twist have mostly been used by researchers in this regard. In this paper, the feasibility of application of various deformation variables for experimental determination of the critical buckling load is investigated in case of I-beams with different initial geometrical imperfections undergoing the elastic lateral-distortional mode of buckling. It is demonstrated that lateral displacement of the I-beams is directly coupled with different deformation variables such as web transverse and longitudinal strains, vertical deflection, and angles of twist of top and bottom flanges. Based on these key findings, the Southwell, Massey, Modified, and Meck Plot methods are applied on these various deformation variables, and consequently satisfactory and reliable estimates are obtained for the critical buckling loads. Finally, it may be concluded that the application of the extrapolation techniques does not actually need to be limited to some certain deformation variables.     

A simplified analytical method for unbraced composite frames with semi-rigid connections

The original Muto’s method for the lateral force analysis of conventional unbraced bare steel frames is first modified to incorporate the effects of semi-rigid beam-to-column connections. From which, equations for the calculation of the inter-storey drifts of these frames are formulated using first principles. By comparing the frame analytical results calculated from a rigorous finite element programme, it is shown that the proposed method gives reasonably accurate internal forces and inter-storey drifts estimations of a sway bare steel frame with semi-rigid connections. The proposed modified Muto’s method is then further developed for the manual analysis of unbraced composite frame systems by introducing an improved equivalent beam stiffness to account for the variation of the beam stiffness in the hogging and sagging moment regions. The accuracy of this simplified analytical method is verified by the rigorous finite element analysis of an unbraced composite frame with semi-rigid connections. Last, a parametric study is conducted to quantify the effects of semi-rigid connections on the inter-storey drifts of unbraced composite frames under lateral loads.     

轴压比超限时框架柱的恢复力模型研究

根据收集到的28根轴压比超限时框架柱在固定轴向荷载和水平反复荷载作用下的试验结果,分析回归出试验柱的各位移角和刚度与剪跨比、轴压比、配箍特征值等之间的关系。根据轴压比超限时框架柱屈服的截面平衡条件推出的屈服柱顶水平位移和荷载、按《混凝土结构设计规范》(50010—2002)计算的柱顶水平峰值荷载及有关试验回归公式确定出框架柱的恢复力模型。研究表明:轴压比超限时试验骨架曲线强化段在水平轴上的投影长度和屈服位移的比值与配箍特征值成正比,与轴压比和剪跨比成反比;框架柱的强化刚度与弹性刚度的比值随着轴压比增大而逐渐减小,而随着配箍特征值增大和剪跨比增大而逐渐增大。框架柱最大荷载时的刚度及退化刚度与弹性刚度比值的绝对值随着配箍特征值增大而逐渐减小,而随着轴压比增大和剪跨比增大而逐渐增大;框架柱的卸载刚度和再加载刚度随位移幅值增加而退化,且在不同位移幅值下卸载刚度和再加载刚度的退化率与轴压比有关。本文建议的恢复力模型既可     

不同自由端约束条件下双轴对称工字形截面悬臂梁的稳定性

对自由端侧向位移和扭转简支约束、自由端仅扭转约束和自由端仅简支扭转弹性约束条件下,双轴对称工字形截面悬臂梁的稳定性进行研究,并提出临界弯矩的计算公式。这一计算公式能够考虑悬臂梁长细比的变化,以及横向荷载作用点沿截面高度方向变化对其临界弯矩的影响。通过与有限元结果的比较可以发现,所提出的计算公式可以获得精度良好的临界弯矩值。     

Lateral–torsional buckling of steel web tapered tee-section cantilevers

In this paper an analytical model is presented to describe the lateral–torsional buckling behaviour of steel web tapered tee-section cantilevers when subjected to a uniformly distributed load and/or a concentrated load at the free end. To validate the present analytical solutions finite element analyses using ANSYS software are also presented. Good agreement between the analytical and numerical solutions is demonstrated. Using the present analytical solutions, the interactive buckling of the tip point and uniformly distributed loads is investigated and a parametric study is carried out to examine the influence of section dimensions on the critical buckling loads. It is found that web tapering can increase or decrease the critical lateral–torsional buckling loads, depending on the flange width of the beam. For a beam with a wide flange (width/depth = 0.96) the critical buckling load is increased by 2% by web tapering, whereas for a beam with a narrow flange (width/depth = 0.19) web tapering reduces the buckling load up to10% and 6% for the tip point loading and the uniformly distributed load respectively.     

Out-of-plane elastic buckling load and strength design of space truss arch with a rectangular section

The out-of-plane stability of the two-hinged space truss circular arch with a rectangular section is theoretically and numerically investigated in this paper. Firstly, the flexural stiffness and torsional stiffness of space truss arches are deduced. The calculation formula of out-of-plane elastic buckling loads of the space truss arch is derived based on the classical solution of out-of-plane flexural-torsional buckling loads of the solid web arch. However, since the classical solution cannot be used for the calculation of the arch with a small rise-span ratio, the formula for out-of-plane elastic buckling loads of space truss arches subjected to end bending moments is modified. Numerical research of the out-of-plane stability of space truss arches under different load cases shows that the theoretical formula proposed in this paper has good accuracy. Secondly, the design formulas to predict the out-of-plane elastoplastic stability strength of space truss arches subjected to the end bending moment and radial uniform load are presented through introducing a normalized slenderness ratio. By assuming that all components of space truss circular arches bear only axial force, the design formulas to prevent the local buckling of chord and transverse tubes are deduced. Finally, the bearing capacity design equations of space truss arches are proposed under vertical uniform load.     

Buckling of circular plates under intermediate and edge radial loads

This brief note is concerned with the elastic buckling problem of a circular plate subjected to both intermediate and edge radial loads. The stability criteria, in the form of transcendental equations, are derived as a function of the location of the intermediate load and the ratio of the magnitudes of the intermediate load and the edge load. Sample buckling results are presented for this new buckling problem.     

Buckling analysis of thin-walled members with closed cross sections

In this paper, an analytical procedure for the elastic buckling problems of thin-walled members with closed cross sections by the transfer matrix method is presented. The transfer matrix is obtained from the differential equations for a plate subjected to axial load, and the extended transfer matrix for closed cross sections is derived from that for branched panels. The analytical local and overall elastic buckling loads for thin-walled members with closed cross sections can be obtained simultaneously. Furthermore, a technique to estimate the buckling mode shapes of these members is also shown. To investigate the accuracy and efficiency of this method, some numerical examples are presented.