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Wire arc additive manufacturing (WAAM) is a method of metal additive manufacturing (AM) that can be used to fabricate optimised large‐scale structural elements of complex form in a cost‐effective manner, opening up opportunities to unleash the true potential of structural optimisation. To demonstrate this potential, two optimised steel cantilever trusses were fabricated by means of WAAM and tested to investigate their structural performance. The tested specimens comprised members of tubular cross‐section of varying diameter and thickness. The geometric features of the tested specimens were determined using 3D laser scanning, while digital image correlation was employed to monitor the displacement and strain fields during testing. Full details of the experimental programme are provided; the obtained results are analysed and comparisons against equivalent conventional reference designs are made. The results of complementary numerical simulations, undertaken to gain further insight into particular features of the structural response of the examined specimens, are also presented. The structural efficiency of all optimised trusses, as measured by the capacity‐to‐mass ratio, was found to be at least 80% higher than that of the corresponding reference designs, underlining the benefits in terms of structural efficiency that can be achieved by the combination of WAAM with advanced optimisation methods. [ABSTRACT FROM AUTHOR]

The behaviour and design of homogeneous and hybrid high strength steel (HSS) beams are addressed in the present study. Six in‐plane three‐point bending tests on three different welded I‐sections were first conducted. Following the experimental investigation, a finite element (FE) modelling programme was performed, which included a validation study confirming the accuracy of the developed FE models in replicating the flexural behaviour of HSS welded I‐section beams, and a parametric study generating additional FE data on HSS welded I‐section beams over a broader range of cross‐sectional slendernesses, steel grades and loading configurations. Then, the suitability of the current Eurocode 3 cross‐section slenderness limits for HSS homogeneous and hybrid welded I‐sections in bending were evaluated. It is shown that the current Eurocode Class 2 and Class 3 slenderness limits are suitable for the classification of the outstand flange (in compression) and internal web (in bending) elements of both HSS homogeneous and hybrid welded I‐sections subjected to major axis bending, while stricter Class 1 slenderness limits are considered necessary to achieve sufficient rotation capacity for plastic design. The findings indicate that plastic design can be used for HSS structures, provided the proposed stricter Class 1 slenderness limits are employed. Further work is underway to develop advanced design approaches using second‐order inelastic analysis with strain limits for HSS welded I‐section members. [ABSTRACT FROM AUTHOR]

A comprehensive numerical investigation into the cross-sectional behaviour and ultimate capacity of non-slender welded I-sections, made of both normal and high strength steels (NSS and HSS), under combined compression and uniaxial bending is presented. Finite element (FE) models were initially established and validated against test results collected from the literature. Subsequently, parametric studies were conducted using the validated FE models to generate extensive numerical data considering different steel grades, cross-section geometries and loading combinations. The obtained numerical data, together with the test results collected from the literature, were utilised to assess the accuracy of the traditional European (EC3) and North American (AISC) design provisions, as well as the Continuous Strength Method (CSM), for NSS and HSS welded I-sections under combined loading. The assessment indicated that the CSM was able to provide more accurate and consistent resistance predictions than the current EC3 and AISC design provisions owing to its ability to capture the spread of plasticity and strain hardening in a systematic, mechanics-based manner. Finally, the reliability levels of the different design methods were statistically evaluated in accordance with EN 1990:2002. [ABSTRACT FROM AUTHOR]

Portal frames are a common form of structural system used to create efficient wide‐span enclosures. Over the past few decades, the design of portal frames has evolved in the direction of longer, lighter and taller structures, while cladding systems have become more substantial. Although this allows larger spaces, it can also result in heightened structural challenges, such as increased frame flexibility, a greater portion of the load being borne by the cladding and greater differential thermal movement. This has led to general concerns related to the effects of scale. To investigate these concerns, with a particular focus on the contribution of the cladding to the global strength and stiffness of the structure (also referred to as the stressed skin effect), and the resulting demands placed on secondary components, this study presents the development of a numerical model for the accurate representation of the structural response of full single‐span portal frames. The model is validated herein in two ways; (i) through validation of each individual frame component against relevant physical experiments, and (ii) through validation of the full model against full scale frame tests. Finally, a preliminary parametric investigation is presented. [ABSTRACT FROM AUTHOR]

In this paper, the flexural‐torsional buckling response of austenitic stainless steel I‐section beam‐columns is investigated. Finite element simulations are performed with the aim of analysing the parameters affecting the behaviour of the studied members. The accuracy and safety of the existing design provisions of the European structural stainless steel design standard EN 1993‐1‐4 (2015) are assessed. A new method providing more accurate ultimate resistance predictions for austenitic stainless steel I‐section beam‐columns undergoing flexural‐torsional buckling is put forward. The higher accuracy, safety and reliability of the proposed design rules relative to EN 1993‐1‐4 (2015) are demonstrated and the new proposals have now been included in the revised Eurocode – prEN 1993‐1‐4 (2021). [ABSTRACT FROM AUTHOR]

An accurate and consistent approach to the out‐of‐plane stability design of steel beams and structures utilising geometrically and materially nonlinear analysis with imperfections (GMNIA) and strain limits is proposed. The method is implemented using computationally efficient beam elements, with the ultimate structural resistance defined either by (i) the ultimate load factor or (ii) the load factor at which a strain limit, determined on the basis of the continuous strength method (CSM), is attained, whichever occurs first. In the present paper, the scope of this method is extended from the in‐plane design of steel structures and structural components to scenarios in which out‐of‐plane stability effects, with a focus on lateral‐torsional buckling (LTB), govern. First, two shapes and corresponding amplitudes of equivalent imperfections, which consider the combined influence of both geometric imperfections and residual stresses, for the out‐of‐stability design of steel and stainless steel members by GMNIA are established. The existing CSM strain limits for in‐plane design are then extended to allow for the influence of shear, torsion and warping, to enable their general applicability to three‐dimensional buckling problems. It is shown that employing the proposed design method together with the proposed equivalent imperfections generally provides more accurate and consistent ultimate strength predictions than traditional Eurocode design calculations and also streamlines the design process by eliminating the need for cross‐section classification and individual member design checks. [ABSTRACT FROM AUTHOR]

