Research On Fatigue Behavior Of Diaphragms For Orthotropic Decks Of Steel Bridge Structures

With the implementation of the China-proposed One Belt and One Road Initiative(i.e.the Silk Road Economic Belt and the 21st-Century Maritime Silk Road Initiative),the construction of traffic system,traffic flow and traffic volume in Eurasia keeps growing.Therefore,the demand for designing and constructing new bridges,as an indispensable part of the land transport system,will be significantly increased.Due to their low dead weight and high bearing capacity,orthotropic steel decks(OSD,hereafter)have been widely used in modern steel bridge.However,the fatigue problem caused by repeated traffic loads on OSDs cannot be overlooked.The most serious fatigue problem that influences the application and development of OSDs is fatigue cracking in diaphragms.This paper investigates the fatigue behavior of diaphragms for OSDs through theoretical analysis,laboratory and field tests and numerical modeling,from the viewpoint of fatigue performance of steel,fatigue analysis method and vehicle load spectrum.The contents and results of the investigation deals with the five aspects as follows:(1)Fatigue tests are carried out on steel Q345 qD,which is commonly used in steel bridges in China.Based on the fatigue tests on all the smooth standard specimens made of steel Q345 qD,its strain-life curve(?-N)is obtained.Coffin-Manson Formula of steel Q345 qD is established;therefore,fatigue life of the steel is determined,providing an important basis for accurate fatigue assessment of OSDs.(2)Field static tests are carried out on an OSD in the example bridge under one test vehicle,and characteristics of stress distribution of diaphragms for OSDs is recorded.By using finite element software ABAQUS,a numerical model of steel box girders is set up to simulate the tests.The calculated results show good agreement with the experimental results.Thus,the stress behavior of diaphragms for OSDs under vehicle load and the influence of loading area on stress distribution of diaphragms are well understood.(3)Three prototype scale specimens(i.e.specimens in 1:1 geometric scale factor)are designed and made based on the actual diaphragms in typical OSDs,on which static and fatigue loading tests are carried out later on.The results of static loading test show that the local stress distribution of the diaphragms of the specimens is consistent with that of the corresponding field diaphragms.The characteristics of fatigue cracking,in diaphragms for OSDs,are identified by the analysis of the test results.It is found that fatigue cracks are only observed at the edge of the cut-outs in diaphragms under U ribs,penetrating generally in horizontal direction.No fatigue cracks are observed in diaphragm to rib joints or welding holes in diaphragms,through which U rib to deck plate joints pass.(4)Local Strain Approach and Theory of Critical Distance are used to assess the fatigue life of the prototype scale specimens through finite element numerical modeling of diaphragms for OSDs.It is seen that the prediction accuracy of both approaches is acceptable after comparing with test results.For Theory of Critical Distance,recommendations are given for the selection of fatigue parameters and analytical methods adopted.S-N curve for the cut-outs in diaphragms under U ribs in OSDs based on Theory of Critical Distance is also presented in this paper.(5)Traffic surveys are performed on the first grade highway to which the example bridge belongs.Eleven groups of vehicles axle weight,axle-bases and frequencies of occurrence are evaluated.The load spectrum composed of five model vehicles is set up on the principle of equivalent fatigue damage.This spectrum can be better applied to first grade highways,on which large trucks and trailers account for a large proportion of traffic flow.Based on the results of finite element analysis on steel box girders,the stress spectrum of local diaphragms is estimated.On the basis of S-N curve of the cut-outs in diaphragms under U ribs in OSDs established in this paper and simplified stress spectrum,fatigue assessment is done to diaphragms for OSDs,which give an important reference to fatigue assessment of OSDs of the same type.