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Fatigue Assessment of Large-Size Bolting Assemblies for Wind Turbine Support Structures

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The fabrication of constructional steelwork with manual or automated welding takes place in a controlled and safe environment.
© Christoph Schuhknecht, Düsseldorf

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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.

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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.

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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.

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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.

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News: Eurosteel Conference postponed until 1-3 September 2021

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For Schorgast Bridge Full Locked Coil Rope high strength tension components have been designed and implemented as stay cables. As part of the revision of EN 1993-1-11 the fatigue design rules for tension components have been modified and adjusted. An article in this edition provides insight and background information (Annan, R. et al., pp. 61-75) © FATZER AG

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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.

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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.

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