This paper aims to assess the influence of geometrical imperfections and residual stresses on the reliability of the stability design rules for steel columns in Eurocode 3 considering a full probabilistic approach and further validate the new buckling curves in the scope of the ongoing revision of the Structural Eurocodes. A reliability assessment of major- and minor-axis flexural buckling of high-strength steel (HSS) welded I-section columns was performed, considering all basic variables as random, including the geometrical and material imperfections, in addition to the material properties of steel and the geometry of the cross-section. An advanced finite element model calibrated with experimental test results is used to perform a very large (290,126 simulations) parametric study covering the majority of practical geometries. Subsequently, Monte Carlo simulation is used to estimate the design values of the buckling resistance that correspond to the target probability of failure of the Eurocodes. Finally, these values are compared to the proposed buckling curves for HSS columns, showing good agreement and supporting their adoption in the revised EN 1993–1-1. It is also concluded that it is on the safe side to carry out a reliability assessment with deterministic reference values for structural imperfections.

With the growing demand for sustainable, cost-effective and overall-efficient building solutions, the need for dependable modular construction systems is steadily on the rise. In the present paper a novel hybrid modular construction system named INNO3DJOINTS is introduced, employing cold-formed welded steel tubular columns, fabricated according to EN 10219, and cold-formed steel thin-wall section based truss-girders, joined by the innovative plug-and-play (P&P) connector, designed to provide ease-of-assembly and -disassembly. The experimental investigation conducted on isolated sub-frame configurations of the novel system is presented, where 6 full-scale specimens were subjected to horizontal and vertical loading. The test configurations differed in the P&P joint socket thickness and the absence/presence of the light steel framing (LSF) wall, encased with oriented strand board (OSB). In addition, a numerical model for predicting the system's global behaviour is proposed, developed in SAP2000. Initially, the behaviour of the employed P&P joint configurations, categorized as partial-strength, is characterized using experimental and validated ABAQUS finite element model (FEM) data, resulting in a spring model implemented into the global FEM. Finally, the numerical and experimental results are compared and discussed, leading to conclusions regarding the system's 2D structural performance, identified behaviour governing phenomena, P&P joint influence, LSF wall and OSB contribution, as well as the capabilities of the developed FEM.

This paper deals with the face plate component in steel joints and addresses the full characterization of the nonlinear behaviour of the face plate component. An equivalent beam strip model is proposed that tackles the connection of a beam to the column web of open I-sections in a minor axis joint or the face of tubular columns, covering endplate or fin plate joint typologies. Closed-form analytical solutions are obtained for the elastic large displacement and the elastic-plastic large displacement behaviour of the equivalent beam strip. Criteria for the establishment of the design resistance using the continuous strength method are also proposed. It was concluded that the model is easy to apply, was validated against a large parametric study using finite element beam models, leads to accurate solutions and demonstrates the need to consider membrane effects in design.

This paper presents the development, implementation, and validation of a macro-element suitable for the linear analysis of innovative 3D plug-and-play joints between tubular columns and lightweight steel truss-girders. The macro-element is based on the component method, accounts for the three-dimensional interaction between the tube faces, and its components have a clear physical meaning. Simplified procedures are developed for the closed-form computation of the stiffness matrix of the macro-element based on the geometric and mechanical properties of the nodal zone. This facilitates practical application in everyday design scenarios. Furthermore, the macro-element's architecture is implemented in the framework of OpenSees as a standalone beam-to-column joint finite element. Validation of the conceptual design is accomplished through parametric studies, comparing its performance with models generated in higher-order finite element commercial software, Abaqus. This research offers a valuable resource for the linear analysis and design of innovative 3D plug-and-play joint connections in structural engineering, enhancing efficiency and reliability in construction practices.

