Truss structures made of directly welded hollow sections under predominantly static loading are used in many areas of structural steel engineering and infrastructure, such as pedestrian bridges, as well as in plant- or mobile crane construction. Due to their direction-independent resistance and stiffness values, low risk of buckling and high torsional resistance, hollow sections are economical profile shapes. The use of high-strength steels can lead to economical lightweight structures with high load-bearing capacities.

The design of connections made of high-strength hollow sections is currently being redefined in the revision of Eurocode 3, Part 1-8. Due to the lack of test results, it is intended to meet the safety requirements by applying conservative general reduction factors. Consequently, the economic use of high-strength steels is strongly limited.

To achieve these objectives, the research project comprised extensive fabrication-related, experimental and numerical investigations of T-, X- and K-joints made of circular and rectangular hollow sections in the high-strength steel grades S460NH, S620QH and S770QLH. As part of this work, the University of Stuttgart conducted large-scale static tests on a truss with exchangeable K-joint test specimens at the Materials Testing Institute University of Stuttgart (MPA). These investigations provided the basis for evaluating the observed failure mechanisms, joint resistances and existing codified design models.

These conclusions also serve as a basis for the formulation of a National Annex to the future German edition of EN 1993-1-8, which can confirm the necessary restrictions, but also reduce the requirements. The following objectives were pursued:

• Specification of reduction factors for the determination of the load-bearing capacity from joints made of high-strength hollow steel sections
• Assessment of the need to take reduced strength into account
• Recommendations for the consideration of secondary bending moments

The research project evaluated the design rules for T-, X- and K-joints made of hollow sections with respect to the individual failure modes, possibly with an adjustment of the reduction factors for high-strength steels. Recommendations were developed that can also serve as a basis for future standardised regulations in structural engineering, thereby contributing to a more accurate, reliable and economical design of welded hollow section joints made of high-strength steels. This enables the SME-dominated steel construction sector and the quality-driven steel industry to exploit both economic and environmental benefits, including cost and energy savings through efficient material use.

• 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
• Münch, A.; Weidner, P.; Ummenhofer, T. (2023) Experimental and numerical investigations on welded high-strength hollow section joints, Tubular Structures 18: proceedings of the 18th in-ternational symposium on tubular structures, Beijing, China, October 17-20, 2023
• 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.