In structural engineering, system-based advanced design methods are increasingly recognised as sound and efficient design approaches for structural steel and stainless steel structures thanks to the reliability-based design formulations developed recently. However, the extension of such methods to aluminium structures still remains unresolved. Aluminium frames, assembled with extruded members, result not only in lower modulus of elasticity, but also in different material uncertainties than those found for steel or stainless steel alloys. Therefore, it is necessary to carry out independent reliability calibrations for this particular metal. This study represents the first effort towards extending the Direct Design Method (DDM) to aluminium portal frames under gravity loading, offering a straightforward design methodology that accounts for the key properties that influence the behaviour of these structural systems. System reliability calibrations are presented for two aluminium portal frames covering two aluminium alloys from the 6xxx series using advanced numerical simulations and First Order Reliability Method. From the calculated system reliability indices for ultimate limit states, suitable system safety factors gamma M,s and system resistance factors phi s are proposed for the direct design of aluminium portal frames according to the European and US design frameworks under gravity loads. Furthermore, the reliability of these structures is assessed at the system level for serviceability limit state, which is found to be a governing consideration for the investigated aluminium frames. This work provides valuable insights into the application of system-based design methods to aluminium portal frames, laying the groundwork for future research and development of consistent design codes across different materials.

Curved steel plate girders are often utilized in bridges that are built in congested urban areas where complex plan alignments are required. However, the design of curved steel plate girder bridges is more complex than that of equivalent straight ones due to the difficulties associated to the simplification of the design procedures. The current and upcoming versions of Eurocode 3 for plated structures (EN 1993-1-5) do not address steel plate girders curved in plan, especially regarding their behavior against concentrated forces and shear. In that sense, the guidance provided for patch loading and shear design is only applicable to straight girders. Hence, current design guides seem not to cover curved plate girders subjected to patch loading and shear appropriately, or to provide only limited and simplified guidance on how to design them. Therefore, it is necessary to carry out more research in order to achieve a better understanding of the behavior of curved steel plate girders so that suitable design approaches can be developed. This paper presents a comprehensive numerical study on curved steel plate I-girders subjected to patch loading and shear, considering the material and geometric nonlinearities as well as initial imperfections. The numerical models are developed by means of the advanced finite element soft ware ABAQUS and validated against experimental tests available in the literature. From the assessment of the numerical results, practical recommendations are proposed to extend the applicability of the design provisions prescribed in the next version of EN 1993-1-5 for straight steel I-girders to the case of curved steel I-girders.

Bridges curved in plan are commonly employed in urban areas, particularly in urban interchange structures that require intricated layouts. As a result of this curvature, these structures present significant complexities, both in their performance and design, when compared to equivalent straight girders. This fact explains the limitations of current design specifications, which appear to either inadequately address the design of such girders under different loading scenarios or to offer limited and simplified guidance only. AASHTO adopts a simplified and empirical approach, providing some design guidelines for curved steel plate girders. In contrast, the current and upcoming versions of Eurocode 3 for plated structures (EN 1993-1-5) lack specific provisions for curved steel plate girders, especially regarding their response to patch loading, and the adoption of the provisions for straight girders has been shown to provide rather inaccurate and overly conservative predictions. To address this issue, this paper presents a practical procedure for estimating the patch loading resistance of curved steel I-girders. The work is based on a comprehensive finite element investigation on curved steel plate I-girders subjected to patch loading. The new proposal features a novel expression for calculating the buckling coefficient that considers the effect of curvature for a more accurate prediction of the elastic critical load of curved steel I-girders, in addition to a novel resistance function specific to these structures. The new proposal is found to offer a more precise and consistent resistance prediction, by incorporating curvature effects, that the current provisions for straight girders.

The new generation of structural codes for stainless steel allow carrying out global plastic analyses on certain types of stainless steel structures with plastic cross-sections at room temperature if the joints conforming the structure are classified as full-strength joints, a capability that has been long recognised for carbon steel structures. However, the requirements for the fire design of carbon and stainless steel structures in current and upcoming European design standards are still primarily based on the resistance of individual members, disregarding the redistribution of internal forces and strain hardening effects. Recent studies have proven that carbon and stainless steel frames with plastic cross-sections and full-strength joints are capable of redistributing internal forces at elevated temperatures, and failed describing global plastic collapse mechanisms under fire situation, suggesting that system effects are also relevant at elevated temperatures and could be taken into account for more efficient designs. On this basis, this paper develops an extensive numerical study focused on carbon and stainless steel frames under fire situation, and proposes a new methodology based on plastic global analysis to estimate the resistance of such structures under fire situation accounting for their redistribution capacity, improving the accuracy of the predicted fire time resistances significantly for the two materials.