Wire Arc Additive Manufacturing (WAAM) has gained popularity due to its speed and cost-effectiveness. However, the current knowledge on the cyclic deformation behaviour of WAAM materials is limited. To address this issue, this study investigates the cyclic deformation behaviour of WAAM ER70S-6 carbon steel. Coupons were extracted from printed walls with horizontal and vertical orientations and from surface and interior locations. Low-cycle fatigue tests were performed under fully reversed strain-controlled conditions, spanning strain amplitudes from 0.20 % to 1.50 %. Regardless of the group, the material exhibited cyclic hardening for strain amplitudes above 0.60 % and cyclic softening for smaller strain amplitudes. Significant non-Masing hysteretic behaviour was also observed. Cyclic stress–strain curves and fatigue-life relationships written in terms of stress, strain and energy-based parameters were derived for all groups. Fatigue life was not significantly influenced by printing orientation or location across thickness. However, horizontal orientation resulted in a slightly superior fatigue response. A statistical analysis revealed no significant difference in the accuracy of fatigue-life relationships between the groups. Finally, a comparative study between WAAM materials and conventional steels showed promising results. Yet, conventional steels outperform WAAM carbon steel, at least doubling the fatigue life for the same value of the damage parameter.

The use of 3D printed stainless steel requires a deep knowledge of its mechanical properties. This paper presents material characterisation of 316LSi austenitic stainless-steel coupons manufactured by CMT-WAAM, considering different deposition directions. The specimens were tested according to ISO 6892-1, the fractures surfaces were examined by SEM for machined and as-built conditions. The material was subject to hardness test and deep microstructural analyses, to assess the anisotropy in material properties at the micro and macro scales, respectively. A thermal analysis performed by infrared thermography of the material deposition in CMT-WAAM was also performed to establish the influence of the temperature evolution (versus time and position) on the microstructural and mechanical properties of the deposited walls. Finally, a statistical assessment was carried out, including results available in the literature and a material model available in the literature was adjusted to the test results, enabling to conclude that it is possible of accurately reproducing the uniaxial stress-strain behaviour, therefore providing a necessary input for the design of steel structures with 3D printed stainless steel.

The lateral-torsional resistance of prismatic double-symmetric I-section beams is accurately predicted using a mechanically consistent Ayrton-Perry approach, combined with a calibrated generalized imperfection. The corresponding design formulation was recently adopted in the revised version of Eurocode 3. However, for prismatic mono-symmetric I-section beams, the General Case shall be used while for non-prismatic beams only the General Method is available. Both methods present a very large scatter and highly underestimate the lateral-torsional buckling resistance. This paper proposes an extension to the General Formulation for non-prismatic beams with arbitrary boundary conditions, partial lateral restraints, and arbitrary loading for mono-symmetric I-sections. Using an advanced numerical model calibrated with experimental test results, a large parametric study is undertaken, and its results are used to assess the available design methodologies and the proposed method. It is concluded that the General Formulation provides excellent safe-sided estimates of the LTB resistance, and it is confirmed the very poor performance of the General Case and the General Method.

The design of pin connections between steel members has been established for many years in design codes. However, recently, in the scope of the revision of Eurocode 3, Part 1–8 (EN 1993–1-8), questions were raised concerning the safety of the corresponding design verifications. This paper identifies two main aspects that require revision, namely: (i) the possibility to design a pin as a bolt in shear and (ii) the verification of the resistance of the pin itself. Based on a thorough literature review, experimental tests and a parametric study, a new proposal submitted to CEN as an amendment to the code, is presented to solve these two identified issues.

Tubular structures are rather efficient systems due to the ease of fabrication, erection, and the high strength-to-weight ratio of their elements. However, they have seen limited use due to difficulties associated with the design and execution of joints. This scenario has changed over the years as more research focused on the behavior of these structural elements, leading to the appearance of design recommendations for some tubular joint configurations. Recently, a new plug-and-play joint configuration suitable for hybrid modular construction systems, comprising tubular columns, lightweight cold-formed steel truss-girders and cross-laminated timber slabs, has been developed in the scope of the INNO3DJOINTS project (RFCS No. 749959). The mechanical behavior of the structural system was experimentally analyzed, and macro-element models suitable for global structural analysis, explicitly including the joint behavior, were developed. In tubular structures, the out-of-plane behavior of the column’s faces is influenced by the condition of each individual face and the three-dimensional interaction between them. Currently, there is no universally accepted modelling approach for this interaction under various loading conditions. This study addresses this issue and presents a general three-dimensional macro-model for the beam-to-column joints based on the component method and accounting for the interaction between the faces of the tubular column. The architecture of the macro-element of the joint is implemented in a finite element object-oriented software framework, OpenSees, as a standalone joint finite element, and its robustness is verified by using high-order finite element commercial software.