Structural design rules for prestressed cold-formed steel beams, considering both the prestressing and imposed vertical loading stages, are presented herein. In the proposed approach, the cold-formed steel member is designed as a beam-column using linear interaction equations in conjunction with the Direct Strength Method (DSM), while the prestressed cable is designed by ensuring that its tensile capacity is not violated during the two loading stages. In the present paper, the design approach and the failure criteria, which define the permissible design zone for the prestressed system, are first introduced. The suitability of the design recommendations is then assessed by comparing a set of parametric finite element (FE) results for several combinations of prestress levels, beam geometries and cable sizes, with the corresponding design predictions. Finally, following reliability analysis, the implementation of the design recommendations is illustrated through a practical worked example. • Design recommendations for prestressed cold-formed steel beams developed. • Linear interaction equations in conjunction with the Direct Strength Method utilised. • Capacities of cold-formed steel beam and cable to be checked for both loading stages. • Design rules assessment through comparisons with parametric finite element results. • Implementation of design rules illustrated through a practical worked example. [ABSTRACT FROM AUTHOR]

Highlights • Results of experimental study on stainless steel I-section beam-columns presented. • FE models validated against new and existing test results. • Numerical parametric studies carried out to generate further data. • Existing structural design provisions for stainless steel beam-columns assessed. • New design provisions developed, and reliability assessed against EN 1990 requirements. Abstract The stability and design of laser-welded stainless steel I-section beam-columns are explored in this study. Owing to the high precision and low heat input of laser-welding, structural cross-sections produced using this fabrication method have smaller heat affected zones, lower thermal distortions and lower residual stresses than would typically arise from traditional welding processes. Eighteen laser-welded stainless steel beam-columns were tested to investigate the member buckling behaviour under combined compression and bending. Two I-section sizes were considered in the tests: I-50 × 50 × 4 × 4 in grade EN 1.4301 and I-102 × 68 × 5 × 5 in grade EN 1.4571 austenitic stainless steel. The two cases of minor axis bending plus compression and major axis bending plus compression with lateral restraints were investigated. The initial loading eccentricities in the beam-column tests were varied to provide a wide range of bending moment-to-axial load ratios. The test results obtained herein and from a previous experimental study were used to validate finite element (FE) models, which were subsequently employed for parametric investigations to generate further structural performance data over a wider range of cross-section sizes, member lengths and loading combinations. The obtained test and FE results were utilized to evaluate the accuracy of the beam-column capacity predictions according to the current European and North American design provisions and a recent proposal by Greiner and Kettler. Finally, an improved approach for the design of stainless steel I-section beam-columns is proposed. [ABSTRACT FROM AUTHOR]

Abstract Stainless steel is widely used in construction due to its combination of excellent mechanical properties, durability and aesthetics. Towards more sustainable infrastructure, stainless steel is expected be more commonly specified and to feature in more substantial structural applications in the future; this will require larger and typically welded cross-sections. While the structural response of cold-formed stainless steel sections has been extensively studied in the literature, welded sections have received less attention to date. The stability and design of conventionally welded and laser-welded austenitic stainless steel compression members are therefore the focus of the present research. Finite element (FE) models were developed and validated against a total of 59 experiments, covering both conventionally welded and laser-welded columns, for which different residual stress patterns were applied. A subsequent parametric study was carried out, considering a range of cross-section and member geometries. The existing experimental results, together with the numerical data generated herein, were then used to assess the buckling curves given in European, North American and Chinese design standards. Following examination of the data and reliability analysis, new buckling curves were proposed, providing, for the first time, design guidance for laser-welded stainless steel members. Highlights • Finite element models of welded stainless steel I-sections are developed • Finite element models are validated against existing experimental data • Parametric studies are presented for both laser-welded and conventionally welded columns • Existing design provisions are described and evaluated • New design proposals are made and assessed through reliability analysis [ABSTRACT FROM AUTHOR]

Cold-formed steel purlin systems with overlapped or sleeved connections are alternatives to continuous two-span systems and exhibit different degrees of continuity. Both connection types are highly favourable in practice since they are both strategically placed over an interior support to provide additional moment resistance and rotational capacity where the corresponding demands are at their largest, thus improving the overall structural efficiency. Until recently, full-scale testing has been the most common way of investigating the structural behaviour of such systems. In this study, numerical modelling, capable of capturing the complex contact interactions and instability phenomena, is employed. The developed finite element models are first validated against data from physical tests on cold-formed steel beams featuring sleeved and overlapped connections that have been previously reported in the literature. Following their validation, the models are employed for parametric studies, based on which the structural behaviour of the examined systems is explored, while the applicability of conventional plastic design as well as of a previously proposed design approach is investigated. Finally, the efficiency of these systems in terms of load-carrying capacity is compared with their equivalent continuous two-span systems. [ABSTRACT FROM AUTHOR]

Highlights • FE models validated against test results on cold-formed steel SHS/RHS. • Validated FE models used to perform parametric studies considering key parameters. • EN 1993-1-1 and AISC-360-16 provisions assessed using test and FE data. • CSM extended and modified to cover the design of cold-formed steel SHS/RHS. • Reliability of different methods evaluated by means of statistical analyses. Abstract Cold-formed steels typically exhibit a rounded stress-strain response with gradual yielding merging into strain hardening. This form of stress-strain curve is at odds with the elastic, perfectly plastic material model that underpins many of the provisions set out in current structural steel design standards. In particular, the beneficial influence of strain hardening on cross-section capacity is neglected. The continuous strength method (CSM) is a deformation-based design method that enables material strain hardening properties to be exploited, thus resulting in more accurate and consistent capacity predictions. The aim of this study is to extend the CSM to the design of cold-formed steel non-slender tubular cross-sections subjected to compression, bending and combined loading, and to verify the proposals through comparisons with existing test data from the literature and finite element results generated herein. The finite element models were first developed and validated against test results on cold-formed steel cross-sections collected from the literature. An extensive parametric study was then conducted to generate additional data over a wider range of cross-section geometries, slendernesses and loading conditions. The numerical results together with the experimental results were then compared with capacity predictions, calculated according to the current design rules in European Standard EN 1993-1-1 (2005) and American Specification AISC-360-16 (2016) as well as the CSM. The CSM is shown to provide more accurate and consistent design predictions for cold-formed steel cross-sections under different loading conditions than those obtained from existing design methods. The improvements arise from the use of the continuous deformation based design approach, as well the rational exploitation of strain hardening. Finally, the reliability levels of the different design methods were assessed by conducting reliability analyses in accordance with EN 1990 (2002). [ABSTRACT FROM AUTHOR]

