Trusses made of hollow sections are among the most frequently used structures in steel construction. The use of high-strength steels enables construction methods with lower material usage and high load-bearing capacities. In order to utilise this potential for improvement in terms of competitiveness and resource efficiency, efficient design rules are required.

The current design rules for connections with hollow sections were validated by tests on normal-strength steels up to S355. These rules are based on the assumption that local stress peaks as a result of non-uniform stiffness distribution in the joint are reduced by plastic load redistribution and can therefore be disregarded in the calculation. The question arises as to whether the local stress peaks must be taken into account for high-strength steels, as their high yield strength and limited ductility only permit limited plastifications.

The reliability of the existing reduction factors for K-joints made of high strength steel was determined by extensive experimental and numerical investigations. To achieve this goal, large-scale tests under static load were carried out on 19 K-joints and 12 X-joints made of the high-strength steel grades S460NH, S620QH and S890QLH. The tests on K-joints were carried out in a test rig at the Materials Testing Institute, University of Stuttgart where any combination (compression and tension) of chord and brace forces can be applied with three independent load cylinders.

Advanced numerical simulations using innovative damage mechanics models made it possible to simulate the failure mechanisms of the hollow section connections, which enabled further comprehensive analyses and investigations.

The research project aimed to check design rules for hollow section joints with K-shape, which refer to the individual failure modes, possibly with an adjustment of the reduction factors for high-strength steels. Recommendations for practical application were developed that serve as a basis for normative rules in the construction industry and enable efficient design of welded hollow section joints with K-shape made of high-strength steels. This will open up economic and ecological advantages for the SME-orientated steel construction sector and the quality-orientated steel industry, for example through cost and energy savings with low material costs.

FOSTA P1503 - High-strength K-joints 
Efficient design of high-strength hollow section K-joints
Duration: 01.10.2020 – 30.06.2024
Research association: FOSTA - Research Association for steel Application

Research centres RWTH Aachen University
Institute of Steel Construction
Mies-van-der-Rohe-Str. 1
52074 Aachen, Germany

Prof. Dr.-Ing. Markus Feldmann
Dr.-Ing. Helen Bartsch
Mirco Meissner M.Sc. (Researcher) University of Stuttgart
Institute of Structural Design
Pfaffenwaldring 7
70569 Stuttgart, Germany

Prof. Dr.-Ing. Ulrike Kuhlmann
Wigand Knecht M.Sc. (Researcher)

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• Stroetmann, R.; Hensel, J.; Kuhlmann, U.; Ummenhofer, T. et al. (2022) Ausführung und Fertigung von geschweißten, hochfesten Hohlprofilen. Stahlbau 91, H. 10, S. 647–659. https://doi.org/10.1002/stab.202200053
• Kuhlmann, U.; von Arnim, M.; Gölz, L.; Hofmann, G.; Knecht, W.; Mönch, S.; Pourostad, V.; Stempniewski, L. (2024) Steel and composite bridges – Enabling sustainable solutions. Steel Construction, Vol. 17, Issue 1, pp. 1-10. https://doi.org/10.1002/stco.202300035
• Kuhlmann, U.; Knecht, W.; von Arnim, M.; Gölz, L. (2024) Design of high-strength and ultra-high strength steels in the frame of Eurocode 3. Australasian Structural Engineering Conference (ASEC). ISBN 978-1-925627-76-3.