Comparative Study On Fatigue Performance Of Two Types Of Diaphragm Cutout Shape On Orthotropic Steel Bridge Decks

Based on the base-metal fatigue cracking at diaphragm cutout on an orthotropic steel bridge deck(OSBD),both the stress monitoring and finite element analysis are applied to comparatively investigate the fatigue performance of two cutout shape on the OSBD.This paper will focus on the following aspec ts and the conclusions will be listed as follows:Firstly,in order to investigate the stress response characteristics and the extent of out-of-plane distortion at the floorbeam cutout detail under wheel loading,and compare the influence of cutout geometries on fatigue performance of various details of orthotropic deck,simultaneous stress monitoring around two types of cutout geometry,which are presented on two closely located floorbeam under the same traffic lane,are carried out on an orthotropic steel bridge under random traffic flow.Then stress spectrums at the cutout details are extracted and fatigue life are evaluated,followed by the finite element analysis with the measured fatigue truck on the bridge.It is found that stress response at the deta ils,such as the cutout,Rib-to-Floorbeam and Rib-to-Deck is significantly high.Similar to the Rib-to-Floorbeam weld,one axle group can produce only one stress cycle at cutout,but wheel loads could produce a separate and distinct stress cycle at the Rib-to-Deck for each single axle among the axle group.It is found that the stress time histories at the cutout detail are completely compressive under wheel loads.The out-of-plane stress at the cutout detail is significantly lower than the in-plane component.The maximum and equivalent stress range obtained at the new cutout geometry is larger than that at the original one,hence its evaluated fatigue life is shorter than the original cutout.It is concluded that,compared to the original cutout geometry,th e in-site fire-cut new cutout detail on the floorbeam cannot improve the fatigue performance of orthotropic steel bridge deck(OSBD)at the cutout detail.Then,for the two types of cutout shape,multi-scale finite element models are established,followed by comparative analysis of the stress response at the cutout,Rib-to-Floorbeam and Rib-to-Deck under the wheel load.It is found that the local stress effect at those details is significant,and the stress influence line at the two cutout shapes is short.Notable stress concentration appears at the cutout and welding toe.The stress gradient of the new cutout is smaller than the original one,but the total hot spot stress is greater than the original.The evaluated fatigue life of base-metal at cutout detail,according to the S-N curves of fatigue performance evaluation proposed by International Institute of welding of Fatigue Grade FAT140,is consistent with that observed on the bridge.And the evaluated fatigue life of new cutout is less than the original,which is consistent with the measured results.Finally,the influence of different cutout geometries on the structural details is analyzed by considering whether the weld at the connections simulated.It is found that the weld simulation has a certain inf luence under the near load on the cutout detail,while little influence on the details of RF-1 and RF-2.And it also show great impact on RF-3 structural detail,mainly because the weld toe has obvious stress concentration and gradient effect.In addition,the finite element analysis of the cutout types respectively proposed by specifications of AASHTO LRFD and Eurocode 3 are compared with the two types of cutout studied in this paper.It is concluded that the change of cutout geometry has an important influ ence on the cutout details.