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This paper deals with the numerical analysis aimed at study the bearing capacity of pinended steel built-up columns under axial compression. Finite element (FE) models were performed for the columns presented in the literature. The main problem discussed in the article is the shape and magnitude of geometric imperfections introduced into the numerical FE model, necessary to obtain the load capacity consistent with the experimental strength tests. Three types of numerical analysis that can be used inAbaqus program to calculate the load bearing capacity have been described. The imperfections possible to introduce for built-up columns were presented and an equivalent imperfection corresponding to both imperfections recommended by Eurocode 3 (global of the entire column and local of the chord) for built-up members was proposed. The results of the geometrically and materially nonlinear static analysis were compared with the calculations according to the code procedures (Eurocode 3 and PN-B- 03200:1990) and the results of experimental tests.

This paper aims to develop design recommendations for closely spaced built-up stainless steel columns, based on findings gained in performed research at the University of Belgrade. The research focuses on pin-ended built-up columns formed from two press-braked channel chords oriented back-to-back and addresses their flexural buckling capacity about the minor axis. The impact of overall and local chord slenderness, interconnection stiffness, geometric imperfections and material nonlinearity is evaluated. In order to fully exploit their structural performance, two separate approaches for the design of built-up columns with welded or bolted interconnections are defined that include different formulas for shear stiffness.

This paper describes the use of stainless‐steel as a structural material, discusses current structural design considerations, reviews recent research work, and highlights important findings and developments. Over the past 25 years, structural design codes for stainless‐steel have been introduced or substantially altered or are still being modified, while at the same time there is a growing interest in the use of stainless‐steel in construction. Realistic modeling of structural elements of stainless‐steel plays an important role in the analysis and design of steel framed structures. Structural element modeling and optimization using stainless‐steel, in case a built‐up column, can contribute to existing research on stainless‐steel. The object under consideration is a stainless‐steel built‐up column, which is classified as atypical structural elements. The purpose of this optimization problem is to perform the minimization of the weight of the selected built‐up column, when the height of the column and the load acting on it are known. The built‐up column is considered with two different types of cross‐sectional chords: at first with circular (CHS) cross‐sectional chords and then with elliptical (EHS) cross‐sectional chords. A genetic algorithm based optimum design is presented for such a built‐up column. The design algorithm obtains a built‐up column with the least weight by selecting appropriate sections from standard set of duplex stainless‐steel hollow elliptical cross‐sections. The optimization model accounts for the strength and stability constraints. The built‐up column is under action of compressive axial force, bending moment and combination compressive axial force and bending moment. The parameters under consideration include the height and width of the outer cross‐section of the column's chords, the ratio of the outer cross‐sectional dimensions to the thickness and the initial eccentricity of the load. The computation results showed that the chosen optimization model and genetic algorithm allow efficiently obtaining the column of minimum weight, where the variables are column chord cross‐section and diagonals cross‐section. The design algorithm used to solve the problem is described in detail in this work. Until today, there is known that the lack of information prevented the recommended methodology from being properly established. Because of this, the obtained results confirmed that these types of structural elements of the steel in question are needed to develop a more efficient design of stainless‐steel structures in the design method and thus contribute to its wider use in construction. [ABSTRACT FROM AUTHOR]

This paper focusses on the buckling resistance of compression members made of back‐to‐back connected angle sections. The studied built‐up members are connected through bolted packing plates. Due to the flexibility of this connection, these members generally do not fully work as a single member but the connection stiffness has to be accounted for. In Eurocode 3 Part 1‐1 it is currently admitted to neglect the effect of the connection stiffness if the packing plates possess a maximum distance of 15 imin (imin is the minimum radius of gyration of one individual angle section, also called "chord"). Elsewise the influence of the packing plates on the member resistance should be accounted for. The limit distance of 15imin appears to be much lower than the past practice, consequently leading to a more conservative design and a complexification of the fabrication. This paper therefore presents a sensitivity analysis focusing on the influence of several parameters as: the distance of packing plates, the presence of clearance in bolt holes and the influence of preloaded bolts. These parameters are studied with a numerical model based on solid finite elements. Owing to the use of solid elements, it is possible to model explicitly the bolts, the packing plates and all contact regions. [ABSTRACT FROM AUTHOR]

