Fatigue Performance Research Of Diaphragm Cutout On Orthotropic Steel Bridge Decks

Based on the base-metal fatigue cracking phenomenon of diaphragm cutout on an orthotropic steel bridge deck(OSBD), Finite element method and field measurement are both applied to explore the fatigue property of structural detail in the paper. Main contents and conclusions are stated as follows:Firstly, multi-scale finite element models are set up and loaded with standard fatigue truck in AASHTO LRFD specification. Then stress response of diaphragm cutout under the moving wheel loading on the deck is obtained, and fatigue life assessment is carried out via nominal stress method as well as hot-spot stress method, respectively. In addition, the influences of diaphragm thickness on structural detail stress are also researched. It is discovered that the influence line length of the diaphragm cutout in bridge longitudinal direction equals to the distance of two times diaphragm space, and loaded with only middle or rear axle is acceptable for the detail, resulting in two or three stress cycles produced at cutout under the passage of an AASHTO standard fatigue truck. It is also found that the nominal stress is hard to be determined at the cutout due to significant stress concentration and serious difference in various nominal stress positions, hence hot-spot stress according to the S-N curve of FAT140 in IIW is preferred to assess its fatigue performance, and stress at the place 6mm away from cutout edge can also be extracted as nominal stress. Since diaphragm thickness presents notable effects on cutout stress, right chooses on them are justified. It is concluded that the bridge cutout cracking is due to the thin diaphragm and high truck volume, while poor control on fabrication, large residual stress among the detail may also contribute to cracking.Secondly, cutout shape parameters, such as diaphragm cutout types and cutout heights, are studied to compare the stress at the welding of diaphragm-to-rib connection and the base metal of diaphragm cutout. Some conclusions can be used as a reference for later design in steel bridge.Finally, the nominal stress time-history curve of structural detail is obtained by field measurement. Rain-flow Counting Method and Miner Accumulated Fatigue Damage Rule are used to get equivalent stress range of diaphragm cutout. Then, fatigue life of structural detail is assessed by S-N curves in AASHTO LRFD specification, and some factors to fatigue cracking of cutout are pointed out.This paper mainly employs FEM to analyze the causes of cutout cracking, meanwhile, the data from field measurement is servered as contrast, and fatigue life of structural detail is obtained. Moreover, the design parameters of diaphragm cutout shape are also investigated, and some related recommendations are proposed for bridge construction and maintenance.