An innovative concept, whereby the load-carrying capacity and serviceability performance of cold-formed steel beams are enhanced by utilising prestressing techniques, is presented. The prestressing force is applied by means of a high-strength steel cable, which is housed at a location eccentric to the strong geometric axis within the bottom hollow flange of the cold-formed steel beam, inducing initial stresses in the beam that are opposite in sign to those introduced during the subsequent loading stage. As a consequence, the development of local instabilities during loading is delayed and thus the capacity of the beam is enhanced. Furthermore, the pre-camber induced during prestressing, as well as the contribution of the cable to the bending stiffness of the system, decrease the overall vertical deflections of the beam. The conceptual development of prestressed cold-formed steel beams and a study investigating the potential benefits are presented. The mechanical behaviour of the proposed beams in both the prestressing and imposed loading stages is described in terms of analytical expressions, while failure criteria for the design of the cold-formed steel beam and the cable are also developed by employing interaction equations in conjunction with the Direct Strength Method. Geometrically and materially nonlinear finite element analysis with imperfections is employed to simulate the behaviour of the proposed beams. Sample numerical results are presented and compared with the developed analytical expressions and failure criteria, demonstrating the substantial enhancement in moment capacity and serviceability performance offered by these beams. [ABSTRACT FROM AUTHOR]

The findings from a numerical investigation into the degree of composite action that may be mobilised within floor systems comprising cold-formed steel joists and wood-based particle boards are presented herein. Finite element models have been developed, simulating all the components of the examined systems, as well as the interaction between them. The models include initial geometric imperfections, the load-slip response of the fasteners employed to achieve the shear connection as well as both geometric and material nonlinearities. The developed models were first validated against 12 physical tests reported in the literature, which showed them to be capable of accurately capturing the load-deformation curves and failure modes exhibited by the tested specimens. Parametric studies were then performed to examine the influence of key parameters on the structural behaviour of these systems, including the depth and thickness of the cold-formed steel section, as well as the spacing of the employed fasteners; in total, about 100 systems have been examined. Significant benefits in terms of structural response have been identified from the presented numerical study as a result of the mobilisation of composite action; for the systems investigated, which were of typical, practical proportions, up to 140% increases in moment capacity and 40% increases in stiffness were found. The presented research reveals the substantial gains in structural performance and the influence of the key governing parameters for this novel form of composite construction. [ABSTRACT FROM AUTHOR]

A stiffness reduction method for the design of laterally restrained web-tapered steel structures fabricated through the welding of individual steel plates is presented in this paper. Stiffness reduction functions for welded members, accounting fully for the deleterious influence of the spread of plasticity and imperfections on the structural resistance, are developed. The method is implemented through (i) dividing tapered members into prismatic segments along their lengths, (ii) reducing the flexural stiffness of each segment by means of the developed stiffness reduction functions considering the first-order forces and cross-section properties of each segment, (iii) performing Geometrically Nonlinear Analysis and (iv) making cross-section strength checks. Essentially, it is proposed to replace the current typical approach to structural design of conducting a simple elastic (with nominal stiffness) structural analysis followed by elaborate member checks with an integrated process utilising more sophisticated second-order analysis (with stiffness reduction) but very simple design checks. The distribution of internal forces within the structure is captured more accurately due to the allowance for imperfections, residual stresses and plasticity through stiffness reduction and the allowance for frame and member instability effects through the use of second-order analysis. The need for determining effective lengths and for conducting member buckling checks is also eliminated. Verification of the proposed approach against the results obtained from nonlinear shell finite element modelling is presented for various tapering geometries, slenderness values and loading conditions. Assessment of the proposed method against the European and North American steel design codes for tapered steel structures is also provided. [ABSTRACT FROM AUTHOR]

The stability and design of steel equal-leg angle section members subjected to uniaxial bending are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and physical tests on hot-rolled steel equal-leg angle section beams is first presented. The test results, in combination with existing experimental data on steel equal-leg angle section beams collected from the literature, are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Next, a numerical parametric study is presented considering both hot-rolled and cold-formed steel angle section beams with a wide range of slenderness values. In major-axis bending, both lateral-torsional and local buckling were observed, with the former characterised by lateral deflection and twist of the cross-section along the member length but no cross-section deformation and the latter by relative twist and transverse bending of the outstands. In minor-axis bending, lateral-torsional buckling and Brazier flattening were observed, with the latter characterised by splaying of the outstands. Note that, for equal-leg angles under minor-axis bending, lateral-torsional buckling is dominant when the cross-section tips are in compression, while Brazier flattening is more influential when the cross-section tips are in tension. When designing for major-axis bending according to Eurocode 3, both local and lateral-torsional buckling are considered; it is shown herein that equal-leg angle section beams under major-axis bending can be designed using a normalised slenderness based on the minimum of the local and lateral-torsional elastic buckling moments, while also considering the ratio of the local to the lateral-torsional elastic buckling moments. For minor-axis bending, Eurocode 3 only requires cross-section checks; this is found to result in unsafe predictions in some cases. It is shown that both safer and more accurate resistance predictions can be achieved when lateral-torsional buckling and Brazier flattening are accounted for in the design of equal-leg angle section beams under minor-axis bending, using a normalised slenderness, based on the minimum of the lateral-torsional and Brazier flattening critical buckling moments. Finally, new design proposals for steel equal-leg angle section beams, covering both major- and minor-axis bending, are developed and verified against the experimental and numerical results. The proposed design rules are shown to offer substantially more accurate and consistent resistance predictions compared to existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor γ M1 = 1. 0 , is verified following the EN 1990 procedure. • Critical buckling behaviour of equal-leg angles under uniform bending is described. • Experiments on hot-rolled steel equal-leg angle section beams are presented. • Numerical parametric study considering steel angle section beams is described. • Experimental and numerical results are used to develop a new design approach. • Accuracy and reliability of the new proposals is verified in accordance with EN 1990. [ABSTRACT FROM AUTHOR]

