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.

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In April 2020, the world's first submerged full size slip joint was installed in the Borssele V offshore wind farm. An extensive qualification programme in terms of numerical modelling and testing (see photo) was set up prior to installing the slip joint. The Borssele V offshore wind farm offered a unique opportunity to demonstrate to the industry that the slip joint connection is fully feasible, certified and ready for practical implementation (s. paper Suur, S; Hengeveld, F., p. 152 ff.)

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.

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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.

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The Parametric FEM Toolbox is a plug-in for the visual programming environment Grasshopper which implements the RF-COM API of the Dlubal RFEM finite element software to establish a connection between these two platforms. Both the transfer of data from Grasshopper into RFEM and back from RFEM into Grasshopper are supported. Thus, new possibilities are enabled beyond the options of the conventional graphical user interface (GUI) of RFEM: the use of the Rhino 3D modelling tools to create NURBS curves and surfaces; the possibility of the parametric modification of an existing FE model or part of it; the export and processing of FE model data, which sometimes is not even available through the program GUI, e.g. 3D shapes of beam elements; etc. With these functionalities, the object-oriented structure and compact GUI, this tool can easily be adapted to numerous workflows and optimization processes. This paper explores which possibilities exist for implementing a commercial FEM software in a parametric design platform. Existing approaches are reviewed, the development of the Parametric FEM Toolbox is described and some of the possible workflows with this new tool are explored through a collection of real-world case studies.

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Double-skin sandwich panels with steel sheetings and low density aluminium foam have an excellent flexural response. Experimental results highligthed the key role of the type of adhesive and its strength on the ultimate performance. Simple equations are provided to predict the stiffness, yield and plastic strength (s. article Latour, M. et al, pp. 73-80)

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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.

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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.