Combined Hot-Spot Stress Procedures for Tubular Joints

An alternative procedure for predicting the combined hot-spot stress (CHSS) at tubular K and Y joints under combined branch loading is presented. The procedure makes use of influence factor (IF) equations developed, as a function of the joint geometry and branch loading, for various potential hot-spot locations on the branch and chord sides of the weld. The CHSS is obtained by lineraly superimposing, at a point, the effects of the axial force and bending moments acting on each branch. The resulting CHSS, therefore, reflects location, orientation and sign of each branch load contribution. Comparisons of predicted CHSS obtained via the new and other procedures to stresses from finite element analyses were made on a large sample of joints. Result show that (1) the new procedure is substantially more reliable than the other procedures studied, (2) none of the procedures consistently predicts conservative CHSS values, and (3) the overriding factor influencing the accuracy of the CHSS calculations appears to be the accuracy of the parametric equations. Although a better stress predictor can be expected to yield more reliable fatigue damage estimates, damage calculations will exhibit broad scatter due to the power function relating damage to stress. Unfortunately, further improvements in themore » accuracy of CHSS based on parametric equations are not likely to be easily achieved, given the large number of variables and locations that need to be considered.« less