Wire and arc additive manufacturing (WAAM) is a method of metal 3D printing that is suited to the requirements of the construction industry in terms of scale, speed and cost. Using this technology, a 10.5 m span footbridge, the first of its kind, has been printed. The testing, analysis and initial verification of the bridge and its components are described herein. The experiments performed included advanced geometric analysis, material testing, compressive testing of cross-sections and full-scale load testing of the bridge at various stages throughout and post construction. Parallel finite element modelling of the full bridge and its constituent elements has also been performed as part of the verification. Confirmation that the bridge was able to sustain its full serviceability design load enabled to the bridge to be unveiled to the public, with controlled access, for Dutch Design Week 2018. Further testing under ultimate limit state design loading is planned before the bridge is placed in its final location and fully opened to the public. The project highlights the potential for metal 3D printing in structural engineering, as well as the necessary considerations for design. Unlabelled Image • Initial verification of the world's first metal 3D printed bridge is undertaken. • Material behaviour of the bridge is determined by tensile coupon testing. • Compressive structural response of the key bridge components is assessed by stub column testing. • Full-scale structural load testing of the bridge both during and after construction under different load cases is described. • Finite element simulations of the bridge are carried out and comparisons are made with the test results. [ABSTRACT FROM AUTHOR]

In current structural steel design specifications, the stability of members is typically assessed through the use of buckling curves which implicitly consider the influence of both initial geometric imperfections and residual stresses. An alternative and more direct approach is to perform a second order analysis of the member or structure with all imperfections included. For modelling convenience, i.e. to avoid the explicit modelling of residual stresses, so-called 'equivalent' geometric imperfections are typically utilised, which consider the combined influence of both geometric imperfections and residual stresses through an increased geometric imperfection amplitude. For columns and beams, equivalent geometric imperfections for use in design by geometrically and materially nonlinear analysis with imperfections (GMNIA) have recently been developed. However, for beam-columns, which are subjected to combined compression and bending, there are currently no specific provisions. Therefore, equivalent geometric imperfections for application in the stability design of steel and stainless steel beam-columns by GMNIA are developed herein. The proposals are calibrated against the results of benchmark finite element (FE) simulations performed on a large number of steel and stainless steel I-section or hollow section beam-columns, with or without intermediate lateral restraints, subjected to various ratios of axial compression and major and/or minor bending moments. Proposals for the shapes and amplitudes of equivalent geometric imperfections are developed for application with both eigenmode-affine imperfections e 0,mod and the direct modelling of sinusoidal bow imperfections e 0,bow. Relative to the traditional Eurocode design calculations, employing the developed equivalent geometric imperfections in GMNIA provides more accurate resistance predictions. The reliability analysis of the design provisions shows that partial safety factors of 1.0 for steel and 1.1 for stainless steel can be safely adopted. • Equivalent geometric imperfections consider the combined influence of both geometric imperfections and residual stresses. • Equivalent geometric imperfections for use in design of steel and stainless steel beam-columns by GMNIA have been developed. • Accuracy and reliability of the proposed equivalent geometric imperfections have been assessed against benchmark FE results. • Employing proposals in GMNIA provides more accurate resistance predictions relative to traditional Eurocode design provisions. • Proposed equivalent geometric imperfections are suitable for use in conjunction with recommended partial safety factors γ M1. [ABSTRACT FROM AUTHOR]

The behaviour and design of pin-ended stainless steel equal-leg angle section members under compression and compression plus minor-axis bending are investigated herein. The studied members are cylindrically pinned about the minor axis. An experimental investigation, including material testing, initial geometric imperfection measurements and physical tests on hot-rolled stainless steel equal-leg angle section members is first presented. Numerical models are developed and validated against the new experimental data. A numerical parametric study is then presented considering both hot-rolled and cold-formed stainless steel angle section columns alongside beam–columns with a wide range of slenderness values. Finally, new design proposals for pin-ended stainless steel equal-leg angle section members under compression and combined compression and minor-axis bending are developed and verified against the results of existing physical experiments, as well as the newly-generated test and numerical results. The proposed design rules are shown to offer substantially more accurate and consistent resistance predictions compared to existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor γ M1 = 1. 1 , is verified following the EN 1990 procedure. • Physical tests on pin-ended stainless steel equal-leg angle columns and beam–columns conducted. • Shell FE models developed in ABAQUS and validated against test results. • Numerical parametric study considering a range of stainless steel equal-leg angle columns and beam–columns described. • Experimental and numerical results are used to develop a new design approach. • Accuracy and reliability of the new design approach is verified in accordance with EN 1990. [ABSTRACT FROM AUTHOR]

The behaviour and design of high strength steel (HSS) beams are addressed in the present study. Six in-plane three-point bending tests on three different welded I-sections − two homogeneous S690 steel welded I-sections and one hybrid welded I-section with S690 steel flanges and an S355 steel web, were first conducted. The beam tests were carried out in major axis bending and a bespoke restraint system was designed and employed in the test programme to prevent lateral-torsional buckling. Following the experimental investigation, a thorough finite element (FE) modelling programme was performed, which included a validation study confirming the accuracy of the developed FE models in replicating the flexural behaviour of HSS welded I-section beams, and a parametric study generating additional FE data on HSS welded I-section beams over a broader range of cross-sectional slendernesses, steel grades and loading configurations. The test results obtained in the present study and collected from the literature, together with the generated FE data from the parametric study, were used to evaluate the suitability of the current Eurocode 3 cross-section slenderness limits for HSS homogeneous and hybrid welded I-sections in bending. It is shown that the current Eurocode Class 2 and Class 3 slenderness limits are suitable for the classification of the outstand flange (in compression) and internal web (in bending) elements of both HSS homogeneous and hybrid welded I-sections subjected to major axis bending, while stricter Class 1 slenderness limits are considered necessary to achieve sufficient rotation capacity for plastic design. The findings from the present study indicate that plastic design can be used for HSS structures, provided the proposed stricter Class 1 slenderness limits are employed. [ABSTRACT FROM AUTHOR]