Recently, the use of built‐up cold‐formed steel (CFS) members in the form of nested channel (NC) compression members is becoming more popular in CFS construction. For NC compression members, the current CFS design standards have been shown to be inadequate, provide over‐conservative predictions, and include some cumbersome design rules relating to maximum allowable screw spacing and end fastener groups. New design guidelines have been proposed by some recent research studies, however, they are based on different assumptions and provide varying capacity predictions. This study has investigated (1) the design guidelines in the current design standards and past research studies, and (2) the effects of screw spacing as well as end fastener groups on the compression behaviour and capacity of NC members. It included a series of compression tests on short, intermediate and slender single channel and NC members, followed by developing validated finite element models and undertaking parametric studies. The results show that: (1) the compression capacity of NC members can be greater than twice that of their individual members, (2) the design rules on the maximum allowable screw spacing and end fastener groups are over‐conservative, and are mostly unnecessary, (3) the design rules in the current standards and past research studies are not suitable for predicting the compression capacities of the investigated NC members, and (4) proposed design guidelines based on the Direct Strength Method in the current CFS design standards can provide reliable predictions for the compression capacities of NC members. [ABSTRACT FROM AUTHOR]

Cold-formed steel (CFS) built-up members are formed by connecting several single members by fasteners. This paper presents experimental, numerical and analytical studies on the effect of fastener configurations on the flexural rigidity of built-up sections. In this research, two zinc-coated steel C-sections are connected back-to-back to form a double-web built-up I-section, which is subject to weak-axis bending. A closed-formed expression for the flexural rigidity of built-up sections as a function of fastener stiffness, number and position of fasteners, and section geometry is then derived. This is validated against the tests and finite element modelling, and can be adapted more generally to determine the stiffness and global strength of CFS built-up members in bending. • The study is about how fastener configurations affect the effective flexural rigidity of cold-formed steel built-up sections. • Tested fasteners are screws, bolts and welds. • The effective flexural rigidity is a function of fastener stiffness, number and position of fasteners, and section geometry. • Effective flexural rigidity affects stiffness and strength of built-up members in bending and members subject to buckling. [ABSTRACT FROM AUTHOR]

The stability characteristics of web panels in two built-up members, i.e. box-sections and I-sections, are investigated here using a higher order plate bending element. At the beginning, the critical buckling loads of rectangular flat panels under various theoretical states-of-stress (pure shear, in-plane bending, shear and in-plane bending and combined stresses) and boundary conditions (simply supported and restrained by flange panels) are investigated. Then, the buckling behavior of an isolated flat panel is correlated with the stability characteristics of the web panels of built-up members under various loading conditions. The results on the stability behavior of the web panels are presented in nondimensional form for easy reference to designers and researchers. [ABSTRACT FROM AUTHOR]

Nowadays, the codified procedure for design of centrically compressed cold-formed stainless steel (CFSS) built-up members is absent in Europe. Besides, there are no published research information and experimental data in this structural area. This paper focuses on a comprehensive experimental investigation of closely spaced CFSS built-up columns and addresses their flexural buckling capacity about the minor principal axis. Two series with a total of 36 built-up members were assembled, each formed by two discontinuously and directly connected channel chords oriented back-to-back to form an I-section. The chords were press-braked from flat strips of austenitic stainless steel EN 1.4301 as non-slender channel sections. The chords were uniformly interconnected by bolts or by welds. To identify the effects of interconnection spacing on chords' composite action, the overall and chord slenderness ratios were varied. Material properties, cross-section response and initial imperfections were quantified before testing. As a result, the overall flexural buckling without any local-overall interactions was observed as the dominant failure mode. All findings were used in the accompanying paper [1] to establish qualitative data base through numerical simulations of flexural buckling tests. Additionally, a quantitative accuracy assessment of the design methods for carbon steel built-up columns provided in Eurocode and American Specification was reported on the basis of their ability to predict flexural buckling resistances of the tested columns. This enabled numerical parametric studies and proposing design rules for CFSS built-up columns with non-slender sections that capture interconnection shear stiffness, overall and chord slenderness ratio and strain hardening effects. [ABSTRACT FROM AUTHOR]

In parallel with an experimental investigation of the flexural buckling behaviour of built-up stainless steel columns presented in the accompanying paper (Dobrić et al., submitted for publication), a detailed Finite Element Analysis (FEA) has been performed to simulate the experiment and identify the key factors affecting the buckling response. The FEA entailed realistic geometry, measured geometric imperfections and material properties of the specimens. Very good agreement was obtained between the experiment and FEA, which proved the capability of the computational approach to replicate experimental results and predict ultimate buckling loads. In the absence of explicit design rules for flexural buckling resistance of stainless steel closely spaced built-up members, the experimental results were compared with design predictions according to the existing European Standard and American Specification for carbon steel structures. The findings indicate that the mentioned design standards may be very conservative regarding the buckling resistance of stainless steel built-up members; this under-prediction may be associated with the impact assessment of chord slenderness and interconnection stiffness on the buckling response. The main purpose of this research is to establish a qualitative data base reliable for the further quantitative numerical parameter analysis and for the development of new design rules for compressed stainless steel cold-formed built-up members. [ABSTRACT FROM AUTHOR]

