# Fatigue Reliability Analysis of Rib-To-Deck Joints Using Test Data and In-Situ Measurements
## 5 Conclusions  
In this paper, the fatigue performance of rib-to-deck (RD) joints in orthotropic steel decks (OSDs) was evaluate in a probabilistic way, using the fatigue test data and in-situ measurements. Two types of OSDs are investigated, including the OSD using conventional U-ribs (CUs) and the OSD using thickened edge U-ribs (TEUs). Fatigue tests were first carried out on the rib-to-deck specimens. Based on the test data of 18 specimens, the probability-stress-life (P-S-N) curve was derived for RD joints in the OSD using CUs and the OSD using TEUs, respectively. Then, an OSD bridge in China was selected as the prototype for further analysis. Through the stochastic traffic model, the in-situ measurement data was employed to derive the stress spectra of the critical RD joints, along with the multi-scale finite element (FE) model of the prototype bridge. After that, an analytic solution-based method was applied to calculate fatigue reliability of the critical joints based on the test data and derived stress spectra. Finally, comparisons have been made between the RD joint in the OSD using CUs and the one in the OSD using TEUs, in terms of the fatigue reliability. According to the above analysis, the following conclusions can be drawn:  
1. As per the fatigue test of rib-to-deck specimens, under the cyclic loading, the fatigue crack initiates at the deck toe and then propagates along the thickness and length of the deck plate until it penetrates through the deck plate and cause failure. The test result also proved the effectiveness of TEUs in enhancing the fatigue performance of RD joints. In the case of mean value, compared with the CU specimens, the fatigue strength of TEU specimens was 21.4% higher in nominal stress and 21.1% higher in hot spot stress. 2. Based on the test data of 18 specimens, P-S-N curves of RD joints were derived for the OSD using CUs and the OSD using TEUs, respectively. In the derivation, the statistics method proposed by IIW was employed after analysis, in which the survival rate of 95% is considered with the confidence of 75% for estimators. As a result, the FAT 90 and FAT 115 curves are suggested for the fatigue check of RD joints in the OSD using TEUs, in terms of nominal stress and hot spot stress, respectively. Meanwhile, for the RD joints in the OSD using CUs, the FAT 70 and FAT 90 curves are, respectively, recommended under the nominal stress and hot spot stress approaches. 3. A probabilistic framework was proposed to derive the stress spectra using the in-situ measurement data. In the derivation, a stochastic traffic model was applied to reflect the in-situ measurement of vehicles. Meanwhile, the multi-scale FE model of the selected prototype bridge was established to transfer the stochastic traffic into stress spectra. Through the large-scale sampling, the stress spectra of two critical RD joints were derived. 4. Based on the S-N approach, the limit state function of fatigue cracking was established. Then, investigations were then performed on the fatigue reliability of the two critical RD  
Figure 17: Reliability in terms of hot spot stress: (**a**) critical detail D1 and (**b**) critical detail D2. points selected, with the consideration of test data and in-situ measurements. The result indicated that the fatigue life of RD joints can be notably improved by using TEUs. For instance, under the target reliability of 2.3, the fatigue life of the two critical joints under the hot spot stress approach is increased by 153% and 155%, respectively. Meanwhile, the result also indicates the conservativeness of the hot spot stress approach over the nominal stress approach. Conceptualization, K.Z. ; Data curation, Y.Z. ; Funding acquisition, K.Z. ; Methodology, J.H. ; Supervision, K.Z. ; Validation, J.H. ; Writing--original draft, X.F. ; Writing--review & editing, J.H. and J.Z.