The post-fire behaviour of eccentrically loaded reinforced concrete columns confined by circular steel tubes, also known as circular steel tube confined reinforced concrete (STCRC) columns, is investigated in this paper. A total of 12 experiments were conducted on eccentrically loaded circular STCRC stub columns after exposure to the ISO 834 standard fire, including both the heating and cooling phases. The temperatures across the tested sections, the axial load versus lateral displacement responses and the strains in the steel tubes were all measured and discussed. A finite element (FE) model was developed using the program ABAQUS, and validated against the test results from the present study, as well as related studies. Parametric studies were then performed to identify the influences of key parameters on the residual capacity of the eccentrically loaded STCRC columns, including eccentricity ratio, heating time, cross-sectional diameter, slenderness ratio, material strengths, steel tube to concrete area ratio and reinforcement ratio. Finally, a simplified method was proposed for predicting the residual load-bearing capacity of the eccentrically loaded STCRC columns after fire exposure. [ABSTRACT FROM AUTHOR]

The structural behaviour and design of cold-formed steel square and rectangular hollow section (SHS and RHS) beam–columns, made of both normal and high strength steels, are investigated in the present study through a comprehensive numerical modelling programme. Refined finite element (FE) models were first established and calibrated against a collection of existing cold-formed steel member test results from the literature. Following this, an extensive parametric study was undertaken to expand the cold-formed steel beam–column data pool, covering a wider range of steel grades, cross-section geometries, member slendernesses and loading combinations. Results from the parametric study were then used to examine the accuracy of the current codified beam–column design methods, provided in the European Standards EN 1993-1-1 (2005) and EN 1993-1-12 (2007) as well as the American Specification AISC 360-16 (2016). The comparisons revealed that the codified design rules provide varying levels of accuracy in predicting the ultimate resistances of cold-formed steel beam–columns depending on the steel grade. An improved design approach, compatible with current codified design rules in EN 1993-1-1 (2005), is proposed that features (1) recently developed column buckling curves that reflect the increasing normalised column buckling resistance with increasing steel yield strength; (2) bending moment resistances calculated using the Continuous Strength Method (CSM) and (3) new interaction curves for cold-formed steel beam–columns, which are anchored to these more accurate end points. The newly proposed design approach is shown to yield more accurate and consistent resistance predictions over current design provisions for cold-formed steel SHS and RHS beam–columns made of different steel grades, and its reliability has also been statistically verified in accordance with EN 1990. • Numerical study on cold-formed steel SHS and RHS beam–columns was conducted. • Modified EC3 column design approach was extended to columns in steel grades up to S1100. • Bending resistance predictions from CSM were adopted in beam–column design. • New beam–column design approach was developed to account for influence of steel grade. • Current EC3 partial safety factor was shown to be applicable to design proposal. [ABSTRACT FROM AUTHOR]

• Equivalent imperfections for use in out-of-plane stability design by GMNIA developed. • Accuracy and reliability of the proposals assessed against benchmark beam FE results. • Proposals provide more accurate resistance predictions compared to traditional method. • Proposals suitable for use with EC3 recommended values of partial safety factor γ M1. In current structural design specifications, such as EN 1993-1-1 for steel and EN 1993-1-4 for stainless steel, the stability of members is typically assessed through the use of buckling curves, which consider the influence of initial geometric imperfections and residual stresses. An alternative, more direct, approach is to perform either an elastic or inelastic second-order analysis of the member or structure with imperfections. For modelling convenience, so-called 'equivalent' imperfections are typically utilised, which consider the combined influence of both geometric imperfections and residual stresses. Equivalent imperfections for the design of columns and beams by second-order elastic analysis, also referred to as geometrically nonlinear analysis with imperfection (GNIA), are provided in the current design specifications. For columns, equivalent imperfections for design by second-order inelastic analysis, also referred to as geometrically and materially nonlinear analysis with imperfections (GMNIA), were recently developed, but for beams that are susceptible to lateral-torsional buckling (LTB), there are currently no appropriate provisions. The aim of this study is therefore to develop equivalent imperfections for use in out-of-plane stability design of steel and stainless steel members by GMNIA. The proposals are calibrated against the results of benchmark finite element (FE) simulations performed on a large number of steel and stainless steel members with geometric imperfections and residual stresses subjected to major axis bending. Two proposals for equivalent imperfection amplitudes are developed: (1) e 0,mod , for use with eigenmode-affine imperfections and (2) e 0,bow , for use with sinusoidal bow imperfections. The latter is applied solely in the lateral direction and as a summation of a half-sine wave and a full-sine wave. Relative to the traditional Eurocode design calculations, employing the proposed LTB imperfections in GMNIA provides generally more accurate member resistance predictions, while remaining safe-sided relative to the benchmark FE results. The reliability of the design provisions is demonstrated through statistical analysis, where it is shown that partial safety factors of 1.0 for steel and 1.1 for stainless steel can be safely adopted. [ABSTRACT FROM AUTHOR]

