Extended keynote paper of Eurosteel 2021
Automated production is finding its way into the fabrication of structural steel. One robot holds attachments (stiffeners, end plates, etc.) on a steel beam or column and another robot produces weld seams. However, welding robots can also be used for Additive Manufacturing (Wire and Arc Additive Manufacturing, WAAM). The wire electrode serves as a printing material. The Institute of Steel Construction and Materials Mechanics in Darmstadt is investigating how typical connecting elements for steel structures can be printed directly on steel beams using Additive Manufacturing with arc welding and robots. Furthermore, structural elements such as nodes for space frames can be printed and even complete structures, e.g. columns and a little bridge, have already been manufactured additively. The main focus is on determining suitable welding and process parameters. In addition, topology optimization is necessary in order to achieve good structures using a small amount of material. This is possible due to the free design prospects of WAAM, which opens up new design and production strategies.

Extended keynote paper of Eurosteel 2021
The robustness of steel structures during severe earthquakes is largely based on the calibration of stiffness, strength and ductility. Such a robust system dissipates the energy by plastic deformations in the plastic members (fuses) made from mild carbon steel (MCS), while the other members remain elastic. In the case of concentrically braced frames (CBF), the fuses can be in the form of buckling-restrained braces (BRB), while for eccentrically braced frames (EBF), the short links could take on this role. In the case of shear walls (SW), slender steel plates can be used. To avoid oversized elastic members, they can be made of high-strength steel (HSS). Structures made from HSS and MCS are called dual steel (DS) structures. In order to ease the post-earthquake intervention, the fuses can be detachable. Furthermore, their replacement is less costly if the structure is re-centred using moment-resisting frames (MRF) connected to the main dissipative system, i.e. a dual frame (DF) structure. This paper presents the concept of DS DF structures and some examples.

Extended keynote paper of Eurosteel 2021
This two-part paper provides a state-of-art report on the most recent findings concerning the behaviour, strength and Direct Strength Method (DSM) design of cold-formed steel (CFS) columns and beams affected by mode coupling phenomena not adequately covered by the current specifications for CFS members, namely local-distortional (L-D), local-distortional-global (L-D-G), distortional-global (D-G) and global-global (flexural-torsional/flexural - FT-F) interaction. The paper addresses experimental tests, numerical simulations and DSM-based design approaches that are intended to i) acquire in-depth knowledge on the non-linear behaviour (elastic and elastic-plastic), load-carrying capacity and failure mode nature of the members under consideration, and ii) make use of that knowledge to develop, propose and assess the merit of efficient DSM-based design approaches to estimate their failure loads/moments. Initially, illustrative column results are briefly presented to help grasp some fundamental concepts, namely the characterisation of i) the aforementioned mode coupling phenomena, ii) different sources of mode interaction that may lead to failure load/moment erosion, and iii) the most detrimental initial geometrical imperfections. The DSM design curves currently codified and two strength curves recently developed for column flexural-torsional and beam distortional failures are presented next. The two-part paper then addresses separately each mode coupling phenomenon dealt with, for columns, but only L-D and D-G interaction for beams - while L-D interaction (in columns and beams) appears in Part 1, the remaining column and beam coupling phenomena (all involving global buckling) are dealt with in Part 2 [1]. For columns undergoing L-D and L-D-G interaction, beams experiencing L-D interaction and angle columns susceptible to FT-F interaction, the work reported includes experimental studies, numerical simulations and DSM-based design considerations and/or guidelines. For the remaining coupling phenomena, only numerical results are reported, but they unveil interesting (and unexpected) behavioural features that will help plan future test campaigns and achieve efficient design approaches. Finally, the two-part paper closes with a few concluding remarks and an outlook regarding future developments in this field.

