# Fatigue Reliability Analysis of Rib-To-Deck Joints Using Test Data and In-Situ Measurements
## 1 Introduction
Taking the 8 mm-thick U-rib as an example, which is commonly used in OSDs, the edge thickness can be increased from 8 to 12 mm (i.e., 50%) when using TEUs. Besides, a smooth transition is generated at the edge of TEUs during the rolling process, as shown in Figure 1b. As a result, no further work is required to create a level for welding, and the stress concentration could also be ameliorated in the RD joint. As shown in Figure 1b, the U-rib is in closed shape, making it very difficult to perform welding works from the inside. Thus, the single-side welding is usually utilized to perform welding from the outside of U-ribs, with a desired penetration rate around 75% to 80% to avoid the negative effect of melt-through [1; 13; 14]. Figure 1: Illustration of thickened edge U-ribs: (**a**) fabrication process and (**b**) view. In the both-side fillet welding technology, the rib wall is welded to the deck plate from both the inside and outside, with the application of a small inner welding machine customized for RD joints only. Intuitively, the fatigue strength of RD joints can be enhanced by the both-side welding since the integrity is improved. However, fatigue test data are still required to prove the effectiveness and feasibility of the both-side welding since little data are currently available. Besides, an additional fatigue-critical point is introduced by the both-side welding, i.e., since an inner weld toe will be formed inside the U-rib [11]. As a result, some negative effects may be imposed on the fatigue strength and is due to be verified by fatigue tests. Unlike the above two methods, in which the target is to improve the fatigue strength directly, the composite deck technology tries to reduce the vehicle-induced stress ranges in RD joints by introducing a layer of ultra-high-performance concrete (UHPC) [15] above the deck surface. The UHPC layer is connected to the deck plate by shear keys, usually shear studs, through which the layer can work together with the deck against the external loads [16]. According to the test data [17], the stress ranges can be significantly reduced in RD joints in composite decks, i.e., about almost 80 percent. However, a new fatigue-critical detail was also found in the composite deck, i.e., the welded joint between the studs and the deck plate. The stud-to-deck joint is found to be highly prone to fatigue failure due to the high-level stress range induced by the applied cyclic loads. After the cracking of studs, the stress range in RD joints would increase to almost the same level as the joint in traditional decks without the UHPC layer. When the loading was continued, a very similar failure model was observed in RD joints due to the rebounded stress ranges. Based on the above analysis, the application of TEUs seems to be both applicable and promising in enhancing the fatigue performance of RD joints. A series of studies was performed on the fatigue strength of RD joints using TEUs through the fatigue tests on full-scale rib-to-deck specimens [10; 18]. According to the test data of 11 specimens, the weld toe-to-deck cracking pattern is observed. The result also suggests that the application of TEUs can notably enhance the fatigue strength of RD joints. Besides, a further experimental study was carried out on two full-scale OSD segment specimens, including one using conventional U-ribs (CUs) and one using TEUs [19]. A similar fatigue failure model of toe-to-deck cracking was observed in both the two specimens, while the fatigue life of rib-to-decks in the CU specimen and the TEU specimen were 4.19 and 8.59 million cycles, respectively. Based on the above fatigue test, the effectiveness of TEUs was proved in enhancing the fatigue performance of rib-to-deck joints. However, due to the limitation in the number of tested specimens, a practical probability-stress-life (P-S-N) curve has yet to be established through statistics analysis on a sufficient set of test data. Another essential work about the TEU is to validate its effectiveness in actual engineering projects.