Thermoelastic flexural–torsional buckling of steel arches

This paper presents the lateral buckling behaviour of steel arch members with a doubly symmetric I-shape cross-section subjected to a linear gradient temperature field over the cross-section. The steel arch is subjected to an in-plane linear temperature gradient field whilst it experiences expansion along its length due to the in-plane temperature gradient producing an in-plane curvature. As the steel arch continues to be subjected to increasing temperature differential and increasing average temperature, the bending moments and axial compressive forces in the steel arch increase and upon reaching a critical value, the steel arch bifurcates from its primary equilibrium position and fails in lateral–torsional buckling mode. A novel non-discretisation mechanical-based methodology developed recently is used to model the behaviour of the steel arch prior to buckling, whilst the classical buckling theory is used to determine the critical temperature which causes flexural–torsional buckling. The proposed methodology allows for the critical temperature gradient and critical average temperature to be ascertained using an iterative method. Using a comprehensive parametric study, the variations of the thermal gradient and the critical average temperature to various parameters are then investigated. The model proposed here provides a closed-form solution for which it forms a platform which can be used for structural steel arch design and evaluation in the development of codified approaches to fire design on a performance based design.     

Flexural–torsional buckling of ultra light-gauge steel storage rack uprights

This paper presents an experimental investigation into the behaviour of ultra light-gauge steel storage rack uprights subjected to compression. Two different types of members with varying lengths are tested and while the combined effects of local and distortional buckling are investigated, special attention is given to longer specimens that fail by flexural–torsional buckling in combination with local and distortional buckling. Deformations experienced during testing by all of the specimens were measured and observations regarding failure modes have been documented. In addition, the geometric imperfections of each member were measured before testing, as were the material properties of the cold-rolled sections and the virgin steel from which the sections were formed. This paper details the observed failure modes, the recorded ultimate strengths and the load-deflection responses. Design capacities calculated from AS/NZS 4084 (2012) [1], RMI (2012) [2] and EN 15512 (2009) [3] specifications are then evaluated and compared to the experimental results obtained.The evaluation of international specificationsdetermined that EN 15512 (2009) [3] is more accurate in predicting ultimate loads of sections undergoing interactive buckling than both AS/NZS 4804 (2012) [1] and RMI (2012) [2].     

Lateral–distortional buckling of monorails

This paper is concerned with the elastic lateral–distortional (LD) buckling of single span steel monorail I-beams and its influence on their design strengths. The distortion of a slender web reduces the elastic buckling resistance of an intermediate length beam below its flexural–torsional (FT) resistance. A finite element computer program was used to study the elastic LD buckling of single span beams. The LD to FT buckling moment ratios were generally higher for simply supported beams with bottom flange central concentrated loads than for uniform bending, and lower for shear centre concentrated loads. For beams with bottom flange loading and unrestrained bottom flanges, there were very significant reductions in these ratios, but they increased when rigid web stiffeners or top flange torsional restraints were provided at the supports. For beams with bottom flange loading and unrestrained bottom flanges, the reductions in the elastic buckling resistance were greater for beams with stocky flanges than for slender flanges. Approximations were found for estimating the reduced resistances which were generally of high accuracy or conservative, and for estimating the increased resistances caused by elastic and rigid top flange end torsional restraints. A method of designing steel beams against LD buckling was proposed and its use demonstrated by a worked example.     

Local buckling of welded steel I-beams considering flange–web interaction

This paper presents an analytical study to investigate the effect of flange–web interaction on local buckling of welded steel I-sections subjected to bending. An inelastic local buckling stability model is presented that accounts for all the geometric and material variables of the problem. The deformation theory of plasticity was used to describe the behavior of steel beyond the elastic limit. The model results were verified through comparison with finite element model results and published experimental ones. A parametric study was conducted to investigate the effect of flange–web interaction on the width to thickness limits. Using the parametric study results, new width to thickness limits were proposed in which all the parameters of the section are reflected.     

