This study investigates the ductile fracture behaviour of coupon-like specimens produced by Wire Arc Additive Manufacturing (WAAM) using AM70 high-strength low-alloy steel wire. Experimental testing under uniaxial tension and shear loading was conducted, supported by digital image correlation to capture strain fields. The material's plastic response was calibrated using a combination of true stress-strain conversion, a weighted average model for post-necking behaviour, and finite element simulations. Ductile damage was implemented numerically to simulate fracture, showing good agreement with experimental results in both failure mode and stress-strain response. The AM70 WAAM material exhibited a fracture strain of 0.65 in uniaxial tension and 0.70 in shear, indicating enhanced ductility compared to high-strength steels and previously reported WAAM references. Despite a moderate reduction in yield strength the material showed promising mechanical performance, particularly under shear-dominated conditions. These findings suggest the suitability of AM70 WAAM components for applications requiring high deformation capacity. The validated modelling approach offers a robust basis for predicting failure, and future work will explore the application of advanced fracture models for improved accuracy across diverse loading conditions.

This paper presents the development and evaluation of a novel plug-and-play beam-to-column connection for steel structures, designed to facilitate rapid on-site assembly, enable structural reuse, and reduce material usage. The connection concept was refined through an iterative design process focused on minimizing material consumption while maintaining structural performance. Finite element analysis was employed at each iteration to evaluate the joint's stiffness and moment resistance, with validation based on experimental data. The final plug-and-play connection was compared to a conventional single extended end-plate bolted connection using equivalent beam and column sections. Results show that the plug-and-play joint achieves a 26.1% reduction in material use while offering higher stiffness and moment capacity. Yielding behavior differs between the two connections, with the plug-and-play joint developing plasticity at higher loads, thereby extending potential for reuse. Classified as semi-rigid and partial strength, the connection supports moment-resisting frame applications. The proposed design demonstrates a viable and efficient alternative to traditional bolted joints, with opportunities for further optimization and expanded applications.

The Ayrton–Perry approach is the basis of Eurocode 3 rules for buckling resistance, with distinct curves and imperfection factors depending on section type, steel grade, and other parameters. For generic members – built-up or not, uniform or not, with complex supports or not – the code allows either the general method (clause 8.3.4 of Eurocode 3) or advanced numerical simulations. Yet, the general method shows wide scatter and often underestimates resistance, while numerical analyses are time-consuming and strongly user-dependent. To overcome these limitations, the general formulation was introduced by Tankova et al. (2018) for members with variable geometry, loads, and supports and recently extended to mono-symmetric I-beams. However, it has only been validated for Class 1 and 2 sections, not for Class 4. This paper extends the general formulation to uniform and non-uniform slender I-section beams subjected to arbitrary loads, boundary conditions, and partial lateral restraints. An advanced numerical model, calibrated against experimental data, was employed to conduct a comprehensive parametric study considering different cross-sections, a range of normalized slenderness values, S460 and S690 steel grades, and different load applications. The proposal was compared with the numerical results, demonstrating a safe-sided and well-calibrated solution for the buckling resistance of high-strength steel slender I-section beams.

In the structural stability design of steel structures, the effects of imperfections arising from steel members' fabrication must be adequately accounted for. In the case of welded members, residual stresses have a substantial impact on the structural performance due to the high thermal energy input during production and subsequent non-uniform cooling. In simplified design methods for steel beams, these imperfections are taken into account by assigning the members to buckling curves and the associated imperfection factors. A review of simplified design methods and their structural efficiency for welded members is timely, given the latest results on residual stresses. This paper presents a study on the lateral torsional buckling behaviour of welded steel beams and the subsequent consequences of the findings for simplified design methods. A comprehensive numerical study analysed the load-bearing behaviour of steel beams made of S235 to S690. Residual stresses were taken into account using the residual stress approach according to current standards, as well as a state-of-the-art approach. The numerical study confirmed that residual stresses have a lesser influence on the structural performance of members made from higher steel grades. To take the identified effects into account for structural designs, an adjustment of the imperfection factors is proposed in this paper.

This study presents a comprehensive characterisation of infill strategies in Wire Arc Additive Manufacturing for the fabrication of thick-walled steel components using ER100S-G wire. The primary objective of this work is to systematically assess how different combinations of edge and infill deposition strategies influence the thermal behaviour, defect formation, surface quality, and resulting microstructural and mechanical properties of solid WAAM components. Eleven deposition strategies, combining edge and infill parameters under two heat input configurations, were systematically evaluated. Thermal analysis based on Δt₈₋₅ cooling times revealed significant heat accumulation in corner regions and extended deposition paths, with cooling times increasing by up to approximately threefold depending on the deposition strategy and location. Microstructural characterisation identified acicular ferrite and bainite with refined grains in faster cooling zones. Electron Backscatter Diffraction confirmed grain growth and local misorientation reduction with increased heat input, alongside a transition from low-angle to high-angle grain boundaries, indicating partial recrystallisation and microstructural recovery. Laser profilometry showed that surface height variation remained below 1 mm in all samples, yet concentric infill strategies and non-weaving conditions resulted in the most irregular surfaces, affecting dimensional precision and post-processing requirements. Defects such as pores, lack of fusion, and overlaps were more frequent in low-energy or non-weaving strategies due to poor material distribution. Finally, hardness measurements confirmed that faster cooling rates (Δt₈₋₅ < 12 s) led to higher hardness values, reinforcing the relationship between thermal history and mechanical response. Overall, the results demonstrate that infill strategy selection plays a critical role in balancing productivity, thermal stability, surface quality, and structural integrity in WAAM-fabricated solid components. These findings offer valuable insights into the process–structure–property relationships in WAAM, providing practical guidance for optimising the production of defect-minimised and structurally consistent solid steel components.

