Formulas for Stress Concentration Factors in T&Y Steel Tubular Joints Stiffened with FRP under Bending Moments

Stress concentration factors (SCF) of steel tubular T/Y-joints strengthened with different types of fiber reinforces polymer (FRP) materials were studied thoroughly under the action of in-plane bending (IPB) and out-of-plane bending (OPB) moments. A comprehensive FE study was carried out through examining different elements for steel substrate and FRP material along with the corresponding contact modeling to attain reliable results substantiated by existing SCF experiments. Shell-to-Solid contact using the node-sharing technique showed the best performance in conformity with the SCF experimental results. Such benchmark modeling was substantiated with additional verifications against the available experimental data on SCFs in T, Y, and K connections. Numerous FE models were then developed to put the most affecting FRP and joint parameters into perspective, while previous studies lack addressing all in one go. It was found that the geometry of the connection has a significant influence on the level of FRP effectiveness. For instance, the sensitivity of SCF in a Y-joint to FRP strengthening is less than a T-joint with similar geometry, due to the different brace-to-chord angles. Justified with analytical interpretations, cumulative effects of all parameters were carefully evaluated to derive SCF formulas for FRP strengthened tubular T/Y-joints under both IPB and OPB moments. Their accuracy and applicability were checked with the well-known statistical indices, and it was seen that the correlation of coefficients was higher than 95%. Moreover, the proposed SCF formulas meet all the acceptance criteria of the Fatigue Guidance Review Panel.