Abstract Within the framework of Connect4C, a European RFCS project, innovative joint solutions are developed to enhance the reusability of structural steel elements. Among the covered joint typologies, the present paper focuses on the latest developments related to the innovative column splice solutions. The latter are first presented. Afterwards, the method allowing for the characterization of the column splices under combined loading, namely combined bending ( M y + M z ) and axial load N , is described. Numerical simulations of the splices are then performed and compared with analytical predictions to validate the proposed approach. Finally, effects of combined loadings on the initial bending stiffness of the column splices are discussed.

Thin-walled cold-formed steel members are widely used in steel construction. However, members with cross-sections that are monosymmetric about their major axis exhibit characteristics that significantly influence their structural behavior. These open sections possess low torsional rigidity and are susceptible to local and distortional buckling, as well as lateral-torsional buckling when not laterally restrained and subjected to major-axis bending. To investigate this complex stability behavior and further enhance the knowledge of such members, an experimental program was conducted consisting of sixteen tests, comprising both 3-point and 4-point bending tests. The study included measurements of initial geometric imperfections and coupon tests in order to characterize the actual geometric and mechanical material properties. Finally, the member design resistance is assessed in accordance with the Eurocode provisions.

Driven by increasing environmental demands within the construction sector, steel-timber composite structures have gained considerable attention as sustainable load-bearing alternatives. Most existing solutions rely on dowel-type fasteners, which often induce premature timber damage during early loading stages, thereby limiting mechanical performance and disassembly and reuse potential. This study introduces a novel shear connection concept aimed at increasing the yield resistance of timber, thereby promoting disassembly and reuse by reducing damage during the loading phase. The experimental programme consisted of three distinct test series: (i) isolated local timber crushing tests to examine failure mechanisms and assess the performance of existing analytical formulations; (ii) push-out tests on two configurations of the proposed steel block connection; and (iii) push-out tests on conventional bolted connections for sake of comparison. The experimental results were subsequently reproduced using analytical formulations and detailed numerical models. Subsequently, a small parametric study was conducted to identify the critical components governing the connection response. The results demonstrate that the proposed steel block connection significantly improves yield resistance and elastic stiffness compared to conventional bolted systems. By enhancing the yield resistance and thereby delaying the onset of irreversible damage to higher load levels, the connection offers improved suitability for reuse and disassembly, which is fully aligned with contemporary circular construction objectives. The study also shows that, although Eurocode-based timber crushing models yield reasonable predictions for isolated crushing behaviour, more advanced formulations are required to capture the connection response with greater accuracy.

This paper presents the findings of an experimental and numerical study on the full-range behaviour of FREEDAM (FREE from DAMage) beam-to-column joints, with particular focus on their response under column loss scenarios. Two full-scale experimental tests were carried out to evaluate the joint performance under (i) bending moment and (ii) combined bending moment and axial force. The experimental results demonstrated that the sliding resistance and overall response are strongly influenced by the initial bolt preload and by preload loss occurring during the slippage phase, leading to an average reduction of approximately 30% in friction resistance. The tests also showed that the shear failure of the damper bolts governs the ultimate capacity of the joint after damper stroke exhaustion. A simplified component-based spring model was validated against the experimental response, proving capable of reproducing the joint behaviour with satisfactory accuracy when preload loss is properly accounted for. Advanced finite element models were further validated and used to investigate local joint mechanisms and to perform parametric studies on bolt preload effects. These results confirm the suitability of FREEDAM joints for robustness-oriented design and provide validated modelling tools for their implementation in global structural analyses.

