Serviceability and ultimate limit states of channels under compression and bi-axial bending

As the demand for light-weight steel constructions continues on the rise, efficient and accurate design procedures become extremely essential. A good understanding and knowledge of the structural performance is a fundamental step to achieve this target. The paper examines the behavior of channel members subjected to compression and bi-axial bending. A modeling strategy is first presented for non-linear analysis of channels under this combined loading arrangement. The interaction between serviceability and ultimate limit states is then highlighted. Cost-effective design spaces are shown for channels under this loading that can be used in practice to achieve economical light-weight designs. Several strategies are also presented that tackle common design challenges encountered in practice. The paper offers to practitioners and steel fabricators procedures that can be used to optimize the channel size under this general loading combination.     

Inelastic behaviour of composite members under combined bending and axial loading

This paper presents an account of the results of a series of tests on partially-encased composite steel/concrete beam-columns. The experimental inelastic behaviour of members is examined under idealised conditions representing extreme lateral loading in combination with co-existing axial gravity loads. Members with three different cross-sectional sizes, utilising Grade S460 steel, were employed in the tests. The specimens were tested under major- or minor-axis bending in conjunction with a constant level of axial loading. The experimental arrangement and test rig constructed for this purpose are described together with the material properties and details of the specimens. The main experimental results from tests carried out on ten composite models are presented and discussed. In addition to providing essential data for validating future analytical and design studies, the experimental results and observations enable a direct assessment of several important factors. Particular emphasis is placed on key parameters related to yield and ultimate capacity as well as on ductility and residual strength considerations.     

Behaviour of reinforced and concrete-encased composite columns subjected to biaxial bending and axial load

An experimental investigation of the behaviour of reinforced concrete columns and a theoretical procedure for analysis of both short and slender reinforced and composite columns of arbitrarily shaped cross section subjected to biaxial bending and axial load are presented. In the proposed procedure, nonlinear stress–strain relations are assumed for concrete, reinforcing steel and structural steel materials. The compression zone of the concrete section and the entire section of the structural steel are divided into adequate number of segments in order to use various stress–strain models for the analysis. The slenderness effect of the member is taken into account by using the Moment Magnification Method. The proposed procedure was compared with test results of 12 square and three L-shaped reinforced concrete columns subjected to short-term axial load and biaxial bending, and also some experimental results available in the literature for composite columns compared with the theoretical results obtained by the proposed procedure and a good degree of accuracy was obtained.     

Nonlinear analysis of short concrete-filled steel tubular beam-columns under axial load and biaxial bending

This paper presents a nonlinear fiber element analysis method for determining the axial load-moment strength interaction diagrams for short concrete-filled steel tubular (CFST) beam-columns under axial load and biaxial bending. Nonlinear constitutive models for confined concrete and structural steel are considered in the fiber element analysis. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a composite section to satisfy equilibrium conditions. The accuracy of the fiber element analysis program is verified by comparisons of fiber analysis results with experimental data and existing solutions. The fiber element analysis program developed is employed to study the effects of steel ratios, concrete compressive strengths and steel yield strengths on axial load-moment interaction diagrams and the C-ratio of CFST beam-columns. The proposed fiber element analysis technique is shown to be efficient and accurate and can be used directly in the design of CFST beam-columns and implemented in advanced analysis programs for the nonlinear analysis of composite columns and frames.     

Theoretical and experimental study on composite steel-concrete shear walls with vertical steel encased profiles

The present work describes the theoretical study and the experimental tests on composite steel-concrete shear walls with steel encased profiles (CSRCW), performed at Politehnica University of Timi?oara, Romania. The composite steel-concrete structural shear walls with steel encased profiles can be used as horizontal resisting systems for buildings that require considerable large horizontal load capacity. The study consists in numerical analysis and tests on 1:3 scale experimental steel-concrete composite elements. The experimental elements differ by the arrangement of the steel shapes embedded in the cross section of the wall and by the cross section type of the steel encased element. Headed steel studs are provided to ensure the connection between the steel profiles and the concrete. The aim of this study is to analyze the nonlinear behavior of elements, focused on the connection between the steel profiles and the concrete respectively, to compare the behavior of shear walls with different steel shape encased profiles with typical reinforced concrete shear walls. Using the tests performed until failure, the maximum load, the deformation capacity and the dissipated energy were evaluated.     

