Ultimate capacity of battened columns composed of four equal slender angles

Cold-formed steel structural members play a great role in modern steel structures due to their high strength and light weight. The behavior and strength of battened column members composed of slender angle sections are mainly governed by local buckling of angle legs or torsional buckling of the angle between batten plates. Moreover, local buckling depends on the interaction between the width–thickness ratio of angle leg, overall slenderness ratio of angle between batten plates and overall slenderness of column. Theoretical study has been carried out by a nonlinear material and geometrical finite element model. Numerous cases of slender battened column sections having different width–thickness angle leg ratios, overall slenderness ratios between batten plates and overall slenderness ratios are chosen in this study. Complete ultimate strength curves are drawn and different failure modes are studied by taking different member lengths, which produce local or torsional buckling of single angles between batten plates or overall buckling of the member. Empirical equations for the effect of shear deformation factor and the ultimate axial load capacities of members formed of battened slender angle sections are proposed. Strengths of axially loaded battened members predicted using finite element as well as the proposed empirical equations is compared with the design strengths obtained using North American and European codes. It is concluded that the design strengths predicted by North American and European codes are generally conservative, and these design rules have been shown to be reliable using reliability analysis.     

Behavior of biaxially-loaded rectangular concrete-filled steel tubular slender beam-columns with preload effects

In composite construction, rectangular hollow steel tubular slender beam-columns are subjected to preloads arising from construction loads and permanent loads of the upper floors before infilling of the wet concrete. The behavior of biaxially loaded thin-walled rectangular concrete-filled steel tubular (CFST) slender beam-columns with preloads on the steel tubes has not been studied experimentally and numerically. In this paper, a fiber element model developed for CFST slender beam-columns with preload effects is briefly described and verified by existing experimental results of uniaxially loaded CFST columns with preload effects. The fiber element model is used to investigate the behavior of biaxially loaded rectangular CFST slender beam-columns accounting for the effects of preloads and local buckling. Parameters examined include local buckling, preload ratio, loading angle, depth-to-thickness ratio, column slenderness, loading eccentricity and steel yield strength. The results obtained indicate that the preloads on the steel tubes significantly reduce the stiffness and strength of CFST slender beam-columns with a maximum strength reduction of more than 15.8%. Based on the parametric studies, a design model is proposed for axially loaded rectangular CFST columns with preload effects. The fiber element and design models proposed allow for the structural designer to efficiently analyze and design CFST slender beam-columns subjected to preloads from the upper floors of a high-rise composite building during construction.     

High strength circular concrete-filled steel tubular slender beam-columns, Part II: Fundamental behavior

There is relatively little experimental and numerical research on the fundamental behavior of high strength circular concrete-filled steel tubular (CFST) slender beam-columns. In a companion paper, a new numerical model for predicting the nonlinear inelastic behavior of high strength circular CFST slender beam-columns under axial load and bending was presented. The numerical model developed accounts for confinement effects on the strength and ductility of the concrete core and on circular steel tubes as well as initial geometric imperfections of beam-columns. This paper presents the verification of the numerical model and extensive parametric studies on the fundamental behavior of high strength circular CFST slender beam-columns. The ultimate strengths and axial load-deflection responses of circular CFST slender beam-columns under eccentric loading predicted by the numerical model are verified by corresponding experimental results. The computer program implementing the numerical model is used to investigate the fundamental behavior of high strength circular CFST slender beam-columns in terms of load-deflection responses, ultimate strengths, axial load-moment interaction diagrams, and strength increase due to concrete confinement. Parameters examined include column slenderness ratio, eccentricity ratio, concrete compressive strengths, steel yield strengths, steel ratio and concrete confinement. It is demonstrated that the numerical model developed is an efficient computer simulation and design tool for high strength circular CFST slender beam-columns. Benchmark numerical results presented in this paper are valuable in the development of composite design codes for high strength circular CFST slender beam-columns.     