The reuse of structural steel components in new buildings represents a key strategy to reduce greenhouse gas emissions from the EU construction sector. However, its implementation demands robust regulatory frameworks and further studies to ensure structural reliability. This paper investigates the performance of a demonstrator warehouse in Spain at system level, which will be built from reclaimed steel members, under vertical load scenarios (including permanent, imposed and snow loads). The variability of the system resistance has been assessed through advanced finite element models and probabilistic characterization of material and geometric properties, imperfections, and load uncertainties, integrated using the First‐Order Reliability Method. Results highlight the strong dependence of failure probabilities on load combinations, with permanent and snow loading being the most critical case. Based on these findings, system partial safety factors required to satisfy the EN 1990 target reliability are derived, which are found to align well with the current recommendations for reclaimed steel design, supporting the use of system‐based design approaches as a pathway to more efficient and sustainable design practices.

Abstract A double‐wire TIG‐WAAM 3D printing technology was successfully developed in the framework of the DWYN project, and used for the manufacturing of a 50· 150· 300 mm rectangular hollow section from ER308L steel with ultimate tensile strength and hardness in the range of the as‐welded material with no considerable differences along the length of the specimen, indicating the homogeneity of the 3D printing process. The specimen presents relatively smaller yield strength and elongation (15.3 and 7.4%, respectively) than the as welded material, which could be due to the inherent anisotropy of the mechanical properties in 3D printed parts caused by the layer‐by‐layer deposition methodology. The printed sample shows no apparent porosity or impurities inside or between the layer boundaries, with an austenitic microstructure with columnar delta ferrite dendrites along the building direction caused by the fast cooling of the welding process. Mechanical and microstructural results indicate the potential of the as‐machined double‐wire TIG‐WAAM 3D printed specimens for structural applications, although anisotropic mechanical properties should be taken in consideration at the same time that a further optimization of the process parameters and additional mechanical studies need to be carried out to completely assess the suitability of the technology for its use in highly demanding applications.

Stainless steel is a structural material with increasing demand for its aesthetic and mechanical properties, as well as its potential in seismic design due to its ductility and strain hardening. However, research on the behaviour of stainless steel under seismic loads is scarce, especially when focused on the global response of structures. One of the key parameters governing the seismic response of systems is the rotation capacity of structural members and compacts, for which recent investigations have proposed predicting expressions under cyclic bending. However, in more complex loading cases such as members subjected to constant axial loads (i.e., gravity loads) and cyclic bending (i.e., seismic actions), the existing expressions for predicting the rotation capacity are very complex or only applicable to a limited range of cases. In this context, the present investigation proposes simple analytical expressions to predict the full moment-rotation diagrams of austenitic stainless steel hollow section elements under combined axial loading and cyclic bending. The proposal is based on an extensive numerical parametric study considering a range of elements under different local slendernesses and several levels of axial loading. The rotation capacities and corresponding bending moment resistances are related to the key affecting parameters, such as the axial load, section stiffness and local slenderness, from which the key features of the full moment-rotation diagrams can be obtained for the characterisation of plastic hinges, allowing a more efficient seismic design of stainless steel structures.

This paper delves into the transformative journey from advanced analysis in steel structures to establishing a design paradigm for the construction sector-responsible for 50 % of raw material extraction, 40 % of all energy consumption and 36 % of greenhouse gas emissions-, aligned with the European Green Deal requirements. In this context, the utilization of novel design and assessment methods, empowered by the use of digital technologies, and the establishment of a comprehensive framework for the reuse of reclaimed steelwork represent the two main paths towards a reliable and environmentally responsible steel structural design. The paper explores and underscores the main challenges that the research community needs to address for improving the sustainability and circularity of the steel construction sector, and presents two specific research projects that will contribute to address such shortcomings: the implementation of a fully documented demonstrator to showcase the reuse process for decarbonization, and the development of a holistic Decision Support System that integrates system-based advanced design methods, Structural Health Monitoring and Life Cycle Assessment. Ultimately, this paper aims to provide insights into multifaceted aspects of smart, sustainable steel structures, advocating for system-based approaches to reshape the future of structural engineering and to ensure a resilient and environmentally conscious built environment.