This article presents the first experimental study reported on the flexural behaviour of cold-formed ferritic stainless steel (CFFSS) closed-section built-up beams. Each closed-section built-up beam has been formed by connecting two lipped channels in the face-to-face arrangement and spot welded at intermediate locations along the beam span. Sixteen specimens have been fabricated and tested till failure under three-point bending and four-point bending. Cross-sectional aspect ratio, plate slenderness of the compression flange and moment gradient were the critical parameters varied for investigating their influence through tests. The CFFSS chemical, material properties, geometric imperfections, ultimate moment capacity, failure modes and deflection responses were recorded for bringing out meaningful interpretation of the test results. The ultimate moment values obtained from the tests have been compared with the nominal design moment capacity predicted by the current Australian Standard (AS/NZS), American Specification (SEI/ASCE-8-02), European Code (EN 1993-1-4) and Direct Strength Method (DSM) in North American Specification (AISI-S100:2016) and Continuous Strength Method (CSM). The test results of the current study compensated for the important reliable data on the flexural behaviour of CFFSS closed-section built-up beams, missing from the literature. • Flexural tests were performed on CFFSS closed-section built-up beams. • Material properties, flexural strengths, moment–deflection response were measured. • Rotation capacity and failure modes at failure were reported. • Comparisons of test flexural strengths with design strengths were presented. [ABSTRACT FROM AUTHOR]

Cold-formed steel (CFS) battened columns with adequately spaced chords have structurally outperformed the conventional CFS built-up columns with closely spaced chords, but have witnessed limited research previously. The relative slenderness of the unsupported chord (between the battens) to the built-up column, is a critical parameter that governs the behavior of battened columns, and has not been appropriately addressed in previous studies, and unlike hot-rolled steel battened columns, its limiting value for a wide range of column slenderness is missing in the literature. Therefore, the various design rules proposed by different researchers for the same may not be credible. The present study focuses on the axial performance of pin ended CFS battened columns composed of two adequately spaced plain channels to form closed built-up sections. The aspect ratio of the built-up sections was fixed as unity. Finite element (FE) models were developed using ABAQUS platform, and were validated against the relevant test results available in the literature. After rigorous validation, the robust FE model developed was used to conduct an extensive parametric study by varying the vital parameters like sectional compactness of the chords, relative slenderness of the unsupported chord and overall slenderness of the battened columns. The effect of each parameter on the behavior of battened columns has been discussed extensively. Furthermore, the strengths of CFS battened columns were predicted using the current North American and European codes. It was noted that both these codes over predict the strengths of CFS battened columns, particularly when the sectional compactness of the chords is low and the relative slenderness of the unsupported chord is high. Also, a range of relative slenderness of the unsupported chord has been identified for which the prediction of the current codes is satisfactory. Based on the wide pool of data points developed, new design rules were proposed for safe and reliable predictions for CFS battened columns. The proposed design rules were verified through a reliability analysis and also the test results on CFS battened columns available in the literature. • Practical range of unsupported chord's relative slenderness (λ chord /λ) is studied. • Behavior of stub, short, intermediate and long columns is discussed. • Limiting value of λ chord /λ to prevent panel distortion is brought out. • NAS/EC3 can be adopted safely if λ chord /λ ≤ 0.25. • Simplified design rules are brought out for λ chord /λ ≤ 0. 50. • Stub columns with λ chord /λ ≤ 0.25 exhibit superior post-peak performance. [ABSTRACT FROM AUTHOR]