Flexural–torsional performance of thin-walled steel hollow box columns subjected to a cyclic eccentric load

This paper investigates the seismic performance of steel hollow box sections. The test results obtained from the combined load tests were used to define the relationship among member performance, sectional aspect ratios and the load combinations. It was found from the comparisons that the flexural strength of members decreased when the torsion magnitude was increased. It was also observed that the reduction in flexural strength increased when the sectional aspect ratio was increased. An empirical expression for estimating member performance under three-dimensional loads is proposed for design references.     

Effective flexural stiffness of slender reinforced concrete columns under axial forces and biaxial bending

Most of the design codes (ACI-318-2008 and Euro Code-2-2004) propose the moment magnifier method in order to take into account the second order effect to design slender reinforced concrete columns. The accuracy of this method depends on the effective flexural stiffness of the column. This paper proposes a new equation to obtain the effective stiffness EI of slender reinforced concrete columns. The expression is valid for any shape of cross-section, subjected to combined axial loads and biaxial bending, both for short-time and sustained loads, normal and high strength concretes, but it is only suitable for columns with equal effective buckling lengths in the two principal bending planes. The new equation extends the proposed EI equation in the “Biaxial bending moment magnifier method” by Bonet et al. (2004) [6], which is valid only for rectangular sections. The method was compared with 613 experimental tests from the literature and a good degree of accuracy was obtained. It was also compared with the design codes ACI-318 (08) and EC-2 (2004) improving the precision. The method is capable to verify and design with sufficient accuracy slender reinforced concrete columns in practical engineering design applications.     

Dynamic stability analysis of composite skew plates subjected to periodic in-plane load

Here, the dynamic stability characteristics of simply supported laminated composite skew plates subjected to a periodic in-plane load are investigated using the finite element approach. The formulation includes the effects of transverse shear deformation, in-plane and rotary inertia. The boundaries of the instability regions are obtained using the Bolotin's method and are represented in the non-dimensional load amplitude-excitation frequency plane. The principal and second instability regions are identified for different parameters such as skew angle, thickness-to-span ratio, fiber orientation and static in-plane load.     

Analysis of equal leg single-angle section beams subjected to biaxial bending and constant axial compressive force

Single-angle section beams are generally loaded parallel to their geometrical axes and their cross-sections are not symmetrical to their principal axes. Even equal leg angle beams have only one symmetrical axis. Many types of loading cause biaxial bending and axial forces in these members. Since single-angle section beams are slender members, they also need to be analyzed in terms of flexural buckling, lateral torsional buckling and local buckling effects. In this study, a calculation procedure is presented to analyze the nominal loads of equal leg angle section beams loaded vertically to the axis of the beam. It is assumed that the axial force is composed of a constant compressive force. The constant axial force is only taken into consideration for the uniform compressive stress and the second-degree effects caused in the cross-section. Thus only the biaxial bending moments remain. The first yield, full plastic and critical lateral torsional buckling moments for biaxial bending are calculated with respect to the slenderness of the beam and the axial force. The nominal design force on the cross-section is calculated according to the load and resistance factor design rules. The analysis proposed for the constant axial load can also be used for other axial forces, by using an iterative calculation procedure.     

Cyclic axial response and energy dissipation of cold-formed steel framing members

This paper summarizes results from an experimental program that investigated the cyclic axial behavior and energy dissipation of cold-formed steel C—sections structural framing members. Fully characterized cyclic axial load–deformation response of individual members is necessary to facilitate performance-based design of cold-formed steel building systems. Specimen cross-section dimensions and lengths were selected to isolate specific buckling modes (i.e., local, distortional or global buckling). The cyclic loading protocol was adapted from FEMA 461 with target displacements based on elastic buckling properties. Cyclic response showed large post-buckling deformations, pinching, strength and stiffness degradation. Damage accumulated within one half-wave after buckling. The total hysteretic energy dissipated within the damaged half-wave decreased with increasing cross-section slenderness. More energy dissipation comes at the cost of less cumulative axial deformation before tensile rupture.     