The structural behaviour of S960 ultra-high strength steel (UHSS) welded I-section beam–columns is studied in the present paper. Nonlinear finite element (FE) models were first created and validated against relevant test results collected from the literature; the models were then used to carry out parametric studies to obtain additional FE data over an extensive spectrum of cross-section dimensions, member lengths and loading combinations. Given the lack of existing design rules for S960 UHSS structures, the codified beam–column interaction curves prescribed in the current European (EC3) and American (AISC 360) design specifications for lower strength steel members, were assessed for their applicability to the studied S960 UHSS non-slender welded I-section members, utilizing the generated numerical data. It was found that EC3 yielded conservative failure load predictions for Class 1 and 2 S960 UHSS welded I-section beam–columns buckling about either principal axis; similar results were found for Class 3 members under combined compression and strong-axis bending, with the mean FE to predicted failure load ratio being 1.29, the corresponding COV being 0.12, but rather conservative strength predictions arose for Class 3 members under combined compression and weak-axis bending, with the mean FE to predicted failure load ratio being 1.50. AISC 360 led to overall accurate and consistent failure load predictions for S960 UHSS non-slender welded I-section beam–columns, but resulted in some unconservative strength predictions for members under combined compression and weak-axis bending. New design proposals, adopting the format of EC3 design interaction curve, but with more accurate compression and bending end points, interaction factors and buckling reduction factors determined from the continuous strength method (CSM), were proposed and shown to lead to improved design accuracy. • The structural behaviour of S960 UHSS non-slender welded I-section beam–columns is studied. • FE models are developed and validated against test results and then used to perform parametric studies. • The current design rules are assessed for their applicability to S960 UHSS non-slender welded I-section beam–columns. • A CSM-based design approach is proposed and shown to lead to improved design accuracy. [ABSTRACT FROM AUTHOR]

An accurate and consistent approach to the out-of-plane stability design of steel beams and structures utilising second-order inelastic analysis with strain limits is proposed. The method is implemented using computationally efficient beam elements, with the ultimate structural resistance defined either by (i) the ultimate load factor or (ii) the load factor at which a strain limit, determined on the basis of the continuous strength method (CSM), is attained, whichever occurs first. Thus far, the method has been established for the in-plane design of steel structures and structural components; in the present paper, its scope is extended, for the first time to the scenarios in which out-of-plane stability effects, with a focus on lateral–torsional buckling (LTB), govern. The accuracy and safety of the method are assessed against the results of nonlinear shell finite element (FE) modelling. It is shown that the proposed method consistently provides more accurate results than the traditional LTB design method of prEN 1993-1-1. In addition to its accuracy, the proposed approach also streamlines the design process by eliminating the need for cross-section classification and member design checks. [Display omitted] • Advanced out-of-plane stability design method for steel beams and structures proposed • Provisions for considering shear-torsion effects for different sections presented • Accuracy of proposed design method verified against shell finite element results • Proposed method consistently provides more accurate ultimate strength predictions [ABSTRACT FROM AUTHOR]

The structural response of stainless steel I-sections in fire is investigated in this paper. Finite element models of stainless steel I-section members, capable of replicating their cross-section behaviour at elevated temperatures, are created and validated against existing experimental data from the literature. The validated finite element models are then utilised to perform comprehensive numerical parametric studies, where over 1000 numerical simulations of the response of stainless steel I-sections in fire are carried out, considering different cross-section dimensions, loading conditions, stainless steel grades and elevated temperature levels. On the basis of the findings from the parametric studies, the existing design rules of the European structural steel fire design standard EN 1993-1-2 and the recent design recommendations of Xing et al. [1] , together with the plastic effective width method of Bambach and Rasmussen [2] , are assessed in terms of their accuracy and reliability. It is observed that relative to the existing fire design rules set out in EN 1993-1-2, the design methods of Xing et al. [1] and Bambach and Rasmussen [2] are able to provide more accurate and reliable ultimate cross-section resistance predictions for stainless steel I-sections in fire, providing further verification of the suitability of the design provisions of Xing et al. [1] for inclusion in the next revision of EN 1993-1-2. • Local buckling behaviour of stainless steel I-sections in fire is investigated. • FE models replicating local buckling of stainless steel I-sections in fire are created. • Extensive parametric studies are performed to assess accuracy of design provisions. • Higher accuracy of the Xing et al. method relative to Eurocode 3 are observed. [ABSTRACT FROM AUTHOR]

• Prestressing levels providing different levels of pre-cambering are determined. • Sensitivity to imperfections in conjunction with pre-cambering levels studied. • Optimal prestressing levels found to maximize structural efficiency for ultimate behaviour. • Doubly-symmetric arrangements perform better with no pre-cambering. • Mono-symmetric arrangements perform better with upward pre-cambering. The behaviour and structural performance of imperfect beam-columns with crossarms and externally prestressed cable stays are studied numerically, where the combination of bending and compression is assumed to be derived from the system self-weight acting orthogonally to the applied axial load. Both doubly-symmetric and mono-symmetric systems are studied. Sensitivity of the structural response to varying prestressing levels, pre-cambering and initial imperfections is investigated. Different initial imperfection levels and combinations are considered to facilitate the exploration of interactive buckling. The optimum prestressing force in terms of ultimate resistance and two structural efficiency indicators is also studied. It is found that relatively small crossarm lengths, stay diameters and crossarm length ratios should be avoided. Moreover, mono-symmetric cases are more sensitive to the level of pre-cambering than their doubly-symmetric counterparts. Considering both load-carrying capacity and structural efficiency, doubly-symmetric cases perform best with zero pre-cambering, but mono-symmetric cases perform best with upward pre-cambering. As for the true optimum prestressing levels, these are recommended to be significantly above the linearly obtained optimum to maximize the structural efficiency. [ABSTRACT FROM AUTHOR]

• A design method performed by second-order inelastic analysis with CSM strain limits is proposed. • CSM strain limits are adopted to account for local buckling and control the spread of plasticity. • Applied to 3680 members with various taper ratios, loading conditions and member slendernesses. • The accuracy is verified against benchmark shell FE results as well as a number of experiments. • The proposed design method is consistently more accurate than current EN 1993-1-1 design. A consistent design approach, performed by second-order inelastic analysis using beam finite elements with strain limits, is proposed for web-tapered steel members. In the proposed design approach, a geometrically and materially nonlinear analysis with imperfections (GMNIA) of the tapered steel member is carried out and the ultimate strength of the member is signified by reaching either the strain limit defined according to the Continuous Strength Method (CSM) or the peak load factor, whichever occurs first. To consider the beneficial effect of strain gradients along the lengths of the members on local cross-section resistances, the strains are averaged over the local buckling half-wavelength. The accuracy of the proposed design approach is verified against results from nonlinear shell finite element modelling as well as a number of experiments on tapered members considering various taper ratios, loading conditions and member slenderness values. The proposed method provides more accurate and consistent ultimate strength predictions than EN 1993-1-1 [1] , because the following aspects, which are ignored in traditional design methods, are captured: (1) the interaction between cross-section elements for the consideration of local buckling, (2) the influence of local moment gradients on cross-section resistance, (3) the partial plastification of cross-sections and (4) strain hardening. [ABSTRACT FROM AUTHOR]