The present work aims to numerically investigate the performance of 4-bolts T-stubs and the influence of geometrical parameters, such as the flange thickness, on both 4-bolts T-stubs capacity and ductility, which have not yet been investigated in the existing literature. Advanced numerical models were properly developed, and their effectiveness was verified against experimental tests performed on a 4-bolts T-stub that had previously been presented in the literature. Sixty 4-bolts T-stubs specimens were designed and modelled, featuring variations in flange thickness and width, the distance between the internal bolt and the web, the spacing between the bolts and the distance between the outer bolt and the edge of the plate. Moreover, the response of all the specimens investigated was compared against two analytical models that had already been presented in the literature. Finally, in order to better understand the influence of the additional external bolts on the overall performance of the T-stubs, the numerical results were compared to those obtained on traditional 2-bolts T-stubs properly introduced. The results demonstrate that the flange thickness exerts a significant influence on the T-stub capacity and ductility, while the influence of the other geometrical parameters, such as the flange width and bolt positioning, is determined by the T-stub failure mode. Finally, in function of the awaited failure mode, the use of 4-bolts T-stubs could allow an increase in specimen resistance and/or ductility with respect to the traditional 2-bolts T-stub.

This study examines the axial compressive performance of multi-material composite columns consisting of concrete-filled steel tubes with embedded CFRP-confined timber cores. A data-driven framework integrating theoretical model, finite element simulation and machine learning prediction is established to address the limited accuracy and scalability of conventional dual-material designs. An analytical bearing-capacity model is derived by accounting for steel confinement, CFRP hoop restraint, and timber orthotropy, of which results match FE results well with 5% deviations. Parametric investigations show that increasing steel yield strength and tube thickness would enhance the capacity of the composite columns, whereas CFRP confinement improves the post-crushing response and ductility of the timber core. The columns with circular cores exhibit better deformability than those with square ones. For axial bearing capacity prediction, a theory-residual-modified XGBoost model is proposed, in which theoretical estimates are corrected via SHAP-guided residual learning, achieving higher accuracy than single learners and ensemble baselines. A lightweight design tool is further developed for single/batch evaluation, automatic capacity-to-self-weight assessment, and interpretable prediction, enabling up to 22% self-weight reduction. The proposed methodology provides a validated and practical route for optimizing sustainable, lightweight multi-material composite columns.

The challenges the construction industry is facing to reduce its environmental impact and to efficiently use the available resources lead to an extensive search for sustainable construction alternatives. In the last decades, competitive engineered wood products (EWPs) appeared in the market. The synergy between EWPs and steel, in the so-called steel-timber composite (STC) form, can be an efficient and sustainable solution for the execution of building floors and decks. Various shear connections for STC have been developed. However, the majority relies in the dowel-type connectors, with limited mechanical performance strongly affected by the flexibility of the EWPs. Furthermore, the nonlinear and permanent deformations often developing in a low range of loading may hinder reuse of the materials and strongly limit the nondestructive disassembly at end of life. Thus, this paper presents the development of a new reversible shear connector for efficient STC floor beams meeting the needs of an optimize use of construction materials through reuse. First, a brief overview of existing shear connections for STC is given, identifying their potential and limitations. Then, the concept of the new proposed connection is exposed. Finally, a preliminary assessment through analytical calculations and numerical simulations is done, demonstrating the efficiency of the proposed shear connection towards a designed for disassembly STC composite beam.

Among the existing solutions to reduce material consumption in the construction sector, the steel market is continuously witnessing the emergence of new, ever-stronger steels. The recent improvements of the steelmaking process have indeed enabled to manufacture wide flange sections with improved material properties, with good toughness and weldability performance. The development of high-strength steels contributes to the optimisation of structural designs with the potential for substantial weight, cost and carbon savings. Hot-rolled sections with a yield strength of up to 500 MPa in Europe and 80 ksi (550 MPa) in the United States already exist and comply with the product standards for structural applications. Although research has shown an insignificant increase in carbon footprint with rising yield strength, demonstrating the environmental benefit of developing and using higher steel grades, their adoption remains quite marginal. This can be explained by a lack of information on existing high-performance products and the advantages they offer, as well as a lack of availability resulting from the current low demand for these grades. In addition, higher steel grades are often associated with increased unit material costs, reduced weldability and ductility, and a greater tendency to both local and global buckling instabilities. Consequently, designers are often reluctant to incorporate these innovative materials in their designs and manufacturers hesitant to develop them, particularly in the absence of clear guidelines for assessing their economic viability in specific structural applications. This paper consists of a comparative study on the use of the appropriate steel grade for members under pure compression. In particular, reference member slenderness ratios are established allowing the identification of the relevant field of application for the different considered grades.