Extended keynote paper of Eurosteel 2021
This is Part 2 of a two-part paper providing a state-of-art report of the most recent findings concerning the behaviour, strength and Direct Strength Method (DSM) design of cold-formed steel (CFS) columns and beams affected by mode coupling phenomena not adequately covered by the current specifications for CFS members. This second paper covers interactions involving global buckling modes, namely local-distortional-global (L-D-G), distortional-global (D-G) and global-global (flexural-torsional/flexural - FT-F) interaction - note that local-global (L-G) interaction, already well mastered by the technical/scientific community, is not dealt with. Like Part 1 [1], this paper also addresses experimental tests, numerical simulations and DSM-based design approaches, intended to i) acquire in-depth knowledge on the non-linear behaviour (elastic and elastic-plastic), load-carrying capacity and failure mode nature of the members under consideration, and ii) make use of the above knowledge to develop, propose and assess the merits of efficient DSM-based design approaches to estimate their failure loads/moments. Taking into account the fundamental concepts and DSM design curves presented in Part 1 [1], the paper addresses separately each mode coupling phenomenon dealt with, for columns, and only D-G interaction for beams - recall that L-D interaction (columns and beams) was covered in Part 1 [1]. For columns undergoing L-D-G interaction and angle columns susceptible to FT-F interaction, the work reported includes experimental studies, numerical simulations and DSM-based design considerations and/or guidelines. For columns and beams experiencing D-G interaction and channel columns prone to FT-F coupling, only numerical results are reported - they reveal surprising behavioural features that will be very useful in planning future test campaigns and achieving efficient design approaches. Finally, the two-part paper closes with a few concluding remarks and a perspective about future developments in this field.

This two-part article gives an overview of the developments of the structural member verification in prEN 1993-1-1:2020 “Eurocode 3: Design of steel structures - part 1-1: General rules and rules for buildings”, one of the second generation of Eurocodes. These developments were undertaken by Working Group 1 (WG1) of Subcommittee CEN/TC250/SC3 and by Project Team 1 (SC3.PT1) responsible for drafting the new version of EN 1993-1-1. In the past, WG1 collected many topics needing improvement, and the systematic review conducted every five years also yielded topics needing further development. Based on this, the current version of EN 1993-1-1 has been developed into a new draft version prEN 1993-1-1:2020 enhancing “ease of use”. The technical content of this new draft was laid down at the end of 2019. Many improvements to design rules have been established with respect to structural analysis, resistance of cross-sections and stability of members. This two-part article focuses on member stability design rules and deals with the basis for the calibration of partial factors, the introduction of more economic design rules for semi-compact sections, methods for structural analysis in relation to the appropriate member stability design rules, new design rules for lateral torsional buckling plus other developments and innovations. This second part of the article is dedicated to illustrating the most relevant changes to member buckling design rules.

Many mechanical joints in steel structures use conventional bolts. Nevertheless, this proven joining technology has some significant disadvantages. These basically include the high levels of scatter during application of the assembly preload using the torque-controlled tightening process, the risk of loosening during cyclic loads due to transverse displacement of the components and the low fatigue resistance under axial loading. Lockbolt technology was invented as long ago as the 1930s and mainly used for the aviation and space industry because of its evident advantages. This joining technology has been constantly further developed in response to the most diverse demands from sectors such as aviation, commercial vehicles, rail vehicles, agricultural machinery, defence technology and steel structures. The application of lockbolt technology, which is primarily used in mechanical engineering, was in most cases based on individual studies, since no consistent rules and guidelines were available for the design and execution of lockbolt connections in steel structures. Within the scope of several public research projects funded by the AiF (German Federation of Industrial Research Associations) and conducted by the iGF (Industrial Collective Research) organization as well as through approval investigations, the Fraunhofer Institute for Large Structures in Production Engineering (IGP) has successively developed the necessary design rules according to the EN 1993 standard (Eurocode 3) for use in structural connections. These design rules will be presented within the context of this article in order to make the benefits of this joining technology available to other users. In addition, insights into the use of technical approvals will be presented together with some current applications.