A finite element (FE) investigation into the compressive behaviour of concrete-filled double skin tubular (CFDST) cross-sections with lean duplex and ferritic stainless steel outer tubes is presented. FE models were initially developed and validated against available test results reported in the literature. Upon successful replication of the ultimate capacities, load–deformation histories and failure modes exhibited by the tested CFDST stub columns, a parametric study was undertaken to investigate the influence of key variables, including the local slendernesses of the outer and inner tubes, the concrete strength and the adopted grade of stainless steel, on the ultimate response of the studied CFDST stub columns. Based on the generated FE data pool and the available test results, the applicability of the existing European, Australian and American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections was evaluated. All the examined design rules are shown to yield unduly conservative (less so for the higher concrete grades) and rather scattered capacity predictions. Modifications to the design treatment in relation to the effective area of the outer tubes, to take due account of outward-only local buckling, and the effective compressive strength of the infilled concrete, to reflect the reduced relative effectiveness of higher concrete grades, are considered. The modified design rules are shown to improve the accuracy and consistency of the design capacity predictions. Finally, statistical analyses were carried out to demonstrate the reliability of the modified design approaches. • FE models of concrete-filled double skin sections with stainless steel outer tubes developed. • Parametric studies undertaken on over 300 CFDST specimens. • Applicability of existing design provisions assessed. • Modifications to the existing design rules shown to provide improved capacity predictions. • Reliability of the modified design rules confirmed by means of statistical analyses. [ABSTRACT FROM AUTHOR]

The structural fire response of hot-rolled steel square and rectangular hollow sections (SHS and RHS) under combined compression and bending is investigated in this study through finite element (FE) modelling. The developed FE models were firstly validated against available test results on hot-rolled steel SHS/RHS subjected to combined compression and bending at elevated temperatures. Upon validation, an extensive parametric study was then carried out to examine the resistance of hot-rolled steel SHS/RHS under combined loading at elevated temperatures, covering a wide range of cross-section slendernesses, cross-section aspect ratios, combinations of loading and temperatures up to 800 °C. The numerical data, together with the experimental results, were compared with the strength predictions according to the current structural fire design rules in the European Standard EN 1993-1-2 (2002) and American Specification AISC 360–16 (2016) for hot-rolled steel SHS/RHS under combined loading. The comparisons generally indicated significant disparities in the prediction of resistance of hot-rolled steel SHS/RHS under combined loading at elevated temperatures, owing principally to inaccurate predictions of the end points of the design interaction curves. The deformation-based continuous strength method (CSM) has been shown to provide accurate strength predictions for these end points i.e. the resistances of hot-rolled steel SHS/RHS stub columns and beams at elevated temperatures. In this study, proposals are presented to extend the scope of the CSM to the structural fire design of hot-rolled steel SHS/RHS under combined compression and bending. The CSM proposals are shown to offer improved accuracy and reliability over current design methods and are therefore recommended for incorporation into future revisions of international structural fire design codes. • Numerical models validated against fire test results for SHS/RHS under combined loading. • Validated numerical models used to perform parametric studies considering key parameters. • Current design provisions in EN 1993-1-1 and AISC 360–16 assessed using the experimental and numerical data. • The Continuous Strength Method extended to cover the fire design of SHS/RHS under combined loading. • Reliability of the CSM proposals confirmed by means of statistical analyses. [ABSTRACT FROM AUTHOR]

The local buckling behaviour and design of stainless steel plates in fire are investigated in this paper. Finite element models of stainless steel plates able to mimic their response in fire are created and validated against experimental results from the literature. Parametric studies are then performed and the results are utilised to assess the current design provisions set out in the European structural steel fire design code EN 1993-1-2; shortcomings in the prediction of the local buckling response of stainless steel plates in fire are revealed. A new effective width based design approach able to reflect the variation in strength and stiffness of stainless steel at different temperature levels in the determination of the local plate slenderness and thereby the ultimate resistances of stainless steel plates in fire is put forward. The proposed approach is shown to provide significantly higher levels of accuracy and reliability relative to the current provisions in EN 1993-1-2 for a wide range of plate slendernesses, elevated temperature levels, stainless steel grades and loading conditions. The design rules proposed for the local buckling assessment of stainless steel plates at elevated temperatures in this paper are due to be incorporated into the upcoming version of the European steel fire design standard EN 1993-1-2. • Local buckling behaviour of stainless steel plates at elevated temperatures is investigated. • FE models able to replicate local buckling response of stainless steel plates in fire are created and validated. • Extensive parametric studies are performed, whose results are utilised to assess existing design provisions in Eurocode 3. • New design provisions are established for local buckling assessment of stainless steel plates in fire. • Higher accuracy and reliability of proposed design provisions relative to existing methods in Eurocode 3 are illustrated. [ABSTRACT FROM AUTHOR]

The flexural buckling behaviour and design of stainless steel I-section columns at elevated temperatures are investigated in this paper. Finite element models able to accurately replicate the response of structural stainless steel columns in fire are created and validated. The models are then utilised to carry out extensive numerical parametric studies considering a broad range of stainless steel grades, cross-section geometries, slendernesses and elevated temperature levels. Using the results from the parametric studies, the safety and accuracy of existing design rules provided in the European structural steel fire design code EN 1993-1-2 for stainless steel columns in fire are assessed. It is observed that the existing design rules provide rather scattered and inaccurate ultimate strength predictions for stainless steel columns at elevated temperatures. For the purpose of establishing an accurate and practical means of designing stainless steel columns in fire, a new design approach, compatible with existing design rules in EN 1993-1-2, is proposed. The safety, accuracy and reliability of the proposed approach are illustrated for a wide range of cases against the results obtained through nonlinear finite element modelling. The proposed stainless steel column fire design rules are due to be incorporated into the upcoming version of the European steel fire design standard EN 1993-1-2. • Behaviour and design of stainless steel I-section columns in fire are investigated. • Advanced finite element models able to mimic response of stainless steel columns in fire are developed and validated. • New design rules for stainless steel I-section columns in fire are established. • Proposed design rules lead to considerably more accurate results relative to the current design rules in EN 1993-1-2. • Accuracy, safety and reliability of proposed design rules are verified against extensive numerical data. [ABSTRACT FROM AUTHOR]