Stainless steel is a corrosion resistant iron alloy with great potential in structural engineering due to its excellent mechanical features, durability and aesthetic properties. Since most of the existing research has concerned the behaviour of the material, the response of individual structural members, or the performance of simple structural systems under monotonic loading, one of the challenges that remains is the evaluation of the cyclic performance of members to promote its use in seismic design and exploit the excellent ductility and strain hardening properties of this material. For this reason, an experimental programme on austenitic stainless steel Rectangular Hollow Section (RHS) specimens was carried out at the Department of Civil Engineering of the Università degli Studi di Salerno in collaboration with the Universitat Politècnica de Catalunya. A total of eight RHS specimens were tested around the minor axis under both monotonic (three specimens) and cyclic (five specimens) loading, in an experimental set-up that followed a cantilever scheme. The main purpose of this study was to acquire information on the cyclic performance of austenitic steel structural members, focusing on the comparison between monotonic and cyclic behaviours. The outcomes of the tests included the load–displacement and the moment–rotation curves, the energy dissipation capacity, and the evaluation of the key rotation capacities of the members. The reported data are essential to enhance the still scarce research on the seismic behaviour of stainless steel structures.

Curved steel plate girders are often utilized in bridges built in congested urban areas, where intricate plan alignments are required. Their performance and design is, however, more complex than that of equivalent straight girders, especially regarding concentrated forces: while AASHTO adopts a simplified and empirical approach for the design of curved steel girders, the Eurocode for plated structures EN 1993-1-5 does not address the patch loading design of steel plate girders curved in plan, being only applicable to straight girders. Therefore, it is necessary to carry out further research in order to achieve a better understanding of the behavior of curved steel plate girders so that suitable design approaches accounting for the specific response of these structures can be developed. This paper presents a comprehensive numerical study on curved steel plate I-girders subjected to patch loading, considering material and geometric nonlinearities as well as initial imperfections. The numerical models are developed by means of the advanced finite element software ABAQUS and validated against experimental tests available in the literature. The numerical results demonstrate that the elastic critical buckling loads increase with increasing span-to-plan radii, which can be conservatively estimated using the EN 1993-1-5 provisions for straight steel girders, and that the ultimate patch loading resistance decreases with the radius of curvature. Based on this, practical recommendations are proposed to extend the applicability of the design provisions prescribed in the next version of EN 1993-1-5 for straight steel I-girders to the case of curved steel I-girders.

Stainless steel frames with compact cross-sections are capable of redistributing internal forces at room temperature and forming plastic failure mechanisms, which is reflected in the new generation of structural codes for stainless steel by allowing to carry out global plastic analyses on certain types of stainless steel if the joints conforming the structure are classified as full-strength joints. Nevertheless, the requirements for the fire design of stainless steel structures in current standards is still primarily based on the resistance of individual members, disregarding strain hardening effects and the redistribution of internal forces, mainly because the influence of the performance of stainless steel joints on the response of the frames under fire situation is yet unknown. On this basis, this paper presents a numerical study focused on stainless steel frames with compact rectangular hollow sections, both at room temperature and at elevated temperatures, to analyse the influence of full-strength joints on the frame's response by means of two joint modelling techniques, and to assess the plastic redistribution capacity of stainless steel frames, especially under fire situation. In addition, an exhaustive revision of the failure criteria for stainless steel frames under fire situation is carried out and a new failure criterion is proposed based on the results derived from the parametric study. The results demonstrate that stainless steel frames are also capable of redistributing internal forces and forming plastic collapse mechanisms under fire situations, suggesting a new procedure to predict the response of these structures under fire situation more accurately.