Nominated for the Professor Eduardo de Arantes e Oliveira Award at XII Conference on Steel and Composite Construction in Coimbra 2019
This paper presents the results of a numerical parametric study that investigated the M-V interaction behaviour of box-girder bridge deck specimens. The study made use of an advanced numerical model that was previously verified against the experimental results. Based on the numerical results and the current rules for trapezoidal box girders, new design models are proposed for predicting the bending and shear resistance of cross-sections with a curved bottom flange. Additionally, the force-based M-V interaction equation proposed by Jáger et al. [1], adopted in the new version of EN 1993-1-5 [2], was verified and slightly modified to fit this type of cross-section.

The simplified formula for the web slenderness limit given in EN 1993-1-5 to prevent flange-induced buckling does not usually govern the web design other than for very slender class 4 plate girder webs in steel grades S235 to S355. However, the same formula applied to plate girders designed in S690 high-strength steel (HSS) gives lower slenderness limits likely to govern web design and restrains the possibilities for reducing their thickness in order to profit from the high steel resistance. This paper reviews the background to the simplified formula currently used and compares the web slenderness limits obtained with numerical results from a full non-linear analysis of slender high-strength steel girders.

Although open-to-circular hollow section (CHS) connections are highly encouraged in the current structural steelwork industry thanks to the extensive range of advantages provided by the CHS columns, a complicated and expensive fabrication procedure has limited their application in practice. The additional gusset plates or stiffeners needed to strengthen a conventional open-to-CHS connection lead to excessive welding quantities and localized CHS distortion, thus causing an economic as well as structural disadvantage. However, if designed efficiently, the CHS connections can offer an extensive range of solutions for modern multi-storey structures. To that end, different types of nominally pinned and moment-resisting “passing-through” open-to-CHS connections have been developed using laser cutting technology (LCT) and proposed by the European research project LASTEICON. This current article concentrates on the LASTEICON two-way moment-resisting connections. The non-linear behaviour of the connections is discussed by way of an appropriate understanding of the force transfer mechanism. Furthermore, these innovative connections are compared with directly welded conventional open-to-CHS connections in order to highlight the advantages offered by the “passing-through” approach.

This two-part article gives an overview of the developments of the structural member verification in prEN 1993-1-1:2020 “Eurocode 3: Design of steel structures - part 1-1: General rules and rules for buildings”, one of the second generation of Eurocodes. These developments were undertaken by Working Group 1 (WG1) of Subcommittee CEN/TC250/SC3 and by Project Team 1 (SC3.PT1) responsible for drafting the new version of EN 1993-1-1. In the past, WG1 collected many topics needing improvement, and the systematic review conducted every five years also yielded topics needing further development. Based on this, the current version of EN 1993-1-1 has been developed into a new draft version prEN 1993-1-1:2020 enhancing “ease of use”. The technical content of this new draft was laid down at the end of 2019. Many improvements to design rules have been established with respect to structural analysis, resistance of cross-sections and stability of members. This two-part article focuses on member stability design rules and deals with the basis for the calibration of partial factors, the introduction of more economic design rules for semi-compact sections, methods for structural analysis in relation to the appropriate member stability design rules, new design rules for lateral torsional buckling plus other developments and innovations. This first part of the article primarily serves to explain the general background to the European Commission Mandate M/515 that led to the further evolution of the Eurocodes and to illustrate the developments in prEN1993-1-1:2020 that pertain to new material grades, partial factors, cross-sectional classification and structural analysis. These form the necessary background to the changes to member buckling design rules, which are treated more specifically in the second part.

In Europe, hot-dip galvanizing has hitherto had no appreciable application in bridge construction, which, despite decades of positive practical experience in building construction, is partly due to a lack of knowledge and regulations regarding the influence of galvanizing on the fatigue behaviour. By contrast, diverse examples from non-European countries already confirm the technical and economic significance of galvanized bridge structures. Based on recent research and a guideline, the basics for wider application have been laid in the recent past. Nete Bridge in the Belgian city of Lier is one of the few road bridges with a fully galvanized steel support structure. The current state of the bridge, after more than 20 years in use, is described below - and a positive interim conclusion is drawn. Nete Bridge is in a good technical condition in terms of corrosion protection; no significant maintenance or repair measures have been required in this regard and are not expected in the future either. The example presented illustrates the feasibility of galvanizing bridge components as an alternative to the organic coating systems previously established. This also offers the bridge management authorities responsible economic benefits due to the very high durability and the associated very low maintenance costs.