Lateral–torsional buckling of cold-formed channel sections subject to combined compression and bending

This paper presents an analytical study of the flexural buckling and lateral–torsional buckling of cold-formed steel channel section beams subject to combined compression and bending about their major and minor axes. For channel section beams a bending about the minor axis creates a non-symmetric pre-buckling stress distribution, which has a significant influence on the lateral–torsional buckling of the beams. This kind of feature has not been discussed in the existing literature. The focus of this present study is the interaction between the compression load and the bending moments about the major and minor axes. It has been found that for a section subject to combined compression and the major-axis bending the bending moment will decrease the critical compression load, although the critical value of the largest compressive stress in the section actually increases with the applied bending moment. However, for a section subject to combined compression and the minor-axis bending the effect of the bending moment on the critical compression load depends on the direction of bending applied. For bending that creates a compressive stress in the lips the bending moment will reduce the critical compression load. However, for bending that creates a compressive stress in the web the bending moment has almost no influence on the critical compression load.     

Optimization of cold-formed steel pallet racking cross-sections for flexural–torsional buckling with constraints on the geometry

Starting from a comprehensive set of experimental test results of upright cross-sections in compression, this paper is focussed on how the section can be optimally designed to achieve the highest possible failure load in global buckling. The aim is to optimize a cross-section prototype shape with restrictions on its geometry attending to manufacturing feasibility and assembly constraints. The proposed scheme consists of maximizing the design strength with respect to flexural–torsional buckling, according to European Standard prEN 15512:2008 [European Standard prEN 15512:2008. Steel static storage systems - Adjustable pallet racking systems - Principles for structural design] / Fédération Européenne de la Manutention [Fédération Européenne de la Manutention (Section X): The design of static steel pallet racking. FEM 10.2.02; August 2000] design recommendations. This is a bicriteria optimization problem which is solved by formulating a nonlinear mathematical program. Optimization is performed prior to the finite element method (FEM) nonlinear analysis of the solution design. The procedure herein presented has two advantages: on the one hand it considerably reduces the number of nonlinear analyses, and on the other it ensures that the best design is achieved.     

Lateral-torsional buckling capacity assessment of web opening steel girders by artificial neural networks — elastic investigation

Bridge girders exposed to aggressive environmental conditions are subject to time-variant changes in resistance. There is therefore a need for evaluation procedures that produce accurate predictions of the load-carrying capacity and reliability of bridge structures to allow rational decisions to be made about repair, rehabilitation and expected life-cycle costs. This study deals with the stability of damaged steel I-beams with web opening subjected to bending loads. A three-dimensional (3D) finite element (FE) model using ABAQUS for the elastic flexural torsional analysis of I-beams has been used to assess the effect of web opening on the lateral buckling moment capacity. Artificial neural network (ANN) approach has been also employed to derive empirical formulae for predicting the lateral-torsional buckling moment capacity of deteriorated steel I-beams with different sizes of rectangular web opening using obtained FE results. It is found out that the proposed formulae can accurately predict residual lateral buckling capacities of doubly-symmetric steel I-beams with rectangular web opening. Hence, the results of this study can be used for better prediction of buckling life of web opening of steel beams by practice engineers.     

A numerical investigation of local–overall interaction buckling of stainless steel lipped channel columns

A detailed finite element (FE) model is presented, which was developed with the aim of studying the interaction of local and overall buckling in stainless steel columns. The model incorporates non-linear stress–strain behaviour, anisotropy, enhanced corner properties and initial imperfections. The model was verified against a program of 29 laboratory tests on stainless steel lipped channels, described in a companion paper [Becque J, Rasmussen KJR. Experimental investigation of the interaction of local and overall buckling of stainless steel lipped channel columns. Journal of Constructional Steel Research 2009; 65(8–9): 1677–84] and yielded excellent predictions of ultimate strength and specimen behaviour.The FE model was further used in parametric studies, varying both the cross-sectional slenderness and the overall slenderness. Three stainless steel alloys were considered: AISI304, AISI430 and 3Cr12. The results are compared with the governing design rules of the Australian, North American and European standards for stainless steel structures.     

Numerical investigation of inelastic buckling of steel–concrete composite beams prestressed with external tendons

The ultimate resistance of a continuous composite beam is governed by either distortional lateral buckling or local buckling, or an interactive mode of the two which is sharply different from the torsional buckling mode in a bare steel beam. A finite element model is developed and based on the proposed FE model, inelastic finite element analysis of composite beams in negative bending is investigated, considering the initial geometric imperfection and the residual stress patterns and the FE results are found agree well with the test results. Parametrical analysis is carried out on the prestressed composite beams with external tendons in negative bending. Factors that influence load carrying performance and buckling moment resistance of prestressed composite beams are analyzed, such as initial geometric imperfection, residual stress in steel beams, force ratio, which is defined as the extent of prestressing force and negative reinforcement in the beams, as well as the slenderness ratios of web, flange, and beams. By varying cross-section parameters, 25 groups of composite beams under negative uniform bending with initial geometric imperfection, residual stress as well as different force ratios, 200 beams in total are studied by means of the FE method. The computed buckling moment ratios are drawn against the modified slenderness proposed by the authors and compared with the Chinese Codified steel column design curve. It is demonstrated that the tentative design method based on the Chinese Codified design curve can be used in assessment of buckling strength of composite beams in a term of the modified slenderness defined.     