The component method currently codified in Eurocode 3 Part 1‐8, enables the simplified analytical modelling of the joint behaviour by decomposing connections into basic components. However, this method shows limitations in accurately predicting the actual collapse mechanisms and the deformation capacity. Within this framework, this study addresses these limitations by focusing on the T‐stub component, proposing a unified and simplified methodology to evaluate its ultimate resistance and deformation capacity. Building upon recent advancements, a refined classification of failure modes is adopted, introducing a novel intermediate failure mechanism (MEC‐1.2) and incorporating a more accurate estimation of the contact force location. Closed‐form expressions for strength and ductility are derived for each failure mode, drawing from and extending existing models. These formulations are validated through an extensive parametric analysis involving 146 T‐stub configurations. Results demonstrate satisfactory agreement, with average predictive errors within 10% and Coefficients of Variation below 0.15. This work contributes with a practical and reliable tool to the evaluation of the T‐stub performance and lays the foundation for future developments toward joint modelling with the component method.

Cold-formed thin-walled steel members are extensively used in rack storage structures due to their advantageous geometrical and mechanical properties as well as their lightweight. In multi-deep rack systems, shuttles are usually used to bring pallets to their designated storage position by rolling over elements made of Z sections. Although fatigue problems could occur on these members due to the repeated multiple passages of the shuttles, this aspect is not fully covered by EN1993-1-9, the European normative document dedicated to the fatigue design of steel structures. This paper presents and discusses the outcomes of a large experimental fatigue test campaign, in which 34 members made of thin-walled cold-formed steel Z sections were tested, in order to characterise the fatigue resistance of two specific details as part of the member. The main objective of these tests was to derive the S-N curves associated to both studied details, in accordance with the recommendations of prEN1993-1-9; these curves could finally be used by the practitioners in their design process.

The Component Method (CM), as formulated in Eurocode 3, provides an efficient framework for characterizing the structural response of steel joints, serving as a viable alternative to complex finite element (FE) simulations. However, conventional CM-based models overlook key aspects such as group effects, component interactions, and the variation of shear force along the column web panel height, which are systematically integrated into the proposed enhanced finite element formulation. A major advancement of the proposed approach lies in the implementation of static condensation, which reduces the computational complexity of the model while maintaining its predictive accuracy. It ensures a more practical and computationally efficient tool for design engineers. The model’s validity is demonstrated through comparisons with experimental data and analytical solutions, confirming its compliance with Eurocode 3 and its applicability to various loading conditions.

Local damages to key structural members may originate from different causes, including localised fires or earthquakes, and, as far as the affected area remains small and the damage is contained, the risk is reduced. However, if the structure does not have the capacity to absorb the damage and bridge over the lost components, progressive collapse may be initiated, with serious consequences for the life of the occupants and the costs of losses. Even if some inherent structural properties, like redundancy and ductility, bring a beneficial contribution to the resistance of structures subjected to such scenarios through the activation of alternate load paths, these properties can be affected when working under elevated temperatures, and thus structural integrity can be at risk. This paper investigates the cumulative effects of seismic events and elevated temperature on the progressive collapse resistance of two-way frames with steel and composite steel-concrete floors. Numerical models are calibrated against relevant test data. The results show that, even if fire protection is an effective way in increasing the resistance of structural components under elevated temperatures, the failure may propagate due to the attainment of the bearing capacity of the surrounding elements and connections that are still at ambient temperature. Also, the interaction between concrete slabs and steel beams may provide additional capacity to stop the progressive collapse.