The component method for joint analysis relies on the formulation of stiffness and strength of individual parts to derive the global properties of the joint. One of these components is the web of an open I-section, or the face of a rectangular hollow section, hereby referred to as face plate. It is currently not codified despite its frequent occurrence in the engineering practice. Hereby, a new mechanical model is proposed to estimate the initial stiffness of the face plate component under out-of-plane loading, leading to closed-form analytical expressions. The model is validated against experimental test results and an extensive numerical parametric study, showing excellent agreement.

The use of high strength steel (HSS) has been consolidated around the world because of its numerous advantages mostly for high-rise buildings and large span bridges. As it provides the possibility of a design with slender sections, substantial reduction in the structure weight is expected, but special attention should be given to the stability of the HSS compression members. However, the existing design codes for HSS columns are limited and based on experimental data related to normal strength steel (NSS) materials, which is responsible for an inaccurate resistance prediction. In this sense, the present paper aimed to investigate the buckling behavior of S690 HSS welded I-section columns failing by flexural buckling around the both principal axes and beam-columns failing by lateral torsional buckling. The experimental campaign contemplated 4 pin-ended columns, in which 2 were tested about their major-axis and the other 2 about the minor-axis, and also 2 beam-columns, including supplementary experimental to measure the geometric imperfections, residual stresses and material properties of the S690 welded I-section columns and beam-columns. In the following, a numerical model was built and validated against the experimental results and afterwards employed to perform a sensitivity study considering different membrane residual stress patterns. Finally, the European [1,2], American [3], and Australian [4] codes were assessed by comparing the design resistances with the results from the experimental tests and the collected test data of relevant studies, showing in general a considerable level of conservatism.

High strength steels (HSS) are becoming more common in engineering practice due to their improved qualities. They are standardized by the specific parts of product standard EN10025 and soon they will be also codified in the execution standard EN1090. Regarding design using HSS, EN 1993−1−1 gives stability design rules for columns, beams and beam–columns up to S460, whereas EN 1993−1−12 gives additional guidance for S500 up to S700 (based mainly on numerical work available at the time). Existing studies on flexural buckling of welded H, I and box columns in steel grades S460 to S960, even though limited, show that improved curves can be used for members in high strength steel (HSS). Recently, within the European Project STROBE, evaluation of the European stability design rules was carried out covering columns, beams, and beam–columns. The research was based on experimental programme covering 20 full-scale tests, residual stress measurements, advanced numerical models, analytical derivations, and statistical evaluation. Finally, it was possible to justify new, more accurate recommendations for the buckling curve selection for HSS members. This paper provides a summary of the project conclusions regarding the stability design of steel members in high strength steel.

Steel tubular members are efficient and aesthetic structural solution, yet the bolted connections between then are challenging due to the restricted access to the inside of the tube. There are several solutions, which try to overcome this difficulty, by adoption of special fasteners and/or additional connecting elements. This paper presents the experimental and numerical characterization of a novel plug-and-play joint that was developed as a demountable solution to connect tubular steel columns to cold-formed steel trusses. The research program comprised a large number of experimental tests on components and the full-scale joints, complemented by numerical models calibrated with the experimental tests. These calibrated numerical models were subsequently used to perform a large parametric study on the components and joint that allowed to propose guidelines on the geometries that maximise the performance of these joints.