On elastic–plastic buckling of cones

The paper considers the buckling of short, and relatively thick, mild steel conical shells subjected to the combined action of external pressure and axial compression. Past results on axially compressed cones and on cones subjected to hydrostatic external pressure are compared with fresh test results on equivalent, axially compressed cylinder and with the equivalent cylinder subjected to external hydrostatic pressure.     

Exact lateral–torsional buckling solutions for cantilevered beams subjected to intermediate and end transverse point loads

This paper presents exact stability criteria for the lateral–torsional buckling of cantilever strip beams under intermediate and end point loads. The two-dimensional stability domain is given in closed-form solutions, in terms of Bessel functions. The limit of the convex stability domain tends towards Dunkerley's line when the intermediate load approaches the tip of the cantilever. However, the Dunkerley's line is no longer appropriate when the distance between both loads is sufficiently large. An approximate set of formulas is proposed for quick estimation of the buckling load.     

Effective section method: A general direct method for the design of steel cold-formed members under local–global buckling interaction

Thin-walled steel cold-formed members usually display local–global buckling interaction which strongly affects the structural strength of columns and beams. The local bucking of slender folded sections develops in analogy with single plate buckling, including the interaction between the plate elements of the cross-section and can be identified with appropriate first-order stability analysis and the consequent results of the critical buckling loads and the associated modes. The effective section method, ESM, as an extension of the original effective area method, EAM, was conceived for the design of cold-formed members on the basis of the actual local buckling results of the section, together with calibrated formulations for column and beam resistance. The strength equations were taken from the direct strength method, DSM, as it is presented in the North American AISI standard. In addition, as a consequence of its proposed formulation the effective section method is able to be applied side-by-side with the traditional effective width method, EWM, allowing its inclusion in the main part of the codes and specifications for the design of cold-formed members and improving its dissemination. The ESM was proposed in combination with equations and tables that enables designers to directly access the critical local buckling compressive force and bending moment of usual sections, resulting in a contribution that improves the attractiveness of the method.     

Fatigue performance and stiffness variation of stud connectors in steel–concrete–steel sandwich systems

In this study, a series of fatigue tests on six nominally identical push-shear specimens is conducted. The test specimens were subjected to an initial quasi-static test, up to a predefined maximum load, followed by a fatigue test to failure. For all the fatigue tests the mean applied load was the same while the load range varied to induce fatigue failure. The push-shear fatigue tests indicated that stiffness of the shear connections is gradually decreased during the test. Overall, the test results revealed that the lifetime of steel–concrete–steel sandwich systems under cycling loads could be predicted beforehand through the evaluation of the stiffness reduction in shear connections.     

Lightweight steel–concrete–steel sandwich system with J-hook connectors

This paper investigates a new concept for designing composite structures comprising a lightweight concrete core sandwiched in between two steel plates which are interconnected by J-hook connectors. Specifically, lightweight concrete (density less than 1450 kg/m³) and novel J-hook connectors have been developed for this purpose. The hook connectors are capable of resisting tension and shear, and their uses are not restricted by the core thickness. Push-out tests confirms that the shear transfer capability of J-hook connector is superior to the conventional headed stud connector in achieving composite action between steel plate and concrete core. Twelve sandwich beam specimens have been tested to evaluate the flexural and shear performance subjected to static point load. Parameters investigated include degree of partial composite, concrete with and without fibres and concrete strength. Using Eurocodes as a basis of design, theoretical model is developed to predict the flexural and shear capacity considering partial composite and enable construction of sandwich structures with J-hook connectors. Compared with test results, the predicted capacity is generally conservative if brittle failure of connectors can be avoided. Test evidence also shows that inclusion of 1% volume fraction of fibres in the concrete core significantly increases the beam flexural capacity as well as its post-peak ductility.     