Cyclic tests and finite element simulations were carried out to investigate the performance of partial strength unstiffened end-plate joints that have been designed either for moment frames in low seismic areas or to provide redundancy, ductility, and extra dissipation capacity of dual frames (i.e., braced frames as primary seismic resisting system in combination with moment frames as secondary system) in the framework of the second generation of Eurocode 8. The tested joints have been designed to promote yielding in the end-plate connection and the column web panel in shear, while preventing the failure of the bolts and welds in the range of the required plastic rotation for highly ductile structures. Three beam-to-column assemblies have been considered to account for the influence of the size of members. Moreover, the tested specimens were designed to vary the relative resistances of the column web panel and connection as follows: (i) balanced column web panel and equal strength connection, (ii) balanced column web panel and partial strength connection, (iii) weak column web panel and equal strength connection, (iv) weak column web panel and partial strength connection. The adopted design criteria of the tested joints are described, and the main technological details are also reported. The tested joints were also numerically investigated using finite element simulations. The accuracy of advanced numerical models was verified against the performed experimental tests, and the validated models were used to investigate the local response of the joints, thus highlighting the influence of both geometrical and mechanical parameters.

This study investigates the mechanical behaviour of preloaded beam‐to‐beam joints incorporating long slotted holes, which can be used to provide horizontal adaptability for the joint in the context of steel reuse. Finite element analyses, validated against experimental results, were performed to evaluate the impact of parameters such as plate thickness, bolt preload, bolt diameter, and bolt number on joint shear and slip performance. Results reveal that increasing bolt preload levels from 30% to 70% leads to a marked improvement in slip resistance and a modest increase in overall shear capacity. Force–displacement curves show that higher preload delays the onset of slip and enhances initial stiffness due to improved frictional engagement at the interfaces. Bolt slip was monitored at the individual bolt level and was found to initiate at significantly higher loads when greater preload was applied. The findings emphasize the critical role of preload in controlling interface behaviour and improving connection performance, particularly in the early stages of loading. In the shear resistance, number of rows and bolt diameter had a stronger influence, though gains diminished beyond M24 due to the adverse effects of larger clearance holes on bearing resistance.

Steel angle members bolted through one leg at their extremities are commonly met in lattice towers, where they are mainly used as bracings. Despite their practical relevance, their design relies on empirical expressions due to the incomplete understanding of their structural behaviour. This paper investigates the influence of the end joints on the buckling resistance of such members. A numerical model, validated against experimental results is employed for a parametric study to evaluate the impact on the ultimate buckling load of key parameters, including the size of the profile, the slenderness of the member and the number of the bolts at the end connections. The results highlight the significant influence of the number of the bolts, of the joint plasticity and the amplitude of imperfections on the buckling resistance. Comparisons with the predictions of the European normative standards for lattice towers, i.e. EN1993‐3‐1 and EN 50341‐1 as well as the new version prEN 1993‐3, reveal considerable discrepancies, with the normative predictions being both conservative and unsafe. The outcomes of the study highlight areas where existing standards may be insufficient and provide a basis for improving design recommendations to enhance safety and optimise material use.

Abstract In the context of structural optimisation, high‐strength steels represent the highest strength‐to‐weight ratio of the existing steels, thereby contributing to the optimisation of steel structures by reaching the same level of resistance with a reduced material quantity and resulting in lighter buildings requiring less extensive and costly foundations. New production technologies are already available for the manufacture of hot‐rolled steel sections with a yield strength up to 500 MPa in Europe and 80 ksi (550 MPa) in the United States, which comply with the product norms for civil engineering applications. The selection of the right steel at the right place may result in further investments by manufacturers in the future to develop the optimal material solution for each application. Nevertheless, the use of mild steels is still often preferred for hot‐rolled sections due to a lack of information regarding the existing high‐performance products and the advantages they offer, as well as a scarcity of availability due to the current low demand. Furthermore, using a higher steel grade is often associated with an increase in the unitary cost of steel, accompanied by an increased carbon factor, a reduced weldability and ductility as well as an increased risk of local and global buckling instabilities. This paper demonstrates the considerable weight, cost and carbon savings that can be attained through the selection of an appropriate steel grade for multi‐storey building columns and propose reference slenderness ratios to facilitate the selection of the steel grade.