Purpose: Additive manufacturing (AM) is a rapidly evolving manufacturing process, which refers to a set of technologies that add materials layer-by-layer to create functional components. AM technologies have received an enormous attention from both academia and industry, and they are being successfully used in various applications, such as rapid prototyping, tooling, direct manufacturing and repair, among others. AM does not necessarily imply building parts, as it also refers to innovation in materials, system and part designs, novel combination of properties and interplay between systems and materials. The most exciting features of AM are related to the development of radically new systems and materials that can be used in advanced products with the aim of reducing costs, manufacturing difficulties, weight, waste and energy consumption. It is essential to develop an advanced production system that assists the user through the process, from the computer-aided design model to functional components. The challenges faced in the research and development and operational phase of producing those parts include requiring the capacity to simulate and observe the building process and, more importantly, being able to introduce the production changes in a real-time fashion. This paper aims to review the role of robotics in various AM technologies to underline its importance, followed by an introduction of a novel and intelligent system for directed energy deposition (DED) technology. Design/methodology/approach: AM presents intrinsic advantages when compared to the conventional processes. Nevertheless, its industrial integration remains as a challenge due to equipment and process complexities. DED technologies are among the most sophisticated concepts that have the potential of transforming the current material processing practices. Findings: The objective of this paper is identifying the fundamental features of an intelligent DED platform, capable of handling the science and operational aspects of the advanced AM applications. Consequently, we introduce and discuss a novel robotic AM system, designed for processing metals and alloys such as aluminium alloys, high-strength steels, stainless steels, titanium alloys, magnesium alloys, nickel-based superalloys and other metallic alloys for various applications. A few demonstrators are presented and briefly discussed, to present the usefulness of the introduced system and underlying concept. The main design objective of the presented intelligent robotic AM system is to implement a design-and-produce strategy. This means that the system should allow the user to focus on the knowledge-based tasks, e.g. the tasks of designing the part, material selection, simulating the deposition process and anticipating the metallurgical properties of the final part, as the rest would be handled automatically. Research limitations/implications: This paper reviews a few AM technologies, where robotics is a central part of the process, such as vat photopolymerization, material jetting, binder jetting, material extrusion, powder bed fusion, DED and sheet lamination. This paper aims to influence the development of robot-based AM systems for industrial applications such as part production, automotive, medical, aerospace and defence sectors. Originality/value: The presented intelligent system is an original development that is designed and built by the co-authors J. Norberto Pires, Amin S. Azar and Trayana Tankova.

Residual stresses resulting from the fabrication of welded steel beams have a significant influence on the stability behaviour of steel members. To consider them in advanced numerical simulations, the models according to ECCS publication no. 33 or prEN1993–1-14 are commonly used. While the ECCS model was developed for steel grades S235 and S355, this model was also adapted in prEN1993–1-14 for the application to high-strength steels and scaled with the yield strength of the material. However, numerous investigations show that these models do not adequately represent existing residual stresses. This paper presents experimental studies of residual stresses conducted at the University of Coimbra and Ruhr-Universität Bochum. Based on these measurements, the influencing parameters on residual stresses were identified and subsequently a novel model for the representation of residual stresses was developed. The model was validated with an extensive data set from residual stress measurements from the last 50 years considering steel grades S235 to S890. Finally, numerical stability analyses were executed to investigate the effects of different residual stress approaches on the load-bearing behaviour of steel columns and beams.

This paper analyses the microstructural and mechanical properties of carbon steel coupons produced by CMT-WAAM. The strategy adopted in the fabrication of the test specimens using a robotised facility is explained. Then, the results of the mechanical characterization performed using as-built and machined samples, extracted in several directions (0°, 45°, 90°) relative to the material deposition trajectory, are analysed. The yield and ultimate tensile strengths were determined by performing tensile tests and the Young's modulus was determined using ultra-micro hardness measurements. A deep microstructural characterization was also performed by optical microscopy for establishing a direct relationship between the manufacturing procedures and the registered mechanical properties. The failure micro-mechanisms associated with the building orientation and the surface condition was also examined by scanning electron microscopy. It was found out that the additive manufactured material has isotropic tensile properties, which result from the formation of an annealed microstructure upon cooling from the successive CMT-WAAM thermal cycles. The machined specimens exhibit higher strength and ductility than the as-built ones. The fracture surfaces of both machined and as-built coupons showed ductile failure. The results of the uniaxial tensile tests indicate that the machined and as-built WAAM steel walls satisfy the requirements of a structural steel grade as specified by Eurocode 3.