The expected failure mode of T-joints between rectangular hollow sections subjected to bending moment with a ratio of the brace width (b1) over the chord width (b0) below 0.85 (i.e., β=b1/b0≤0.85) is the chord face failure, produced by yielding. This paper presents the development of a numerical model to reproduce this phenomenon, calibrated against experimental laboratory tests, which is then used to carry out a large parametric study, where the influence of the main geometric parameters involved in the joint resistance –the chord width (b0), the chord thickness (t0), the chord radius (R0), the brace width (b1), the brace height (h1) and the weld thickness (a)– is assessed for two different materials: S275 carbon steel and EN 1.4301 austenitic stainless steel. The results derived from the parametric study suggested that a different yield line mechanism may develop for higher β-values (β≥0.45) than that considered in current European and American codes. Hence, a new formulation for estimating the bending resistance of T-joints exhibiting chord face failure modes is presented based on a different yield line mechanism developed in this paper. This formulation includes additional geometric parameters that are dismissed in the current formulation, such as the chord radius and the weld thickness, which are found to have a non-negligible influence on the joint resistance. Moreover, the proposed formulation is adjusted in order to be used for joints with lower β-values without losing accuracy. Additionally, the new proposed formulation is adapted for stainless steel joints by combining it with the Continuous Strength Method (CSM) formulation with promising results. Finally, the soundness and safety of the proposed formulation is statistically demonstrated.

Stainless steel alloys exhibit a nonlinear stress‐strain behaviour which can be precisely described by the Ramberg‐Osgood material model up to the ultimate stress. However, there is no consensus on the ductile fracture model under monotonic loading, which affects the post‐necking stress‐strain behaviour, to be adopted for stainless steels, even though the fracture model may have clear applications in structural design, such as in predicting joint failure in direct design approaches. This study presents the calibration of the fracture parameters for two of the most common ductile fracture models: the modified critical stress model and the Lee and Wierzbicki model. The calibrations are based on experimental results from tensile coupons tests on austenitic, ferritic and duplex stainless steel, extracted from cold‐formed tubes and sheet material, and on advanced finite element simulations. The calibrated values show a low dispersion for each of the analysed materials, indicating that the fracture parameters are an inherent property of the material, and which have been found to be slightly higher for the more ductile alloys. Average values of the fracture parameters for different stainless steels are also preliminarily proposed for their direct application in finite element simulations.

Current design standards for stainless steel such as ASCE 8-02 and EN 1993-1-4 prescribe provisions for the design of cross-sections and members that account for material nonlinearities and strain hardening, although these features are not considered in the global design of structures. Recent studies have highlighted the need of accounting for material nonlinearities in order to design efficient and safe stainless steel structures, and it is expected that the forthcoming versions of the standards will incorporate updated rules for the global design of these structures. To contribute to this field, this paper presents a Stiffness Reduction Method (SRM) for the in plane design of stainless steel members and frames with stocky sections based on the prescriptions given in the next version of EN 1993-1-4. The proposed approach predicts the ultimate capacity and internal forces in stainless steel structures by performing a second-order elastic analysis in which the stiffnesses of the members are reduced by a set of factors defined in this paper to account for the effect of the spread of plasticity, residual stresses and member imperfections. The accuracy of the presented method is assessed for individual stainless steel structural members (columns, beams, and beam-columns) with different cross-sections and material properties, and for austenitic stainless steel portal frames, against numerical results obtained from nonlinear analyses conducted on finite element models. A comparison between the proposed approach and the Direct Analysis Method prescribed in the upcoming AISC 370 Specification is also provided, showing that the results are comparable in the two approaches.

Steel structures can be consistently and efficiently designed using system-based design-by-analysis approaches such as the Direct Design Method. However, since direct design approaches lead to potentially lighter structural configurations, they can also result in larger deformations under service loads. Thus, greater attention may be required to serviceability limit states in structures designed using design-by-analysis approaches than for structures designed elastically at their ultimate limit state following current two-stage approaches, especially for materials showing highly nonlinear stress vs strain responses such as stainless steel alloys. With the aim of investigating the influence of allowing larger deformations in the ultimate limit state design of stainless steel structures, this paper presents an explicit analysis framework for assessing serviceability reliability at system level. Using this framework, the paper investigates the serviceability reliability of cold-formed stainless steel portal frames designed using the Direct Design Method for different load cases, including the gravity load and the combined gravity plus wind load combinations. The study considers six baseline frames covering the most common stainless steel families and international design frameworks (i.e., Eurocode, US and Australian frameworks), for which the reliability of vertical deflection and lateral drift serviceability limit states is investigated using advanced numerical simulations and First-Order Reliability Methods. From the comparison of the calculated average annual reliability indices and the relevant target reliabilities for the different design frameworks, it was found that the reliability of stainless steel frames appears to be adequate for the serviceability limit states investigated for the Eurocode, US and Australian frameworks.

