Experimental Investigation of Failure Modes and Strength of High-Strength Steel CHS K Joints

When tubular joints are fabricated from high-strength steel, their design resistance, calculated from a resistance equation, should generally be reduced by applying the material factor (joint strength reduction factor) and/or the upper limit on the yield stress used in the calculation (such as fy<0.8fu). Although design rules for high-strength steel tubular joints, including certain strength reduction measures, have been introduced by design standards and guides, validation of these rules has not been thorough because of the lack of available test data. This study notes that test data are particularly scarce on high-strength steel circular hollow section (CHS) K joints. Therefore, laboratory tests are conducted for CHS K joints with a nominal yield stress of 460 MPa to investigate their ultimate static behavior and evaluate relevant design rules according to the recently revised Eurocode 3 (EN 1993-1-8:2024). Notably, the testing program incorporates not only the basic joints where both the chord and branches are made of high-strength steel, but also the “hybrid” joints, for which high-strength steel is applied only to the chord while conventional steel is used for the branches. All tested joints, encompassing both basic and hybrid types with gap or overlap configuration, exhibit experimental strengths exceeding the Eurocode 3 design resistance with a sufficient margin of safety, supporting the suitability of the codified design method for practical use. However, a more in-depth analysis of the Eurocode 3 provisions reveals that, for fracture failures observed in high-strength steel K gap joints, relevant design limit states are not available or fail to provide adequate prediction for the fracture load. Therefore, the necessity of considering the stress nonuniformity (or the “effective width”) is emphasized to handle fracture failures more appropriately. Regarding hybrid use of material, replacing branch members with mild steel is shown to be particularly beneficial for the K gap configuration.