Material Factors for High Strength Rectangular Hollow Section X joints

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An increasing trend is occurring in the use of high strength steel (HSS) in tubular joints. Reason for this trend is the high yield strength of HSS which leads to reduced weights and costs. Characteristic for HSS is the reduced ductility in comparison to normal steels. Therefore, Eurocode 3 part 1-8 (2020) recommends material reduction factors (Cf) for the design of joints made of steel with a yield strength higher than 355 MPa. The standard includes steel grades up to S700. The material reduction factors need to be validated to ensure efficient design of tubular HSS joints. This research intents to investigate the behaviour of X joints made from different kind of steel grades. The goal is to investigate if the material reduction factors for HSS, proposed in the new version of the Eurocode, are justified. Coupon experiments are conducted to determine the material properties of the X joints. Two geometrically identical X joints with varying steel grades of S355 and S500 are experimentally tested. The conducted X joint experiments, which are evaluated by digital image correlation (DIC), provide the necessary evidence to understand the behaviour of the failure mode and the fracture strain. Finite element analysis confirms the experimental evidence. The results from both the experiments and finite element analysis are used to validate the Eurocode design resistance. Furthermore, a parametric study is executed to investigate the influence of the material properties and the brace width over chord width ratio on the numerical resistance of X joints. This results in a total of 39 different X joint models that are included in this research. Lastly, the obtained numerical resistances are compared to the predicted design resistance according to Eurocode.