Welded K-Joints out of High-Strength Hollow Sections: Experimental Investigations

Welded hollow section steel trusses are used in many areas of structural engineering and infrastructure applications, e.g., footbridges, plant construction, and mobile crane construction. The use of high-strength steels (HSS) can promote economic lightweight structures with high load-bearing capacity and large spans. The current design rules have been validated on a semiempirical basis using test results on joints made of mild-strength steels (MSS). These rules are based on the assumption that local stress peaks as a result of nonuniform stiffness distribution in the joint are reduced by plastic load redistribution and can, therefore, be neglected for design purposes. However, to exploit the full potential of HSS, efficient design rules are required for the entire range of steel grades. Compared with MSS, HSS have a lower plastic redistribution capacity due to strain hardening caused by the manufacturing process. Due to a lack of experimental evidence, this behavior is currently taken into account in the design rules by means of simplified reduction factors resulting in conservative structural designs. This paper presents an experimental study on K-joints made of HSS and reports on the reliability of the existing reduction factor-based structural design for steel grades higher than S355. Nineteen large-scale tests on K-joint were carried out, considering circular and square hollow sections with different diameter ratios and chord preloading. The main failure mode obtained in all tests was a through-thickness crack of the chord on the side of the tension brace, referred to as punching shear failure. The test results showed that the reduction factors for HSS facilitate reliable and conservative structural designs. However, amending the design rules is recommended to fully exploit the potential of HSS. To achieve this, the application of innovative damage-mechanics models for numerical modeling of material failure will extend the scope of joint configurations in the future.