Response of steel beam–columns exposed to fire

Steel beams when exposed to fire develop significant restraint forces and often behave as beam–columns. The response of such restrained steel beams under fire depends on many factors including fire scenario, load level, degree of restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam–columns under realistic fire, load and restraint scenarios. The finite element model is validated against experimental data, and the importance of high-temperature creep on the fire response of steel beam–columns is illustrated. The validated model is used to carry out a set of parametric studies. Results from the parametric studies indicate that fire scenario, load level, degree of end-restraint and high-temperature creep have significant influence on the behavior of beams under fire conditions. The type of fire scenario plays a critical role in determining the fire response of the laterally-unrestrained steel beam within a space subframe. Increased load level leads to higher catenary forces resulting in lower fire resistance. Rotational restraint enhances the fire resistance of a laterally-unrestrained steel beam, while the axial restraint has detrimental effect on fire resistance.     

Simplified nonlinear simulation of steel–concrete composite beams

A computationally efficient macro-model approach is proposed for investigating the nonlinear response of steel–concrete composite beams. The methodology accounts for material nonlinearity and interface slip between the concrete slab and the steel beam. The validity of the technique is evaluated through comparison of the macro-model-based simulations with results obtained from experimental testing of composite beams. Four full scale composite beams are tested under monotonic positive and negative bending. The results show that the proposed macro-element-model can capture the essential characteristics of the nonlinear load–deformation response of composite beams. Such an approach is a compromise between simplicity and accuracy and a viable alternative to detailed finite elements analysis. Additionally, a parametric study, including the compressive strength of slab concrete, the yield strength of the steel flanges and web, and the shear connection degree, of the steel–concrete composite beams subjected to positive moment is conducted utilizing the numerical macro-model proposed. The slips and their influences on the behaviors of composite beams during loading process have been analyzed.     

Structural fire performance of earthquake-resistant composite steel–concrete frames

Seismic and fire design of a building structure may be two very demanding tasks, especially if included in a performance based design philosophy. For the time being, the necessary harmonization on the regulations concerning these two design fields is almost missing, thus preventing the effective possibility of an integrated design. Besides, while many countries have already moved towards the use of performance-based codes for seismic design, the application of such methodologies for the fire design of structures is still limited in scope. Within this framework, the development of suitable procedures introducing structural fire performance issues for a comprehensive design methodology is needed.In this paper, a numerical investigation for the assessment of the structural fire performance of earthquake resistant composite steel–concrete frames is presented. With reference to a case study defined in the framework of a European Research Project, a great effort was devoted to the identification of the key structural parameters allowing for a possible correlation between the predictable performances under seismic and fire loadings, when these two are considered as independent actions.At the conceptual design level, the most suitable structural solution with respect to both design actions was chosen, including composite beams and circular steel concrete-filled columns. The frame was designed in order to resist severe seismic action according to the ductile design approach provided by Eurocode 8; the parameters affecting members’ sizing were outlined in this phase. Afterwards, the seismic performance of the designed frame was investigated by means of non-linear static analyses; once the seismic performance objectives were met, in order to evaluate the structural fire performance of the whole frame a set of criteria was defined. To this purpose, thermo-mechanical analyses under different boundary conditions were developed and in order to identify the possible mechanisms leading to structural failure, the state of stress at the critical cross-sections at different times of fire exposure was investigated. Another point of main concern was represented by the assessment of the influence of different restraining conditions on the achieved fire resistance rating and kind of structural failure.Moreover, the proposed methodology allowed making an estimate of the amount of axial restraint provided to the heated beams by the surrounding structure; in this view, the importance of choosing column elements in function of their flexural stiffness was revealed, in order to correlate it with the predictable performances under both seismic and fire loadings.     

Practical moment–rotation relations of steel shear tab connections

An analytical model for shear tab connections in modern steel buildings is proposed. This connection model includes the concrete slab effects and has unsymmetrical behaviors under positive and negative moments. Under positive moment, the flexural strengths are determined by two stress patterns: linear triangular tensile stress along shear tab combined with uniform compressive stress of 0.30f_c′ on slab at yielding state; uniform yielding stress along shear tab combined with uniform compressive stress of 0.85f_c′ on slab at ultimate state. Under negative moment, shear tab plate is the contributor to the flexural strength and the concrete slabs are neglected. Comparisons between experimental and analytical results indicate that the proposed steel shear tab models are able to capture major behaviors with a compatible effort often made in a typical structural design office. Seismic studies of one three-story non-ductile braced frame including proposed models demonstrate the capabilities and simplicities for practical structural engineering application.