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Tubular space trusses for bridge applications use thick-walled tubes. The reduction in fatigue resistance due to geometrical size effects is thus an important issue. In order to carry out a thorough study, both fatigue tests on large-scale specimens and advanced 3D crack propagation modelling were carried out at ICOM/EPFL. The study is limited to circular hollow sections (CHS) K-joints. An alternate current potential drop (ACPD) system is used to measure crack depth on nodes of the tested truss specimens. The results obtained from the tests are given in the paper in terms of S-N data, crack depth versus number of cycles and deduced crack propagation rates. The numerical model was developed using the dual boundary elements method (DBEM), software BEASYTM, and was validated with fatigue tests data. The stress intensity factors (SIF) along the doubly curved crack front at different crack depths were obtained.With this model, a parametric study investigates the influence of geometry, size and load case on fatigue life. The results of both proportional and non-proportional sizing effects on fatigue strength are presented. The paper shows that size effects (proportional and non-proportional) can be expressed as a function of the non-dimensional parameters and chord thickness. Keywords: Structural steel, fatigue, size effects, large-scale tests, boundary element method.