Higher strength steels (HSS) hollow sections with steel grades from S460 up to and including S700 are becoming increasingly competitive in long span structures. Reduction of self weight is accomplished with the smaller wall thickness of the hollow section, leading to fabrication, transport and execution benefits. The use of HSS has a positive effect on the CO2 equivalent emission, which is aligned with the European Green Deal for the future development of the construction sector. Lack of experimental evidence is identified as one of the main reasons to propose rather pessimistic material factors in the revised version of EN1993-1-8 [5] for design in tubular joints. The revised version, published in 2020, recommends material reduction factors for the design of joints made of steel with yield strength larger than 460 and up to 700 MPa, in the range from 0.9 to 0.8. In this thesis, the behaviour of the X-joints made of steel grade S355, S500 and S700 are investigated experimentally and numerically, and the proposed material reduction factors are discussed. Five full-scale welded X-joints in tension with rectangular hollow section were tested Stevin Lab II, TUD. Additionally, base material and butt-welded coupon specimens are tested to obtain the engineering stress-strain relationship of the base material, weld and heat affected zone (HAZ). Based on the results of coupon tests, finite element software ABAQUS is used to model X-joint and to supplement the limited number of experiments. Furthermore, result of a parametric study is presented in which effects of three parameters: the yield strength, the parameter β (ratio of the width of the brace member to that of the chord member) and the thickness of the chord are investigated to analyse their influence on the structural performance of X-joints. Following properties are thoroughly examined: static strength, stiffness, ductility and failure mode. Finally, conclusions are drawn on the validity of the material reduction factors.

A welded rectangular hollow section (RHS) X-joint exposed to tension loading has three typical fracture-related failure modes: Punching shear failure (PSF), Brace failure (BF), and Chord side wall failure (CSWF). Prediction of these failure modes by finite element (FE) simulations requires modelling of the material damage. An appropriate damage model accurately predicts the behaviour of the fracture zone and provides the necessary information to improve design rules for welded high-strength steel (HSS) RHS X-joints based on parametric studies using validated model. In this paper, the parameters of the Gurson-Tvergaard-Needleman (GTN) damage model are calibrated for the base material (BM) and the heat-affected zone (HAZ) of butt-welded cold-formed RHS connections, no fracture appeared in the weld. A computational homogenisation analysis is carried out using representative volume element (RVE) models to calibrate the pressure-dependent yield surface parameters of the GTN damage model, considering the different combinations of the accumulated initial hardening strain and the void volume fraction (VVF) due to a varying stress triaxiality. The critical and final VVFs are calibrated against tensile coupon tests. Finally, the GTN damage models calibrated for BM and HAZ are used in the fracture simulation of nine welded cold-formed RHS X-joints in monotonic tension. The FE model successfully predicts the experimental load-displacement relationships and fractured zone, indicating the calibrated GTN models could effectively be used in parametric study of welded cold-formed RHS X-joints. Finally, possible improvements to the used FE model are outlined for future studies.@en