Experimental investigation of built-up cold-formed steel section battened columns

This paper presents an experimental investigation on behaviour and design of built-up cold-formed steel section battened columns. The built-up columns were pin-ended and consisted of two cold-formed steel channels placed back-to-back at varied spacing of intersection. The two channels were connected using batten plates, with varying longitudinal spacing. The cold-formed steel channel sections were manufactured by brake-pressing flat strips having a plate thickness of 2 mm. The built-up cold-formed steel section battened columns had different slenderness and geometries but had the same nominal length of 2200 mm. The column strengths, load–axial shortening, load–lateral displacement and load–axial strain relationships were measured in the tests. In addition, the failure modes and deformed shapes at failure were observed in the tests and reported in this paper. Overall, the built-up column tests provided valuable experimental data regarding the column behaviour that compensated the lack of information on this form of construction as well as used to develop nonlinear 3-D finite element models. The column strengths measured experimentally were compared against design strengths calculated using the North American Specification, Australian/New Zealand Standard and European Code for cold-formed steel columns. Generally, it is shown that the specifications were unconservative for the built-up cold-formed steel section battened columns failing mainly by local buckling, while the specifications were conservative for the built-up columns failing mainly by elastic flexural buckling.     

Experimental and code based study of beam-column behaviour of equal leg single-angles

This study intends to establish the behaviour of equal leg single-angles under bi-axial bending and compression experimentally and analytically using nonlinear finite element analysis techniques. In the experimental system, 100×100×10 nominally sized equal leg single-angles were tested. The legs of angles were placed in a flange-down position, and their axes were restrained laterally as a simply supported beam. These simply supported beams were loaded with a concentrated vertical load applied at the middle of the length and an axial load parallel to the beam axis until failure was reached. The FEM model of the experimental system was generated with ABAQUS software. Suitable finite element mesh configurations, boundary conditions and nonlinear analysis solution techniques were determined to identify the system behaviour. The test and ABAQUS software results were interpreted using some code specifications and one design procedure.     

Experimental and numerical analysis on the structural behaviour of cold-formed steel beams

A research study on the structural behaviour of cold formed steel beams with C-, I-, R- and 2R-shaped cross-sections at ambient temperature is presented, based on the results of a large programme of experimental tests and numerical simulations. Firstly, several four-point bending tests were carried out in order to assess mainly the failure loads and failure modes of the beams. Secondly, a suitable finite element model was developed to compare with the experimental results, and finally, a parametric study was undertaken in order to investigate the influence of the thickness, height and length of the beams on its structural behaviour.     

Inelastic behaviour and design of slender I-sections in minor axis bending

Slender steel sections in bending are generally designed by taking the maximum moment as the yield moment. The assumption for the ultimate condition is thus the point at which first yield is reached in the section. Certain types of slender sections, however, have shown significant post-elastic behaviour in attainment of the ultimate moment. Experiments on I-sections in minor axis bending have shown this to be the case, where significant plastic stress distributions are attained in the tension flanges after the compression flanges have locally buckled. Current international steel specifications are unduly conservative when estimating the bending strength of these sections as the yield moment. This paper quantifies this conservatism and presents inelastic design methods whereby the post-elastic strength may be captured. Design equations are proposed for Australian, American and European hot-rolled and cold-formed steel specifications.     

Experimental investigation of composite shear walls under shear loadings

One of the efficient methods for improving the seismic behaviour of high-rise buildings is using Composite Steel Plate Shear Wall (CSPSW). In this paper, extensive experimental studies of one and three-story CSPSWs with the scale of 1:3 and 1:4, together with stress equations of each element are reported. The experimental results indicate that this system has reliable behaviour if the columns have high bending stiffness. Also bolts spacing to plate thickness ratio has direct relationship with system ductility. However, plate yield load has an inverse relationship with this ratio. In this system, plate stiffening requirement is obtained with minimum reinforcement for reinforced concrete, though for damage prevention high strength concrete is preferred. Also, the results show a good agreement for the recommended values of (b/t) by an AISC code for preventing plate buckling.     