• A buckling finite element is developed for double angle members in compression. • The Vlasov theory is extended to provide an upper bound for the buckling load. • Standard design provisions are found to overestimate the buckling strength. • The finite element is used to determine the buckling load for 1250 cases. • A simple equation is developed to determine the elastic buckling strength. The present study investigates the elastic buckling resistance of compression members consisting of non-slender hot-rolled double angles connected back-to-back with discrete interconnectors. Towards this goal, the study (a) formulates a thin-walled beam finite element buckling solution that treats both angles as independent members away from the intermediate connector locations while constraining the displacements for both angles at interconnector locations, and (b) develops an extension of the Vlasov thin-walled beam theory, originally intended for monolithic members, to determine the effective torsional/warping sectional properties for back-to-back double angle members. The validity of the thin-walled beam finite element model is then verified against experimental results and shell finite element models and its predictions are shown to asymptotically converge to the Vlasov theory extension developed in the present study as the number of interconnectors is increased. A systematic parametric study consisting of 1250 finite element runs is then conducted and the database of results generated is used to propose a simple equation to predict the elastic compressive buckling resistance of double angle compression members. [ABSTRACT FROM AUTHOR]

Abstract: This study provides a direct experimental verification of the AISC slenderness ratio formulas for built-up compressive members. The comparison on various code-specified slenderness ratios or provisions, which used in the AISC-ASD, AISC-LRFD, AS-4100, and CSA S16-01, are presented. The 0.75 rule, which states that the slenderness ratio of component element of built-up member should not exceed three-fourths times the governing slenderness ratio of built-up member, seems justified according to the tests. The governing slenderness ratio of built-up member could be either the modified or the unmodified one — as specified in the AISC Specifications. The test results indicate that the built-up members with component slenderness ratio of 0.75 to 1.0 times the governing slenderness ratio (modified or unmodified) could also furnish a quite encouraging design outcome. The use of separation ratio in built-up compression members results in the decrease of design strength when compared to one with no use of separation ratio. [Copyright &y& Elsevier]

Abstract: A wide experimental investigation on laser welded connections based on both lap-shear and tension tests was performed at the University of Naples. The aim of the study was to assess the possibility of using laser welding in innovative built-up cold-formed steel beams. The experimental programme was designed in order to evaluate the effects of different parameters on the strength and ultimate displacement of the selected connections. In particular, the influence of sheet thickness, gap, loading direction, weld shapes and zinc coating were analyzed. The results of the experimental tests were compared with the predicted resistance values according to Eurocode 3, with the aim of validating for laser welding the mechanical models provided for spot welds. [Copyright &y& Elsevier]

Abstract: The results of a research project aimed at designing and development a built-up cold-formed steel beam assembled by laser welding are described in this paper. The research activity was concerned with the evaluation of the applicability of laser welded connections to cold-formed members and with the assessment of the load bearing capacity of the assembled beams. With this aim, both lap-shear and U-tension tests were carried out on laser welds in order to assess the influence on connection strength of different parameters such as the gap between the steel sheets and the zinc coating. Finally, the load bearing capacity of laser welded built-up cold-formed members was investigated by four-point bending tests. Four full-scale prototypes were manufactured with different spacings of connections along the flanges in order to evaluate the effects of weld configuration on the load bearing capacity. [Copyright &y& Elsevier]

The paper presents methods of analysis of built-up sections in which the discrete locations of fasteners is accounted for explicitly, rather than by smearing their effect using continuous shear flexibility as in current approaches. By considering fasteners at discrete points, it is possible to analyse the effects of placing additional fasteners at the ends (end fastener groups), to account directly for actual support conditions and to determine the optimum locations of fasteners. The paper first outlines the linear analysis of beams in flexure and introduces the notion of the effective flexural rigidity to account for partial composite actions. Closed form solutions are provided for five load and end support cases to demonstrate the application of the analysis. Next, the paper describes the linear analysis of built-up sections in torsion, considering first uniform torsion followed by non-uniform torsion. Closed form solutions are obtained for the effective torsion rigidity (GJ eff) of built-up sections featuring closed loops. A framework is also presented for determining the effective torsion rigidity (EI w , eff) of open built-up sections in non-uniform torsion. The paper concludes with the analysis of built-up sections subject to flexural buckling. A general variational buckling equation is derived followed by an energy-type method for calculating buckling loads for common end support conditions, including columns supported on flexible end tracks. Closed form solutions are presented for up to seven rows of fasteners longitudinally. While by nature approximate, the solutions are shown to be highly accurate. Comparisons are made between the presented closed form solutions and buckling load predictions obtained using current design provisions. • Set out a consistent theory for analysing built-up cold-formed steel members subject to flexure or torsion. • The theory accounts explicitly for the discrete nature of intermittent fasteners. • Closed-form solutions are presented for the flexural buckling of built-up cold-formed steel columns. • The influence of end support conditions is highlighted through worked solutions. [ABSTRACT FROM AUTHOR]