Seismic behaviour of square CFT beam-columns under biaxial bending moment

This paper investigates the behaviour of square concrete-filled steel tubular (CFT) beam-columns subjected to biaxial moment. Nine tests on beam-columns are reported here under a combined loading of constant axial load and cyclic lateral load applied at varying angles to the axis of the cross-section, referred to as ‘diagonal’ loading. The specimens were prepared in order to evaluate the influence of different parameters on the overall structural response, their ductility and their energy dissipation ability; the parameters included the effects of axial load ratio, width-to-thickness ratio, concrete compressive strength, slenderness ratio and load angle on the moment strength. The experimental results indicate that the ductility and energy dissipation ability of biaxially loaded square CFT columns decrease with increasing the axial load ratio. Their ductility and energy dissipation ability was also observed to decrease as the concrete compressive strength increased while the ductility was barely affected by the load angle. An increase in the load angle of biaxially bent square CFT beam-columns led to a slight decrease of the moment strength. Both EC4 and AIJ code provisions were shown to predict with reasonable accuracy the moment strength capacity observed in the tests, while the ACI-predicted moment strength gave to slightly conservative values. On the other hand, the LRFD code provisions greatly underestimated their moment strength.     

Resistance of rectangular concrete-filled tubular (CFT) sections to the axial load and combined axial compression and bending

This paper describes the development of the direct strength method (DSM) for concrete-filled tubular (CFT) sections. The axial and flexural strength of CFT sections with local buckling are proposed based on previous test results. Although Eurocode4 does not allow the use of slender steel skins for CFT sections, the limit of the width-to-thickness ratio for the steel skin has recently been extended to slender sections in AISC specifications. A simple formula for the axial and flexural strength of CFT sections for the DSM is proposed to account for the local buckling of a thin steel skin and for the enhanced compressive strength of concrete from the confining effect of the steel skin. The squash load predicted by the proposed formula is compared with test results and those predicted by AISC specifications and Eurocode4. A formula for strength interactions of CFT members under combined compression and flexure is proposed and is compared with test results. The comparison confirmed that the formula for axial and flexural strength and that for strength interactions can conservatively predict the resistance of CFT columns to the axial load and combined compression and bending.     

钢筋混凝土Z形截面双向压弯柱抗震性能试验研究

为了研究Z形截面钢筋混凝土柱的抗震性能,对缩尺比为1∶2的9根试件进行了拟静力试验,2根试件进行了单调荷载试验。分析了轴压比（n取0.30、0.45）、加载方向（加载角α为0°、45°、90°、135°）、肢高肢厚比（3∶1、4∶1）、纵筋强度等级（HRB400、HRB500）等参数对其抗震性能的影响。研究了Z形截面柱的破坏特征,得到了试件的荷载-位移滞回曲线及骨架曲线、承载力、位移延性系数等力学性能指标。研究结果表明：试件在横向反复荷载作用下的主要破坏形态为弯曲型破坏,破坏主要发生在与加载方向平行的跨中截面,滞回曲线对称、饱满,试件延性好,具有良好的抗震能力。采用数值分析方法编制了Z形截面柱承载力电算程序进行计算,计算结果与试验结果吻合较好。     

钢管高强混凝土压弯构件抗震性能的试验研究

本通过对8根钢管高强混凝土压弯构件试件在恒定轴力和水平周期反复荷载作用下抗震性能的试验研究，阐述了混凝土强度、轴压比和含钢量对钢管高强混凝土压穹构件延性性能的影响．由试验结果分析了影响构件延性性能的主要原因及因素。并提出建议。     

新型预制低层装配式剪力墙抗震性能试验研究

提出了一种适用于低层建筑的装配式剪力墙结构,并对3个足尺预制剪力墙试件进行了低周往复加载试验.结果表明:在合理范围内增加轴压比,墙体的承载力有明显提高,两个试件都是延性破坏,延性和耗能能力均有所增强;减小剪跨比会使墙体开裂荷载和原始刚度变大,但延性变差,水泥砂浆层出现脆性破坏.     