Experimental behavior of steel beam-columns subjected to fire-induced thermal gradients

Fire tests were performed to investigate the mechanics and capacity of steel beam-columns that develop a thermal gradient through their depth when exposed to fire. Wide-flanged specimens were loaded axially and tested vertically in a furnace recently commissioned at Michigan State University. The placement of insulation simulated a realistic three-sided heating scenario such as that experienced by a column on the perimeter of a building frame. Specimens were tested with several combinations of load level, fire scenario, and direction of the thermal gradient (which dictates the direction of bending). The different combinations of tested parameters had a significant influence on the fire response of these columns, which all failed by full section yielding due to a combination of axial load (P) and moment (M). These columns developed bending moments in response to through-depth thermal gradients as well as a moment reversal due to a shift in the section’s center of stiffness. The plastic resistance to combinations of axial load and moment was also affected by the thermal gradients such that the critical section, located in the hottest region along the column length, was where moment was the smallest (not the largest, as would be intuitively expected). The experiments and computer models showed good agreement with the predicted demands (i.e. bending moment reversal) and capacity (i.e. changes in the plastic P-M capacity).     

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.     

Experimental study of steel single unequal-leg angles under eccentric compression

A total of 26 steel unequal-leg angle specimens were tested under eccentric compression with respect to either major or minor principal axis of the cross-section to study the steel single angle beam-column behavior. Results suggested that for angles subjected to major principal axis bending causing the angle short leg in compression, the presence of moment, in some cases, resulted in the angle ultimate load higher than its concentric compressive capacity. In the case of the major axis bending causing the long leg in compression and minor axis bending, an increase in eccentricity resulted in a reduction in the angle ultimate load but the extent of the reduction was more pronounced for specimens under minor axis bending. Overall, the effect of eccentricity on the ultimate load decreased as the slenderness increased. The design of angle beam-columns as proposed in AISC Specification 2005 and the Direct Strength Method was evaluated using the test results.     

Experimental behaviour of eccentrically loaded slender circular hollow steel columns in-filled with fibre reinforced concrete

This paper is based on the experimental study of twelve slender steel tubular columns of circular sections filled with both plain and fibre reinforced concrete. The specimens were tested under eccentric compression to investigate the effects of fibre reinforced concrete on the strength and behaviour of slender composite columns. The slenderness ratio was considered to be the main test parameter. Hollow steel sections of similar specimens were also tested as reference columns. The test results were illustrated by load-deflection and load-strain curves. Various characteristics such as strength, stiffness, ductility, energy absorption capacity and failure mode are discussed. Interpretation of the experimental results indicates that the use of fibre reinforced concrete as infill material has a considerable effect on the strength and behaviour of slender composite columns.     

Experimental investigation of cold-formed lean duplex stainless steel beam-columns

This paper describes a test program on cold-formed lean duplex stainless steel members in combined compression and minor axis bending. The test specimens were cold-rolled from flat strips of lean duplex stainless steel grade EN 1.4162. In this study, square and rectangular hollow sections were compressed at different eccentricities, in order to obtain a beam-column interaction curve for each series of tests. Initial overall geometric imperfections of the members were measured prior to testing. The ultimate loads and the failure modes of each specimen were obtained. The observed failure modes include local buckling, flexural buckling and interaction of local and flexural buckling. The test strengths obtained from this study and other available data were compared with the design strengths predicted by the American Specification, Australian/New Zealand Standard and European Code for stainless steel structures. It should be noted that these specifications do not cover the material of lean duplex stainless steel. Therefore, the suitability of the beam-column design rules in these specifications for lean duplex stainless steel is assessed in this study. Generally, these specifications are capable of predicting the beam-column strengths of the lean duplex stainless steel test specimens, and the design rules in the specifications are considered to be reliable. It is observed that the European Code generally provides quite conservative predictions for the beam-column specimens compared to the American Specification and Australian/New Zealand Standard predictions.     

Behaviour of short and slender concrete-filled stainless steel tubular columns

In this paper, a series of tests were carried out on short and slender concrete-filled stainless steel tubular columns to explore their performance under axial compression or combined actions of axial force and bending moment. Empty short steel hollow sections were also tested for comparison. The test results showed that the performance of the composite columns was quite good and have the potential to be used extensively as structural members. Comparisons of the test results were also made with several existing design methods for conventional concrete-filled carbon steel tubular columns as presented in Australian standard AS 5100 (2004), American code AISC (2005), Chinese code DBJ/T 13-51-2010 (2010), and Eurocode 4 (2004), which indicates that all the codes are somewhat conservative in predicting the load-carrying capacities of both short and slender columns.     

