A Lego-like steel-framed structural system has been developed within the frame of an RFCS project, REDUCE, to facilitate 1) deconstruction of composite structures, 2) circularity at structure and element levels, and 3) serial production in construction by promoting a greater standardization of structural elements. The system utilizes innovative demountable shear connections for composite flooring solutions with precast concrete elements, and adjustable steel connections for use in both beam-to-beam and beam-to-column connections. The first use case of the structural system has been realized in the Petite Maison project which is a demonstration project for circularity and contributes to the event ESCH2022. Each construction element is linked to a digital database and remains available for future reuse, as a result of the plug-and-play, demountable and robust features of the developed system. This paper presents the proposed demountable system, the results from experiments and finite element analyses on the behaviour of shear connections, composite beams, and steel connections, and indicates the analysing methods for structural engineers to open a pathway for full implementation of the structures built into digital tools, fabrication, and construction.

In the framework of the revision of Eurocode 3, Part 1-1, several amendments have been proposed and accepted in order to improve the rules for the resistance to member buckling. For clarification, a flow chart connecting the global analysis (first or second order), the imperfections and the type of verification has been implemented for ease of use. Since the publication of the standard in 2005, many research projects have been conducted across Europe on this topic and their results have contributed to provide appropriate answers to problems identified in practice. Therefore, the revised code provides new design rules for stability. Important works of calibration have been performed in these different projects to derive appropriate values of the partial factor on the resistance side. For example, a new formulation for lateral-torsional buckling has been introduced for the calculation of the reduction factor. The consequence is a reduction of the discrepancy between the results obtained by these new methods and those from experimental or numerical tests. In order to extend the scope of Eurocode 3, Part 1-1, additional methods have been implemented in an annex to cover the stability of members with mono-symmetric cross-section under compression axial force, biaxial bending, with or without torsion. The format of the resistance criteria remains similar to the format of the current interaction formulae so that the designers can easily identify the evolution of the rules. This article presents in a systematic way the new implementations in the formal vote version FprEN 1993-1-1 of the code.

For the stability assessment of members and structures according to EN 1993-1-1, the equivalent member method, the geometrical non-linear calculation with equivalent geometrical imperfections or the GMNIA analysis with geometrical imperfections and residual stresses can be used alternatively. The second possibility requires a corresponding model for the cross-section resistance. For the verification of lateral torsional buckling, bow imperfections e0 out-of-plane are defined, which lead in combination with the given loading in-plane and the geometrical non-linear analysis to bending moments Mz and torsion of the members. The amplitudes of the imperfections are highly dependent on the nature of the approach (e.g., scaling of the buckling shape, assumption of bow imperfections) and the verification method for the members. Within the framework of the scientific work supervised by the TU Dresden and TU Darmstadt, extensive parametric studies were conducted to calibrate imperfections for lateral torsional buckling based on the GMNIA. After determining the nature of imperfections and the design models for section resistance, this article presents results of these parametric studies and shows the calibration of imperfections for a standardisation proposal based on EN1993-1-1. The evaluation of the data in combination with the necessary simplifications for the design practice leads to appropriate definitions of imperfection values e0, LT and the necessary differentiations.

The buckling behaviour of panels may be determined according to EN 1993-1-5 [1]. Most of the design rules relating to stiffened panels in EN 1993-1-5 were derived on the basis of open-section stiffeners. Several recent investigations have shown that the application of the design rules according to EN 1993-1-5 considering the torsional stiffness of the stiffeners may overestimate the resistance of the panels. Therefore, the recent Amendment A2 to EN 1993-1-5 states that the torsional stiffness of stiffeners should generally be neglected in determining critical plate buckling stresses. In addition, prEN 1993-1-5 [2] provides rules for considering the torsional stiffness of stiffeners. However, in this article it is shown that even the rules of prEN 1993-1-5 are not sufficient to overcome the safety deficiencies. The article focuses on the investigation of the buckling behaviour of stiffened panels with closed-section stiffeners subjected to constant longitudinal compression stresses. Improved rules have been developed that allow to consider the torsional stiffness of the stiffeners. Based on an extensive numerical parametric study, a new interpolation equation between column- and plate-like behaviour is proposed. In comparison to [3], the investigations have been extended to the effective width method. They show that the proposal provides a safe and economic solution for the reduced stress method and the effective width method when considering the torsional stiffness of stiffeners by calculating the critical plate buckling stresses.