Many mechanical joints in steel structures use conventional bolts. Nevertheless, this proven jointing technology has some significant disadvantages. These basically include the high levels of scatter during application of the assembly preload using the torque-controlled tightening process, the risk of loosening during cyclic loads due to transverse displacement of the components and the low fatigue resistance under axial loading. Lockbolt technology was invented as long ago as the 1930s and mainly used for the aviation and space industry because of its evident advantages. This jointing technology has been constantly further developed in response to the most diverse demands from sectors such as aviation, commercial vehicles, rail vehicles, agricultural machinery, defence technology and steel structures. The application of lockbolt technology, which is primarily used in mechanical engineering, was in most cases based on individual studies, since no consistent rules and guidelines were available for the design and execution of lockbolt connections in steel structures. Within the scope of several public research projects funded by the AiF (German Federation of Industrial Research Associations) and conducted by the iGF (Industrial Collective Research) organization as well as through approval investigations, the Fraunhofer Institute for Large Structures in Production Engineering (IGP) has successively developed the necessary design rules according to the EN 1993 standard (Eurocode 3) for use in structural connections. This paper presents connections with lockbolts in steel structures. Following an introduction to lockbolt technology and the assembly preload of lockbolts, the securing effect and corrosion protection of lockbolts are presented.

Cylindrical steel storage tanks are thin-walled significant engineering structures. This study investigated the buckling conditions of ground-supported cylindrical steel liquid storage tanks according to roof shapes and shell thicknesses. The roof shapes selected were open-top, flat-closed, conical-closed and torispherical-closed-top tanks, and shell thicknesses were 4, 6 and 8 mm. These tanks may be exposed to several types of failure during earthquakes, such as elephant-foot buckling, diamond-shape buckling, overturning and uplift. Great financial loss and environmental damage can also ensue when steel liquid storage tanks are damaged in an earthquake. The seismic analyses were conducted under Kobe earthquake conditions for cylindrical steel tanks with different shell thicknesses and roof types. To investigate dynamic behaviour of the tanks accurately, the hydrodynamic response of each tank was divided into impulsive and convective components. Directional deformation and buckling results are presented for both impulsive and convective masses. As a result, the deformation of the tank is significantly reduced when the top of the tank is in the shape of a torispherical dome. The flat-closed tank has a maximum directional deformation in both impulsive and convective modes. Results also show that when shell thickness was increased, buckling deformation decreased, but different deformation states were observed on the wall and roof components depending on the type of roof.

This paper discusses an innovative approach to the design of planar steel frames composed of prismatic and/or non-prismatic members. The method evaluates the member stability limit states using an inelastic eigenvalue buckling analysis configured with column, beam and beam-column inelastic stiffness reduction factors derived from the ANSI/AISC 360-16 Specification. The resulting procedure provides a relatively rigorous evaluation of all member strength limits accounting for moment and axial force variations along the member lengths, non-prismatic geometry effects, general out-of-plane bracing conditions and beneficial end restraint from less-critical adjacent, unbraced segments and/or from end conditions. The approach uses a geometric non-linear pre-buckling analysis based on the AISC direct analysis method to estimate the in-plane internal forces. Given these forces, an eigenvalue buckling solution is conducted to evaluate the overall member stability. Other limit states are addressed through cross-section strength checks using the internal forces determined from the geometric non-linear load-displacement analysis. Calculations using this approach are compared with results from recent experimental tests.