The General Formulation has proven to be a practical, design-focused solution for the stability design of different member configurations - built-up or not, uniform or non-uniform, with complex or simple support conditions. It was recently extended for mono-symmetric I-section beams, but its applicability to slender section members remains unexplored. This work aims to expand the scope of the General Formulation to include Class 4 I-section beams. A numerical model was calibrated to assess the lateral-torsional buckling resistance of beams. A parametric study was conducted on S690 HSS slender I-section beams under uniform bending moment, considering different cross-sections, normalized slenderness, prismatic and non-prismatic beams, simply-supported and arbitrary boundary conditions. Finally, the numerical results were compared to the analytical ones of the General Formulation and Eurocode 3, supporting the applicability of the General Formulation due to its good correlation with the numerical model.

Additive manufacturing (AM) rapidly expands to all research areas due to its multiple advantages, such as the freedom and flexibility in achieving any geometry. Using AM as a fabrication technique, the design process has almost no limitations, blending considerably well with the irregular geometries that may result from topology optimization. Yet, in practice the application of AM together with topologically optimized geometries is not as straightforward. In this research, a hollow square t-joint is used as a case study to investigate and understand the difficulties in the design and manufacturing of steel parts using wire arc additive manufacturing (WAAM). The case study showcases from the application of two optimization methods with various parameters to find an optimal geometry; numerical analysis (FEM) on non-optimized and optimized models; a procedure called “re-engineering” that adjusts the optimized geometry to structure efficiency and AM effectiveness; dynamic slicing and path planning; an adjustment of the welding parameters to enhance the material properties and accuracy of the final specimens; and experimental assessments on non-optimized and optimized printed t-joints to validate the entire process. The application of this process allowed the manufacture of a complex optimized geometry, which have more resistance than the non-optimized T-joint.

Bolted flange connections in wind turbine towers are subjected to cyclic loading, making fatigue a critical concern for their structural integrity. Bolt preload helps mitigate fatigue damage, but actual preload levels often deviate from design values due to uncertainties in the tightening process and geometric imperfections. This study evaluates the fatigue life of bolts L-flange connections under varying preload levels using a numerical fracture mechanics approach. A comprehensive three-dimensional finite element analysis (FEA) is conducted to assess the effects of preload on the stress intensity factor (SIF), crack propagation behaviour, and load transfer function (LTF). Additionally, the influence of thread helix angle, as well as combined axial and bending loads, on SIF and crack front evolution is examined. Experimental validation of the numerically obtained LTF is performed. A methodology for predicting S-N curves is proposed by deriving normalised solutions for LTF and SIF. The results indicate that increasing preload up to 90 % significantly reduces the SIF range, thereby decelerating crack growth and enhancing fatigue life. However, beyond 90 %, the improvement in fatigue life becomes less pronounced. Furthermore, the findings suggest that Eurocode 3 provides conservative fatigue life predictions, as it neglects bending effects, which are less detrimental than axial loading. Notably, even minor preload loss considerably shortens fatigue life, an effect that becomes more pronounced at higher preload levels. This research contributes to the development of predictive fatigue models for the bolted L-flange connection, providing insights into incorporating preload effects into fatigue life assessments.

The paper addresses the fatigue crack growth behaviour of untreated and heat-treated WAAM ER70S-6 carbon steel. Specimens were extracted from the printed wall along different directions (vertical and horizontal) and tested under mode-I loading at two stress ratios (R=0.05 and R=0.25). Crack closure was measured using Digital Image Correlation (DIC). The microstructure of the untreated material mainly consisted of polygonal ferrite and intergranular lamellar pearlite. After heat treatment, pearlite decomposed, allowing ferrite to grow and reducing hardness. The load ratio influenced fatigue crack growth rates due to variations in crack closure levels. However, the loading direction relative to the print layer orientation did not significantly affect the crack growth rate. Fracture surfaces were examined by scanning electron microscopy to identify the main fatigue crack growth mechanisms associated with the different loading orientations and material conditions. Fractographic analysis revealed a mixed fracture mechanism, characterised by cleavage in the harder pearlite-rich regions and fatigue crack propagation striations in the softer ferrite-dominant areas. Minor manufacturing defects, such as inclusions and porosity, were also observed. The tested WAAM carbon steel exhibited slightly lower performance than conventional steels of a similar grade, aligning closely with the existing literature for WAAM ER70S-6 carbon steel.

Robotic welding and additive manufacturing (AM) processes have an intricate design space influenced by numerous configurable process parameters. Currently, the precise impact of each parameter or a combination of them on the variability and dimensions of deposited material is unclear due to the stochastic nature of the process, which is affected by factors like arc stability, temperature gradients and other in-process changes. In AM and various cases of welding like cladding, quantifying these variations is necessary for developing path planning strategies that produce components without defects. This study presents a framework that automates process data collection and scanning of the weld bead and analysis of the point cloud, based on the design of experiments principals towards building representative machine learning models. In comparison to alternative approaches, this framework incorporates spatial variation along the deposited length by utilising location-based binning of measurements, thereby enabling more detailed analysis of various deposition stages including arc ignition and extinction regions. The framework is tested with single pass bead-on-plate weld beads deposited with different process parameters followed by spatial–temporal matching. Variations were noted in relation to travel speed and welding current when subjected to identical heat input values. Machine learning models for prediction of height and width account for non-linearities and are validated with additional experimental data. These models have demonstrated a high degree of accuracy in predicting in-process variations within the deposited material.

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.

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.

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.

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.

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.

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.

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.

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 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.