Fatigue of Diaphragm-Rib Joints in Orthotropic Steel Bridge Decks: A Nodal Force-Based Framework

Diaphragm-rib (DR) welded joints in orthotropic steel bridge decks are complex structural details undergoing both proportional and non-proportional multiaxial stress states under traffic loading. A nodal force-based framework is introduced in this paper to determine the equivalent structural stress components and corresponding multiaxial fatigue damage assessment methodology is demonstrated by a case study. The most critical DR joints of a long-span suspension bridge are considered for this case study. Potential fatigue crack mechanisms are suggested. Three assessment methods are implemented: nominal stress, hot spot stress and the equivalent structural stress (ESS) adopted by the ASME Div Code. The nodal force-based framework is used to determine the equivalent structural stress components along the four potential crack planes. The Path Dependent Maximum Range (PDMR) cycle counting method is implemented to account more accurately for the stress path dependency of the fatigue accumulation process. The cumulative fatigue damage estimated based on nominal stress approach is the lowest of the three methods. The hot spot stress method identifies limited levels of fatigue damage. The ESS combined with the PDMR cycle counting method reveals the largest damage indexes of the three approaches, thereby illustrating the importance of accounting for the presence of non-proportional multiaxial stress states in the fatigue evaluation of DR joints.