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.

Due to the commonly eccentric connection on only one angle leg (bolted or welded), additional bending moments are acting on the angle member under compression axial force, leading to a complex load-carrying behaviour with flexural and/or flexural torsional buckling phenomena. Furthermore, type and size of rotational restraints at the member's ends (provided by the adjacent structure) significantly influence the compression capacity of these members. In this article, a recently developed design model for angle members in compression with welded or bolted end connection is presented. The design model considers the accurate rotational spring stiffness of three common joint types. The presented procedure allows for calculating the internal forces based on elastic second-order theory for an individual member with eccentricities, equivalent geometric imperfection and rotational spring stiffness at both ends. The detailed analytical equations for the rotational spring stiffness of all studied joint configurations are summed up. In addition, calibration factors fDi are presented, to get accurate compression capacities with the design model. Finally, the accuracy of the design model for all three studied joint types of angle members with welded connections is shown through comparison with sophisticated finite element calculations, code provisions (EN 1993-1-1, AISC) and experimental tests from the literature.

The galvanizability of high-strength, hot-finished hollow sections was investigated as part of a test programme. Here, square sections made of steel grades ranging from 355 to 620 MPa were galvanized using two processes: a classic, quasi-rare zinc melt and a zinc-5 % aluminium alloy. To determine the influence of galvanizing on the material properties, samples were taken from the galvanized specimens, tensile tests and Charpy-V-tests were performed on them, and the results were compared with those in the ungalvanized initial state. Furthermore, we investigated the optical and microstructural characteristics of the zinc coatings, as well as possible cracking due to liquid metal embrittlement (LME). The results show that the two galvanizing processes used in the test programme produce no negative effect, but rather tend to have a positive effect on the mechanical properties of the steels tested. In all cases, stable zinc coatings are formed that exhibit the expected characteristics. Cracking as a result of LME could not be detected.

Bolted joints in steel structures are generally subject to combined actions. The research on ultimate capacity of bolted joints under combined tension and shear actions, twin shear actions, and combined tension-twin shear actions is relatively limited. The aim of this paper is to calibrate the fracture locus of grade 10.9 bolts using the mesoscale critical equivalent plastic strain (MCEPS). We use ductile fracture simulation to numerically evaluate the ultimate resistance of bolts subject to multiaxial loading and compare the results with the current design standards. The results show that predictions for grade 10.9 bolts subject to multiaxial loading in Chinese code GB 50017 and American code AISC-360 are not on the safe side, while predictions in European code EN1993-1-8:2005 are slightly on the conservative side. We propose two modification factors to improve prediction of bolt strength subject to multiaxial loading; namely the multiaxial loading factor for shear resistance &xgr;v and the multiaxial loading factor for tensile resistance &xgr;t.

In April 2020 the world's first full-size submerged slip joint connection was installed at the Borssele Site V offshore wind farm. This event marked the launch of a new, alternative transition piece/monopile (TP/MP) connection for the offshore wind market. An extensive qualification programme in terms of numerical modelling and testing was set up prior to installing the slip joint. This process was witnessed, reviewed and accepted by DNVGL. It led to a certified slip joint design for Borssele V, with a 9.5 MW wind turbine generator (WTG) on top of the foundation as well as an A-level component certificate for the slip joint in general. The installation of the slip joint for Borssele V was the first use of this connection in a commercial full-size wind turbine.
The implementation of the slip joint connection at Borssele V demonstrated the benefits of the slip joint in terms of simple, safe and fast installation as well as the option for using the slip joint submerged, thus allowing a more optimized weight/length split between MP and TP and the use of smaller installation vessels. A monitoring campaign was executed during and after installation to provide lessons for future slip joint designs.
Reducing the levelized cost of energy (LCoE) requires big steps in technology innovation. The Borssele V offshore wind farm offered a unique opportunity to demonstrate to the industry that the slip joint connection is fully feasible and ready for practical implementation.
The slip joint: simple connection, ingenious design.

