| The orthotropic steel decks(OSDs),as composed of longitudinal ribs,transverse diaphragms/ floor beams and top decks,have the advantages of light weight,large integral rigidity,high bearing capacity,good seismic performance,and rapid construction,and therefore are widely used in highway bridges.Recently,more applications of OSD have been intorduced into railway bridges.However,due to significant differences of weled details,sufrace wearing,and vehicle loads between highway and railway bridges,the fatigue behaviors of railway OSDs have not been revealled and thus are worthy of investigating.By employing Nanjing Da Sheng Guan Yangtz River Bridge as the research object,this paper carried out the fatigue tests and numerical analyses for the welded connections in railway OSDs.The contents and the achievements are as follows:(1)The finite element model of railway OSD segment was constructed by using both solid elements and shell elements,and the stress analysis was carried out accordingly.Three types of welded connections/details named Stringer-Deck-Floor beam(SDF),Stringer-Deck(SD),and Rib-Deck(RD)were found to be the regions most prone to fatigue cracking.Subsequently,individual models of these three details have been established for the refine analysis,based on which the boundary conditions,load cases and critical hot spots of these joints were determined in order to provide foundamentals tothe fatigue tests.(2)Totally 12 specimens,including 2 SDF joints,4 SD joints and 6 RD joints,were fabricated for the testing.Static loading,where centric and eccentric load cases were both considered,was first carried out to measure the stresses near the weld toes and to obtain the hot spot stresses at weld toes by using the extrapolation method.The high-cycle fatigue loading was subsequently conducted.The fatigue load ranges were determined based on the measured hot spot stresses as well as the selected hot spot stress ranges.The loading frequencies vary between 2 Hz and 4 Hz,depending on the real vertical displacement ranges of the actuator,and the load ratios are around0.2.(3)Static test results show that quadratic stress distributions were observed within the extrapolation regions of SDF joints,while linear stress distributions were commonly found in the SD and RD joints.Fatigue test results shows that,for the SDF joints,three cracks initiated at the stringer flange to floor beam web weld toes on the tensile side of the floor beam,and the joints finally failed due to rigidity degradation caused by through-thickness cracks.For the SD and RD joints,fatigue cracks first initiated at weld toes on the bottom surface of deck plates and then extended mainly in their longitudinal direction until the crack tips reached the deck plate edges.Most crack lengths,crack depths and vertical displacements of all specimens remain nearly constant or change slightly during the early stage of the test and then increase rapidly after critical numbers of load cycles.The formulae of crack propagation rate have been proposed based on numerical fitting of experimental data.(4)By comparing the measured fatigue lives with the S-N curves provided in the IIW fatigue design recommendation,the FAT 100 curve could be used to predict the fatigue lives of SDF and RD joints.While the lower FAT 90 curve could also overestimate the fatigue lives of fillet welds between stringers and decks in the SD joints investigated,which suggests that fulljoint pennetration(FJP)or 80% partial joint pennetration(PJP)groove welds,as have been used in RD joints,should be adapoted in these cases.The research findings can provide scientific basis for the fatigue stress analysis and fatigue life estimation of orthotropic steel decks in railway bridges. |
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