Current structural codes for steel and stainless steel structures such as AISC 360-16, AISC 370-21, AS/NZS 4100 and Eurocode 3 are based on the traditional two-step member-based design approach, in which internal actions are first obtained from a structural analysis, usually elastic, and the strength of each member and connection is subsequently checked using a structural design standard. However, the most recent versions of these standards already incorporate preliminary versions of the direct, or one-step, system-based design alternative, which is based on the design-by-analysis concept and allows evaluating the strength of structures directly from numerical simulations, although the standards in their current form do not provide reliability requirements for structural systems. Therefore, it is necessary to build a rigorous structural reliability framework to investigate acceptable target reliability indices for structural systems and to provide adequate system safety factors and system resistance factors. While this framework has been developed based on advanced Finite Element analysis for hot-rolled and cold-formed carbon steel structures in recent years in the form of the Direct Design Method (DDM), the framework does not exist for stainless steel structures. This paper presents an extension of the DDM to the analysis of stainless steel structures, in which system reliability calibrations are presented for six stainless steel portal frames under gravity loads covering the three most common stainless steel families and different failure modes using advanced numerical simulations. From the derived reliability calibrations, suitable system safety factors and system resistance factors are proposed for the direct design of stainless steel frames in the European, US and Australian design frameworks under gravity loads.

Data was generated using the general purpose finite element software ABAQUS and performing advanced nonlinear analyses. The database is comprised of vertical and lateral system stiffness values corresponding to different random samples of six different nominal stainless steel frames under gravity and gravity plus wind load combinations. The values of the random variable assignments are given for each case. The full details of the finite element model can be found in: Arrayago, I.; Rasmussen, K.J.R. Reliability of stainless steel frames designed using the Direct Design Method in serviceability limit states. Journal of Constructional Steel Research 196, 107425, 2022. DOI: https://doi.org/10.1016/j.jcsr.2022.107425 The data included in the dataset corresponds to the vertical & lateral stiffness of each frame under different load conditions. Although the data has been generated using the finite element software ABAQUS, no special software is required to read or interpret the data.

Stainless steel is a structural material with great potential in seismic design due to its ductile and strain hardening characteristics. However, no specific seismic design provisions exist for stainless steel, despite the remarkable differences with carbon steel. Moreover, research on the behaviour of stainless steel under cyclic conditions is scarce, not allowing designers to rely on accurate capacity models able to catch the actual member ductility under seismic excitation. Hence, the aim of this paper is to fill this lack and to provide simple provisions to estimate the total and stable part of the rotation capacity of stainless steel members with rectangular hollow section (RHS). A numerical study on 120 austenitic, ferritic and duplex beams under cyclic loading was performed following the ANSI/AISC 341 loading protocol. From the resulting skeleton curves, information on the ultimate strength and ductility (total and stable parts of the rotation capacity) were extracted. It was found that the ultimate bending moment resistance of RHS stainless steel beams under cyclic loading is accurately predicted by the Continuous Strength Method moment capacity, and that the rotation capacities can be related to the local slenderness by power functions calibrated from the numerical results. Finally, a tri-linear model that describes the full moment-rotation curves of stainless steel beams under cyclic loading is proposed using the calibrated equations, showing a good agreement with numerical moment-rotation curves, and which can be implemented in design software to define the behaviour of concentrated plasticity hinges.

Stainless steel is an excellent construction material due to its high ductility, strain hardening, durability and aesthetic characteristics. To date, most studies on stainless steel have been devoted to understanding its mechanical properties and the behaviour of individual structural members and simple structures under monotonic loading. As a result, next versions of stainless steel codes, traditionally based on carbon steel codes, will enable efficient structural designs under static forces. However, advances related to the performance of stainless steel structural members under cyclic forces are still scarce, and there are no specific rules for the seismic design of stainless steel structures in Eurocode 8 in spite of the notable differences between this material and other steels. On this basis, an experimental programme on austenitic stainless steel hollow section elements subjected to cyclic loading has been recently conducted. A total of nine specimens with different local and member slenderness values were tested under cyclic bending around their major axis following a cantilever loading scheme. This paper describes the experimental set-up adopted for the tests, including the loading scheme and instrumentation, and discusses the load–displacement, moment–rotation, degradation of stiffness and energy dissipation values obtained in detail. In addition to providing fundamental information on the response of stainless steel members under cyclic loading, the description of the set-up reported in this paper will assist researchers in planning similar experimental programmes, while the results will serve as a reference to validate numerical analyses of stainless steel members and frames subjected to cyclic loading.