This paper describes finite element simulations of the structural deformation and material fracture behaviour of high-strength steel RHS K gap truss joints. The fundamental scope was to examine whether the joint strength predictions based on the behaviour of lower strength and more ductile steel with a yield stress of 355 MPa or less would hold good for higher strength 450 MPa steel with a lower ductility. The FEA reliability analysis indicates that for failure modes associated with local buckling, yielding and deformation (chord side wall failure, chord face plastification and brace failures due to reduced effective width), the existing approach could, with modifications, be extended to cover higher strength tubes, but for failure modes associated with fracture or ductility or modes liable to brittle failure (tearing in the tension brace and chord punching shear), a strength reduction modifying factor was required. The finite element simulations incorporated a damage mechanics approach to calibrate experimental results in both the fracture and deformation modes of failure. The paper proposes a new formulation for strength and incorporates reduced ductility in high-grade steel with a modifier function that is not based on yield stress, instead recognizes the reduced ultimate strains, damage parameter for fracture and ultimate stress of the material.

Fatigue enhancement by way of high-frequency mechanical impact (HFMI) treatment can enable effective design and construction of steel bridges. However, bridges may experience high and varying mean stresses, the effects of which are not covered today by any design recommendation or in the literature on HFMI-treated joints. In this study, fatigue experiments were conducted with realistic in-service bridge loading, which revealed the same high fatigue performance as for constant amplitude loading. The effect of mean stress in spectrum loading was quantified and a method to account for it in an equivalent manner is proposed. A design framework has been developed for design and engineering purposes.

Nominated for Eurosteel 2021 Best Paper Award
The introduction to the market of metal foams represents a significant opportunity to increase the use of metal in the construction market. In fact, the development of new products made of metal foams and steel could improve the performance of conventional buildings in terms of stiffness and resistance. The first structural applications of these materials have already shown the potential of composites made of foam and steel, and demonstrated the feasibility of producing mono-dimensional and bi-dimensional elements with a high resistance-to-weight ratio. Reducing the structural weight can be a significant advantage in many cases, such as infrastructures or buildings in seismic zones. Within this framework, the work presented in this paper aims to assess the response of composite sandwich panels made of steel and aluminium foam using a set of tests that will serve as the basis for developing a new system of dry-assembled composite floors. The tests were carried out at the Strength laboratory (Structural Engineering Test Hall) at the University of Salerno, Italy and were designed to verify the mechanical properties of metal foam panels and double-skin glued sandwich panels. The tests have shed light on the basic mechanical properties of both foams and sandwich panels.

Nominated for Eurosteel 2021 Best Paper Award
The fatigue design of shear connectors for composite girders is based on fatigue strength curves (with 95 % survival probability) derived from tests with one or just a few connectors. According to the Eurocodes, the lifetime of a girder can then be limited by failure of the shear connector with the highest fatigue utilization. The failure probability of a component is thus determined by the failure probability of one shear stud or composite dowel, although a composite girder is an internally highly statically indeterminate system with the capability of redistributing forces in the composite joint. This paper outlines how considering crack propagation, the residual capacity of shear connectors with cracks and the redistribution of forces towards less stressed and damaged connectors can have positive effects on the lifetime of a girder. The failure probability of a girder is determined by several Monte Carlo simulations using a simplified FE lamella model with incremental calculation of the degradation of the connectors. The input parameters with the greatest influence on the fatigue behaviour of the girder are evaluated. Furthermore, the results show the economic potential of a future global safety concept for composite girders, especially those with composite dowels.

Nominated for Eurosteel 2021 Best Paper Award
In recent decades, innovative seismic-resilient structural systems have been proposed to reduce the direct and indirect losses related to seismic events. Among others, steel moment-resisting frames (MRFs) equipped with damage-free self-centring column bases (SC-CBs) represent a promising solution. Although several configurations of SC-CBs have been proposed in literature, only a few research studies investigated how the significant parameters (e. g. number of storeys, frame layout, seismic mass, seismic intensity) affect the seismic performance of MRFs with SC-CBs. To further investigate this aspect, the present work focuses on the influence of an additional parameter (i. e. the combination of seismic mass and acceleration) on their self-centring capability. Three 5-bay steel MRFs with 4, 6 and 8 storeys are considered as case-study frames and designed based on two different values of the seismic mass (i. e. M1 and M2). Numerical models are developed in OpenSees, incremental dynamic analyses (IDAs) are performed to monitor global engineering demand parameters (EDPs), and fragility curves are derived to evaluate the seismic performance of the structures. It is observed that the inclusion of SC-CBs produces beneficial effects in terms of increased self-centring capability on all the investigated case studies. Moreover, the parametric analysis allows some preliminary observations to be drawn regarding the influence of the number of stories and seismic mass.