Steel-concrete composite floor systems represent a common solution for buildings, leading to reduced material consumption and improving the structural strength and flexural stiffness. Lightweight steel frame construction systems, based on the combination of thin-walled cold-formed steel members (CFS) and panels (e.g. wood, cementitious, gypsum plaster), may also benefit from the composite behaviour in the case of concrete slabs. In this context, full-scale experimental tests were conducted to evaluate the structural behaviour of a floor system conceived with 0.95 mm thick CFS trussed beams and prefabricated concrete slabs. An innovative solution for shear connectors was designed and tested in order to ensure full interaction between the CFS trussed beam and the concrete slab. The thin-walled channel (TWC) connector is based on a lipped channel CFS attached to the top chord of the truss by standard self-drilling screws. Experimental results indicate efficient behaviour of the TWC shear connectors with improved bending capacity of the floor system.

It is well known that inelastic behaviour in the panel zone of beam-to-column connections can help to ensure a more stable structural performance, as the panel zone can absorb energy to prevent structural collapse. Although plastic deformation of up to only 30 % in the panel zone can contribute to improving performance according to Eurocode 8, in the Japanese code there is no limit when structural performance is verified. This paper describes full-scale testing of beam-to-column connections where the beam was connected to the column by a T-stub using high-strength bolts. In the test specimens, a shape similar to a T-stub was used for the continuity plate, and partial reinforcement at the panel was expected. The testing allowed the maximum strength and deformation capacity of the beam-to-column connection to be evaluated.

During inspection work, when the pipe is empty, a thin-walled steel pipe is susceptible to shell buckling due to external pressure. Comprehensive research activities, including tests, began in the 1960s, and analytical and empirical formulae were developed for design.
Some of these different analytical design concepts are presented in the first part of the paper, and their results are compared for the whole range of pipe slenderness in practice.
Afterwards, the results of extensive numerical calculations are shown. These deal with different imperfections in practice and are not considered in the current design solutions. In addition, the effect of the radial flexibility of the surrounding rock mass has been studied - an issue ignored up to now. All these results are compared with the design formulae in practice.
It is therefore possible to conclude the paper with an appraisal of the individual solutions in practice and sum up appropriate design rules for external pressure.

Designing fillet welds using the directional method of EN 1993-1-8 requires the consideration of force components, converted to stresses, located in the plane of the weld throat. For circular hollow section (CHS) joints, considering these “stress components” can be exceedingly difficult. An approach to applying the directional method to CHS joints is developed in this paper. The directional and simplified methods of EN 1993-1-8 are then evaluated against available finite-element data, validated from recent tests on weld-critical CHS X-joints according to the standard evaluation procedure of EN 1990. Despite inherent non-uniform loading of the weld around the perimeter of the brace, it is shown that taking the total weld length to be effective is safe for both methods. Procedures are given so that these methods can be used to design fillet welds in CHS joints as “fit-for-purpose”, as permitted by EN 1993-1-8 and ISO 14346, and new design charts are produced for minimum fillet welds that develop the axial capacity of the connected brace.

This paper presents the design considerations for typical photovoltaic panel arrays having aluminium members. Section and member design checks are performed according to Eurocode 9 on the basis of the wind, snow and seismic loads of Eurocodes 1 and 8. Improvements to the design are then sought, starting by reducing the distance between the vertical posts and then by changing the thickness of specific sections. Following that, the effects of member imperfections and connection flexibility are studied using a reduced flexural rigidity and different rotational stiffness values respectively. The degree of dynamic coupling when the array is placed on the top floor of an existing building as well as the influence of founding the vertical posts on compliant ground are also evaluated.

As part of the ongoing revision of EN 1993-1-11 “Design of structures with tension components”, the current fatigue design rules have been subjected to a critical review. This has resulted in modifications and amendments, including the adjustment of several parameters. This paper provides an introduction to as well as background information on those changes.