Abstract The local cross-section behaviour of stainless steel channel sections under the combined actions of axial compression and minor axis bending moment is investigated in the present paper and its companion paper, based on a comprehensive experimental and numerical study. Two channel section sizes were considered in the experimental programme, with the test specimens laser-welded at the two flange-to-web junctions from hot-rolled EN 1.4307 and EN 1.4404 austenitic stainless steel plates. The experiments involved initial local geometric imperfection measurements and 15 eccentrically loaded stub column (combined loading) tests. The loading eccentricity was varied to achieve a range of ratios of axial compression to minor axis bending moment; both orientations of bending (web in compression and web in tension) were considered. The test setup and procedures, together with the key experimental observations, including the load-carrying and deformation capacities, load-end rotation histories and failure modes, are fully reported. A finite element simulation study is then presented, in which the models were first validated against the obtained test results and then employed, in the companion paper, for parametric investigations and the assessment of design provisions. Highlights • Laser-welded stainless steel channel sections under combined compression and minor axis bending moment has been studied. • An experimental programme including 15 combined loading tests has been carried out. • Finite element models have been developed and validated against experimental results. [ABSTRACT FROM AUTHOR]

• A new residual stress model for S235 to S960 steel welded I-sections has been proposed. • Tests on HSS homogeneous and hybrid welded I-section columns buckling about the major axis have been carried out. • Numerical models have been established and validated against test results. • Numerical parametric studies have been performed considering different key parameters. • Current codified design rules on flexural buckling of welded I-section columns have been assessed. • A new design method has been devised providing more accurate and consistent flexural buckling resistances. The flexural buckling behaviour and design of homogeneous and hybrid welded I-section columns, considering a wide range of steel grades, are investigated in the present study. Residual stresses are first examined through the statistical analysis of 71 existing experimental results collected from the literature; on the basis of the findings, a new residual stress model for S235 to S960 steel welded I-sections is proposed. Experiments on a total of five pin-ended homogeneous (S690) and hybrid (S355 web and S690 flanges) welded I-section columns buckling about the major axis are then presented. In parallel with the experimental programme, finite element (FE) models were created and validated against the experimental results obtained from the present study, as well as those collected from the literature. The developed FE models were shown to be capable of accurately replicating the key experimental responses, and were then utilised to carry out extensive parametric studies, through which additional 6000 numerical column buckling data covering a wide range of steel grades, cross-section geometries and member slendernesses were generated. The combined experimental and numerical data were used to evaluate the accuracy of the flexural buckling design rules for welded I-section columns set out in the current European and North American design standards, where shortcomings relating to the consideration of steel grade were identified. A modified Eurocode 3 (EC3) method was devised to reflect the influence of yield strength on the buckling resistances of welded I-section columns more systematically and shown to provide substantially improved resistance predictions in terms of accuracy and consistency; the reliability of the modified approach was statistically verified following the procedure set out in Annex D of EN 1990 and is considered to be suitable for incorporation into future revisions of Eurocode 3. [ABSTRACT FROM AUTHOR]

• 26 ferritic stainless steel beam-column tests undertaken. • Over 2000 numerical results generated for austenitic, duplex and ferritic stainless steel CHS beam-columns over a range of local and global slenderness values. • Existing Eurocode 3 design provisions are evaluated and new design proposals are determined and verified. • Reliability analyses are undertaken for existing and proposed stainless steel CHS beam-column design methods using the new experimental and numerical dataset. The present work was prompted by shortcomings identified in existing design provisions for stainless steel circular hollow section (CHS) beam-columns. First, addressing a lack of existing experimental data, a series of ferritic stainless steel CHS beam-column tests was undertaken at the cross-section and member levels. In total, 26 beam-column tests, including two section sizes (a non-slender class 3 and slender class 4 cross-section), two member slenderness values for each cross-section type and a wide range of loading eccentricities were carried out to investigate the interaction between local and global buckling. Following validation of finite element (FE) models, a numerical study was then undertaken to explore the buckling response of stainless steel CHS beam-columns, covering austenitic, duplex and ferritic grades with a wide range of local and global slendernesses and applied loading eccentricities. Over 2000 numerical results were generated and used to assess new design proposals for stainless steel beam-columns, featuring improved compression and bending end points and new interaction factors. The new proposals are more consistent and more accurate in their resistance predictions than the current EN 1993-1-4 (2015) design approach. The reliability of the new proposals has been verified by means of statistical analyses according to EN 1990 (2005). [ABSTRACT FROM AUTHOR]

Externally anchored prestressed cables can be employed to enhance the stability of steel truss compression elements significantly. To demonstrate this concept, a system comprising a tubular strut subjected to an external compressive load and a prestressed cable anchored independently of the strut is studied. Energy methods are utilized to define the elastic stability of the perfect and imperfect systems, after which the first yield and rigid–plastic responses are explored. The influence of the key controlling parameters, including the length of the strut, the axial stiffness of the cable and the initial prestressing force, on the elastic stability, the inelastic response and the ultimate strength of the system is demonstrated using analytical and finite element (FE) models. To illustrate the application of the studied structural concept, FE modelling is employed to simulate the structural response of a prestressed hangar roof truss. A nearly two-fold enhancement in the load-carrying capacity of the truss structure is shown to be achieved owing to the addition of the prestressed cable. • Analytical modelling of steel struts with externally anchored prestressed cables. • Post-buckling stability dependent on ratio of cable axial stiffness to strut bending stiffness. • Failure evaluated from Perry-Robertson type approach and a rigid-plastic analysis. • Ultimate strength predictions verified through FE modelling of a prestressed truss. • Two-fold enhancement in capacity demonstrated compared to system without cables. [ABSTRACT FROM AUTHOR]