Nominated for Eurosteel 2021 Best Paper Award
Additive manufacturing (AM) is playing an increasing role in the production of complex steel structures. Robot- or machine-guided gas-shielded metal arc welding (GMAW), known as wire and arc additive manufacturing (WAAM), is suitable for this purpose. A small footbridge was designed in shell form at TU Darmstadt. It was completely additively manufactured over a stream on site using a welding robot. The work called for cantilevered manufacturing without support structures, which poses special challenges in manufacturing. This paper describes selected aspects of this project: the preliminary strength studies, the manufacturing strategy to ensure homogeneous manufacturing and the findings from on-site manufacturing. In addition, investigations were carried out which led to a notable increase in the deposition rate with the right choice of gas and wire.

As part of the ongoing revision of the Eurocodes, design provisions in EN 1999-1-1 on the buckling of longitudinally welded aluminium compression members have been subjected to a critical review. The numerical investigations described in part 1 of the paper were conducted because a need for improvement was identified. In part 2 the main observations were presented qualitatively. Part 3, the final part of the paper, presents the statistical evaluation of the numerical investigations and the determination of the buckling curves. Two different approaches are conceivable for these buckling curves: conventional buckling curves with explicitly specified imperfection factor and plateau length parameters, and a modified Ayrton-Perry approach with interpolated parameters. Both approaches are compared with the results of the numerical investigations and with test results.

Vertical web stiffeners partially restrain the warping of the simply supported beam when lateral buckling occurs. The restraint increases the critical moment of the beam. This increase can be shown through the decrease of the warping effective length factor in the general formula determining the critical moment. An approximative expression to determine the warping effective length factor is proposed and verified by numerical analysis. Simply supported beams stiffened by vertical stiffeners, including end plates, which are subjected respectively to uniform bending, uniformly distributed loads, two concentrated loads and one concentrated load are numerically analyzed using COMSOL finite element software. The reliability of the proposed expression is assured by small coefficients of variation and very high coefficients of determination. It can provide a solution to improve the critical moment in the design of the beam without using an intermediate lateral bracing system.

The clearance between the core plate and the restraining part of the buckling-restrained brace using steel mortar planks (BRBSM) considerably affects the failure mode and hysteretic characteristics of the brace. A clearance adjustment construction is proposed for the BRBSM which can control the clearance through partially pasting clearance adjustment materials and polishing the mortar plank surface. First, the clearance adjustment construction workflow and quality control are demonstrated, and it is verified that the clearance can be assured by fabricating specimens. Further, the effects of the difference in clearance between the core plate and restraining part on the structural performance of the BRBSM in terms of failure behaviour, increase strength ratio, buckling mode and cumulative plastic strain energy ratio are evaluated by conducting a cyclic axial loading test.

Increasing loads on the connections of tubular towers that support wind turbines are making the traditional bolted L-flange more challenging to design, while the large mass of components and tools make installation challenging from a safety perspective. The C1 Wedge Connection is a symmetrical connection between tubular sections. The connection consists of a cylindrical lower flange and a fork-shaped upper flange that slides over the lower flange. Both flanges have radial elongated holes distributed along the perimeter allowing the C1 fasteners to pull the two flanges together. The fastener assembly of the C1 Wedge Connection uses a small, horizontally placed bolt to pull two wedges together. The wedges transform and magnify the horizontal bolt force into a larger preload between the flanges by pulling them together. The orientation of the bolt in the fastener prevents significant load fluctuations under external loading. As a result, unlike a bolted L-flange, the C1 Wedge Connection is less susceptible to preload loss, thereby eliminating the need for regular maintenance and inspection. The scalability of the C1 Wedge Connection makes it suitable for larger loads. These larger loads are expected with increasing size of wind turbines and when connections are used in more demanding locations such as hurricane and seismic conditions or where connections are subjected to high dynamic loads caused by extreme wave conditions or on floating foundations.

As part of the ongoing revision of the Eurocodes, design provisions in EN 1999-1-1 on the buckling of longitudinally welded aluminium compression members have been subjected to a critical review. The numerical investigations described in part 1 of the paper were conducted because a need for improvement was identified. In part 2 of the paper, the main observations are presented in qualitative terms. Those observations are: the influence of the allocation of the materials to buckling classes, the influence of the imperfections plus the cross-section geometry including the position and size of the HAZ within the cross-section. Part 3 will conclude this paper by discussing the proposed design approaches in detail.