Extended version of the Tomá Vraný SDSS Award 2019
The method of the equivalent compression flange simplifies the verification of lateral torsional buckling of steel beams to flexural buckling of an equivalent part of the cross-section that is in compression. The current simplified method is based on outdated normative rules and the design results may be both, uneconomic and non-conservative. To remedy these shortcomings, the simplified method is modified based on the basic structural behaviour of steel beams taking into account the effects of material, cross-sectional geometry and residual stresses. This paper presents a comprehensive experimental study on the lateral torsional buckling behaviour and residual stresses of welded doubly- and mono-symmetric I-shaped steel beams. Moreover, an improved simplified method of the equivalent compression flange is proposed for design purposes.

Selected, extended paper from the SDSS 2019 special session ECCS/TC8 - Structural Stability
This work reports on the results of an ongoing investigation of the direct strength method (DSM) design of hot-rolled steel equal-leg angle columns with pinned (spherically hinged) supports and short-to-intermediate lengths, i.e. buckling in flexural-torsional modes. It extends the scope of similar studies involving cold-formed steel angle columns with the same characteristics (but higher leg width-to-thickness ratios). After collecting the experimental and numerical failure loads available in the literature concerning columns with several geometries, the paper addresses the mechanical reasoning behind a DSM-based design approach proposed in the context of fixed-end and cylindrically hinged cold-formed and hot-rolled steel short-to-intermediate angle columns, which must be modified to handle spherically hinged supports. After that, the failure loads gathered are used to assess the quality of their estimates using this design approach, namely, by determining LRFD resistance factors. It is shown that this approach can be successfully applied to predict hot-rolled steel simply supported angle column failure loads, leading to LRFD resistance factors above 0.90.

Selected, extended paper from the SDSS 2019 special session ECCS/TC8 - Structural Stability
Preliminary numerical and experimental investigations by the authors have highlighted the fact that the stiffness of the rotational restraints at the gusset plates near the ends of members are crucial for the prediction of the compression member capacities of bolted angles. Consequently, analytical models for estimating appropriate spring stiffness values have been developed for several practical applications in buildings and two-bolt connections at both ends of a member. The connection details investigated comprise a simply fixed gusset plate, a connection to the flange of an I-shaped section and a connection to the web of an I-shaped section via a gusset plate. In this paper, the formulae developed are presented alongside the detailed background to their derivation. The stiffness values formulated can in future be used to improve the accuracy of the prediction of the compression member capacities of single steel angles with bolted end connections.

Selected, extended paper from the SDSS 2019 special session ECCS/TC8 - Structural Stability
The stability design of members and structures based on a geometrically nonlinear analysis with equivalent initial geometric imperfections (GNIA) is a commonly used method. In the case of lateral torsional buckling (LTB) of members, the regulations in EN 1993-1-1 are unsatisfactory. With the current code, but also with the new draft prEN 1993-1-1 results are achieved, some of which are significantly too conservative as well as lacking in terms of safety. The reasons for this are the specified shape of imperfection as bow imperfections e0 out of plane, an inappropriate differentiation related to the cross-section shape and yield strength, as well as neglecting the influence of the moment distribution over the member lengths. Parameter studies have shown that a distinction is required due to the different structural behaviours of members loaded with pure bending, pure compression, or a combination of bending and compression. This article presents current research results that consider the case of pure bending of I- and H-profiles. The dependencies of the profile shape, the cross-sectional resistance model and the influence of steel grades are analysed with respect to the height of the equivalent geometric imperfection to be applied.

Selected, extended paper from the SDSS 2019 special session ECCS/TC8 - Structural Stability
Cable-stayed columns offer several advantages among traditional solutions. Owing to the additional restraint given by the prestressed cables and cross-arms, they can provide enhanced buckling resistance in comparison to conventional columns. They are also aesthetically appealing, combining a strong architectural effect with a top-notch engineering solution. This paper presents a design approach for the stability design of prestressed cable-stayed columns. This method is based on the well-known Ayrton-Perry format, combining first- and second-order effects. The imperfection factors were calibrated on the basis of experimental results. This method is consistent with design rules for uniform columns in Eurocode 3 and is easy to apply.