Experimental and numerical investigation on the shear behaviour of friction-welded bar-plate connections embedded in concrete

Friction-welded bar-plate connections are a basic structural component of Bi-steel steel-concrete-steel sandwich construction. In Bi-steel members, the bar-plate connections, embedded in concrete, are subject to tension, shear and bending. The static and fatigue behaviour of the embedded connections subjected to bar tension is described in another paper [Xie M, Chapman JC. Static and fatigue tensile strength of friction-welded bar-plate connections embedded in concrete. Journal of Constructional Steel Research [in press]]. This paper presents experimental and numerical studies on the static behaviour of the friction-welded connections with the bar loaded in shear. Finite element analysis is carried out to examine the effects of variations in geometric and material parameters. The experimental results are used to derive an empirical equation for predicting the shear strength of embedded connections, and compared with existing test results and code specifications. Further papers will describe fatigue behaviour of the embedded connections subjected to bar shear, and static and fatigue tests on Bi-steel beams.     

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.     

3D buckling analysis of a cylindrical metal bin composed of corrugated sheets strengthened by vertical stiffeners

The paper presents 3D results of a quasi-static buckling analysis of a funnel-flow cylindrical metal bin composed of horizontally corrugated sheets strengthened by vertical columns. A linear buckling and a non-linear analysis with geometric and material non-linearity were carried out with a perfect and an imperfect real silo shell by taking into account axisymmetric and non-axisymmetric loads imposed by a bulk solid following Eurocode 1 and different initial geometric imperfections and load non-uniformities around the circumference. The calculated buckling forces were compared with the permissible one given by Eurocode 3.     

Experimental study of beam-column behaviour of steel single angles

Twenty-eight rolled steel single angle specimens were tested to investigate their response when required to carry axial compressive loading at various end eccentricities. Results suggested that when eccentrically loaded with respect to the major principal axis, there is a critical eccentricity below which any consequent reduction in the ultimate load is marginal. In contrast, as eccentricity of loading with respect to the minor principal axis is increased, reduction of the ultimate load is more pronounced and no similar critical eccentricity can be identified. Test results, when compared with the corresponding values as determined from the design equations suggested by Adluri and Madugula (1992), in AISC Specification 2000 and AISC Specification 2005, indicated that the former two methods give a conservative estimate of the ultimate compressive capacity of single angles. This conservatism is more pronounced for specimens subjected to eccentric loading with respect to the major principal axis than that resulting from eccentric loading with respect to the minor principal axis. Although intended for doubly symmetric sections, the third method provides improved capacity estimates of single angles.     

Experimental model of the behaviour of bolted angles connections with stiffeners

This paper presents the experimental results of twenty full scale specimens of steel bolted connections beam-to-column with top-and-seat angles in six groups, labelled L91-tp8, L82-tp8, L73-tp8, L91-tp10, L82-tp10 and L73-tp10 groups (L=length of top-and-seat angle, tp=stiffener thickness of top-and-seat angle), under static loading. This study was undertaken to analyse the influence of angles and beams with stiffeners on the behaviour of the beam-to-column joints. The main parameters observed are the evolution of the resistance, the stiffness, the rotation capacity, the ductility of a joint, and the energy dissipation capacity. The aim was to provide necessary data to improve the Eurocode 3.     

Experimental behaviour and strength of concrete-encased composite beam-columns with T-shaped steel section under cyclic loading

Experimental research was conducted to investigate the structural behaviour of concrete-encased composite beam-columns with T-shaped steel section. Specimens were tested under lateral cyclic loading and axial compression. The test parameters included the distribution of longitudinal reinforcement, the spacing of transverse reinforcement, the presence of cross ties, and the axial compressive load level. The test results indicate that the cyclic behaviour and failure modes of the beam-columns are greatly affected by the direction of the bending moment owing to the unsymmetrical cross section. The concrete-encased composite beam-columns can develop stable hysteretic response and large energy absorption capacity by providing cross ties and decreased spacing of transverse ties. The current ACI and AISC-LRFD design provisions were also evaluated by comparing predicted strengths with the test results.