The determination of the bearing capacity of a member under compression, presenting an initial out-ofstraightness and residual stresses resulting from the production process, is a stability problem governed by the geometry of the member and the mechanical properties of its constitutive material. Amongst these, the yield strength plays a key role. Indeed, it has a direct impact on the spread of yielding in the member but, also, the detrimental effect of geometrical and material imperfections decreases as the yield strength increases. The effect of these initial imperfections is considered by an imperfection factor alpha in the design recommendations provided in EN1993-1-1. However, the current and new upcoming versions of this norm give a stepwise evolution of this imperfection as a function of yield strength by only distinguishing grades lower than S460 from the ones equal/ higher than S460. The present paper aims to briefly summarize the existing studies on this topic, to assess the recommendations of current and new upcoming versions of EN1993-1-1 and the models reported in the literature and, finally, to suggest a modified imperfection factor for hot-rolled sections (H-shape and I-shape) for already existing grades (up to S500) but also for grades up to S690, which do not exist yet for this section typology in the European steel market, to evaluate the structural benefit of developing them.

An innovative simplified method for incorporating the behaviour of structural joints in structural analyses of steel parking-structure is proposed in this study. The method is based on the conventional mechanical modelling of joints through special springs that accommodate the arching effect and moment-axial force (M-N) interaction. The characterization of these springs in terms of stiffness and resistance is performed using analytical formulae. The accuracy of the proposed method is validated through comparisons to results from advanced numerical methods conducted on a multi-scale approach. Its implementation in sub-structure models is then demonstrated, with the main advantage of extremely reducing the modelling and computational time. Validated sub-structure models are finally employed to investigate the influence of the joint properties on the response of parking structures under vehicle collision scenarios. The results show that considering the rigid and semi-rigid joints in the research are characterised by the welded and bolted connections in the research, for a structure built in a courtyard, bolted connections (semi-rigid joints) are recommended, while for the same structure built near an urban road, welded connections (rigid joints) should be considered to enhance the overall robustness.

Low-damage beam-to-column steel joints equipped with friction devices have been recently prequalified for European applications within FREEDAM and FREEDAM+ research projects. Despite their excellent seismic performance, these joints may exhibit limited ductility in the case of column loss scenarios. Therefore, a numerical study has been conducted to investigate the behavior of this type of joint under column removal utilizing refined finite element simulations. The results for the seismically prequalified configurations confirmed poor ductile behavior due to the premature failure of the upper T-stub connecting the beam flange to the column flange. Therefore, a local strengthening solution was designed to enhance the T-stub ductility without interfering with the joint seismic performance. The effectiveness of the adopted solution was numerically investigated by means of refined numerical model. The performed analyses showed that the designed strengthening details allow for increasing the ductility of the whole connection without modifying the flexural resistance of the joints.

In recent years, significant production improvements have been observed in the field of steel structures allowing for a constant increase in the yield strength and consequently for a reduction in the weight of structures. However, considering the required production techniques for high-strength steels, which are sometimes more demanding in terms of energy as well as more demanding in terms of alloy content, the manufacturing cost generally increases with the yield strength. Accordingly, designers and engineers are sometimes reluctant to consider higher steel grades as they do not know in which cases their use presents an economic benefit. Evaluating reliable relative prices for higher grades is seen as a necessity to realise a prospective study and prove their economic benefit but, looking to the literature, only a few publications address this topic. Thus, this article aims to establish relative price-strength relationships for different section typologies to help practitioners in choosing cost-effective steel grades for their applications. These investigations are based on previously available producer price lists to establish relative price intervals for existing and future emerging steel grades. The results of this article demonstrate the base price dependency and the key features which govern the relative prices and thus the economic benefit of such grades.