Residual stresses have considerable effect on both the local and the member stability behaviour of steel structures. Previous studies have shown that the structural stability behaviour depends on both the distribution and the amplitude of the residual stresses. The residual stress distribution is affected by the cross-section geometry, the steel grade and the manufacturing process, e.g. flame cutting and the welding procedure. A realistic consideration of residual stresses is necessary for an efficient and safe design of steel structures. This article presents an experimental and numerical study on the influence of residual stresses on the stability behaviour of structural members, i.e. steel columns and beams considering a variety of cross-sectional slenderness, i.e. plastic, compact and slender cross-sections are taken into account. A novel residual stress model for welded I-shaped sections was developed and evaluated using test data. This model takes into account the main influencing parameters, i.e. the cross-sections geometry, the steel grade as well as manufacturing process with thermal cuts or non-thermal cuts of the steel plates. The novel residual stress model is then used to investigate the stability behaviour in terms of a numerical simulation study. The result of the study allows to propose an improvement of the buckling curve selection for welded high strength steel columns and beams.

EN 1993-1-1 gives stability design rules for columns, beams and beam–columns up to S460, whereas EN 1993-1-12 gives additional guidance for S500 up to S700. Recent studies show that high strength steel members may be designed using improved buckling curves, where the enhanced behaviour is usually attributed to the improved material properties but mainly due to the more favourable residual stress distribution. The behaviour of unrestrained beams in HSS has not been widely studied. At present in EN 1993-1-1, the design rules for lateral-torsional buckling of beams are not dependent on the steel grade, meaning that the code does not distinguish between beams in conventional strength steel or HSS. In pursuit of an answer to the mentioned shortcomings, the present research is based on the experimental programme covering 12 full-scale tests, residual stress measurements, advanced numerical models and analytical derivations. The experiments cover different steel grades up to S690, welded and hot-rolled sections, homogeneous and hybrid (flanges in HSS and web in mild steel), double and mono-symmetric sections as well as variations in the cross-section class. This paper provides an overview of the experimental programme, discusses the results for lateral-torsional buckling of beams, and presents an advanced numerical model that was calibrated to the experimental results including the measured residual stress distribution and geometrical properties of the members. The numerical model was explored to assess various assumptions for the member imperfections, and these are further compared with code recommendations.

In recent years, modular construction has been used in low-to medium rise multi-story and even high-rise buildings. Pre-fabrication by off-site manufacture leads to faster and safer construction, improved quality, reduced resources and waste. The research activity summarized in this paper is carried out within the European project INNO3DJOINTS and it aims at developing innovative plug-and-play joints which enable modularity, faster construction and deconstruction. The developed modular construction system is hybrid, whereby tubular columns are combined with cold-formed lightweight steel profiles using plug-and-play connections to provide an efficient structural system. The influence of the type of floor system, the horizontal and vertical load-bearing systems and the type of truss on the structural performance are investigated. The results obtained from numerical analyses allowed optimizing the geometry of the examined hybrid modular system. To highlight its effectiveness and versatility, a case study building was used to compare the proposed hybrid system with other conventional structural solutions.

The authors regret the use of α_p = 0.7 at the end of paragraph “4.6. Critical assessment of the results”. Where it reads “α_p=0.7 and λ₀=0.7” it should be “α_p=0.34 and λ₀=0.7”. This correction does not affect in any degree the conclusions made in the paper. Moreover, the EN1993-1-5, Annex B curve in Fig. 13a) is correctly plotted, i.e. using α_p = 0.34 and λ₀ = 0.7. The authors would like to apologise for any inconvenience caused.