Finite element analysis of CFT columns subjected to an axial compressive force and bending moment in combination

Proper material constitutive models for concrete-filled tube (CFT) columns subjected to an axial compressive force and bending moment in combination are proposed and verified in this paper by using the nonlinear finite element program ABAQUS compared against experimental data. In the numerical analysis, the cross sections of the CFT columns are categorized into three groups, i.e., ones with circular sections, ones with square sections, and ones with square sections stiffened with reinforcing ties.It is shown that the steel tubes can provide a good confining effect on the concrete core when the axial compressive force is large. The confining effect of a square CFT stiffened by reinforcing ties is stronger than that of the same square CFT without stiffening ties but weaker than that of a circular CFT. Nevertheless, when the spacing of reinforcing ties is small, a CFT with a square section might possibly achieve the same confining effect as one with a circular section.     

The effects of axial tension on the hogging-moment regions of composite beams

Structural parts commonly comprised of composite members such as bridge approaches, inclined parking ramps and stadium beams, can be subjected to a combination of high axial loads and bending moments. Steel-concrete composite construction is a popular solution for these types of structures due to the numerous advantages that they offer. Although, current design codes (e.g. Eurocode 4, American code AISC, Australian codes AS2327 and AS5100) provide rules for the design of composite columns subjected to flexure and axial load, however the design of composite beams, which are asymmetric in nature under the combined effects of tension and bending, is not yet fully addressed. This paper investigates the ultimate strength of composite beams under the combined effects of axial tension and negative (hogging) bending moment. An experimental programme carried out in the laboratory of the University of Western Sydney comprised of a total of six specimens representing composite beams and subjected to various levels of axial tension and bending moment. Ultimate failure modes were identified and the resulting interaction diagrams were compared to the results of sectional rigid plastic analysis. Following the tests, three-dimensional finite element models were employed using the ABAQUS finite element software to further investigate the nonlinear behaviour of the composite beams and extend the experimental observations by studying the effects of parameters such as the span length and the effect of partial shear connection. Finally, simple design rules and formulae are proposed for use in engineering practice.     

Interaction formulae for members subjected to bending and axial compression in EUROCODE 3—the Method 2 approach

The final version of EN1993-1-1, EUROCODE 3 [EN1993-1-1. Eurocode 3. Design of steel structures, general rules and rules for buildings. 2005] for Steel Structures provides two alternatives for the buckling check of members subjected to axial compression and bending by interaction formulae, which are called there Method 1 and Method 2. This paper presents the characteristics, the background and the use of Method 2. The analogous presentation of Method 1 has already been given in [Boissonnade N, Jaspart J-P, Muzeau J-P, Villette M. New Interaction formulae for beam-columns in Eurocode 3. The French-Belgian approach. Journal of Constructional Steel Research 2004;60;421-31].The Method 2 formulae have been derived on the basis of the general format of the interaction concept of existing codes, e.g. the ENV-rules; however with advanced numerical background and consistent classification of the buckling modes. In this respect new improved interaction factors were developed from a wide scope of numerical simulations and the concept of the formulae was focussed distinctly on describing the modes of in-plane and out-of-plane buckling for members susceptible to fail either in flexural buckling or in lateral-torsional buckling. As result two sets of formulae are provided, which each cover a clear scope of physical member behaviour. Hereby, the specific effects of intermediate lateral restraints—as often found in steel structures—have also been included.The Method 2 formulae aim at providing buckling rules with compact simplified interaction factors and transparent application for standard cases.