Welding is by far the most widely used metal joining method. High-Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting are two post-weld treatment methods that aim to enhance the strength of the steel. In this paper, the improvement in residual stress and material characteristic obtained with these methods are studied by conducting several experimental investigations such as hole drilling, hardness testing and microscopy. Hole drilling shows that HFMI treatment improves the status of residual stress at the weld toe in the first 1 mm from the surface. Furthermore, Vickers testing shows a remarkable improvement in the hardness values at the weld toe in the first 2 mm. This can be attributed to the reduction in grain size after treatment. Moreover, acicular ferrite and tempered bainite are found to be the main constituents in the fusion and heat-affected zones after TIG-remelting.

Double-I sections are widely used as columns in moment-resisting frames. The conventional welded moment connection between I-beam and double-I built-up column behaves in a partially restrained manner and does not provide the strength and ductility required in special moment frames. In this paper, a modification method is proposed to improve the behavioural characteristics of conventional moment connections. The modification involves replacing the continuous cover plate with two alternative plates having increased width and thickness at the location of the beam flange-plates. The behaviour of the proposed connection was investigated in detail both experimentally, by means of two full-scale specimens, and analytically, by means of eight finite element models. Based on the results obtained, in beams with a modified connection most of the plasticity develops remote from the column cover plate, and the panel zone remains in an elastic state. Accordingly, the deformation of the modified connection is reduced significantly and its stiffness is increased to the level required for a fully restrained connection. The hysteresis curves of the modified connection are also stable during large inelastic deformations with no substantial pinching. The modified connection provides the strength and ductility required in special moment frames.

Particularly efficient steel components can be designed by performing a plastic global analysis of a structure made of high-strength steel (HSS) as well. However, plastic design of HSS structures is not embodied in current standards, which is why a broad research study was carried out to address different aspects of the use of HSS for plastic design. This paper deals with verified numerical investigations of the rotation capacity of HSS I-girders considering realistic true stress-strain curves, local plastic instability and damage mechanics approaches to predict ductile crack initiation. Various influences on the rotation capacity of HSS beams were assessed by varying flange and web slenderness ratios, material characteristics, system dimensions, stiffness of lateral supports and loading conditions so that recommendations for the plastic design of HSS girders can be given. It is apparent that, apart from the slenderness of the flange and the material properties, the slenderness of the web is one of the main influencing characteristics. The results show that the plastic design of HSS structures is possible when limits for cross-section class 1 are more severe for HSS grades over S460 up to S690 compared with the current limits in EN 1993-1-1.

ISO 14346 permits fillet welds in rectangular hollow section (RHS) joints to be designed either (i) to develop the capacity of the connected brace or (ii) to resist the actual load(s) in the brace (i.e. as “fit-for-purpose”). However, no methods for design according to approach (ii) are given in the ISO or any European standard. In this paper, data from 41 weld-critical tests on RHS gapped K, T, Y and X joints under brace axial load(s) are analysed to determine a reliable method for the design of fillet welds as “fit-for-purpose”. “Weld effective length” formulae are evaluated in conjunction with EN 1993-1-8 (directional and simplified methods) according to the standard procedure of EN 1990. A reliable method, based on ISO 14346, is proposed for calculating the design resistance of all-around fillet welds in RHS gapped K, T, Y and X joints. Updates to the minimum fillet weld throat thicknesses required to develop the capacity of a connected RHS brace are also discussed.

Unprotected steel and steel structures exposed to the atmosphere inevitably corrode. For thin-gauged structures especially, corrosion is critical for stability. Although the reduction in thickness caused by corrosion is insignificant for some structures, in other structures corrosion is unacceptable. Therefore, corrosion protection is a crucial issue for steel structures and the steel industry. Over the last decades, many new corrosion protection systems and innovative steel products have been introduced onto the market. Corrosion protection for cold-formed structural steel elements has been standardized through the introduction of the EN 1090-4. This paper introduces the harmonized rules for the corrosion protection of loadbearing thin-gauged structures. The zinc-magnesium-aluminium (ZM) metal coating system developed in recent years is also dealt with.