This paper reports the results of a systematic study of stresses in tubular Y and T-joints based on nearly 900 finite element analyses using thin-shell elements. These encompass a wide range of joint geometries under axial, in-plane and out-of-plane loading. For each mode of loading, and for both the chord and brace sides of the intersection, parametric equations were derived which describe the ratio of bending to membrane stress and stress concentration factor at key locations. Characteristic formulae were also fitted to the stress variation around the intersection By combining these three sets of equations the full stress distribution for a tubular Y or T-joint may be predicted from its geometric parameters and the mode of loading. Although cumbersome to set down on paper, when programmed on a computer the equations allow instant prediction of the stress distribution. This will enable more accurate fatigue life calculations to be performed on cracked tubular members using fracture mechanics.
The jacket structures of steel offshore platforms used for the extraction of oil and gas are composed of tubular joints welded together to form a threedimensional space frame. Such a structure is susceptible to localised fatigue failure at the welded intersections as a result of the high stresses in these regions combined with the large number of stress cycles experienced by the structure during its operational life as it is continually loaded by waves. These stresses may be considered to result from a combination of global and local effects. On a global scale, stresses are induced as a result of differences in the deformation behaviour between brace and chord, while the local stress contribution arises from small variations in surface topography produced by the notch effect at the toe of the weld.
