# Fatigue Reliability Analysis of Rib-To-Deck Joints Using Test Data and In-Situ Measurements
## 1 Introduction  
The orthotropic steel deck (OSD) has been widely employed in steel bridges, along with the evolution more than half a century after it was first invented as a battle deck in the 1930s. Compared with other types of decks, the structural members in OSDs are highly integrated, leading to superiorities such as large loading-capacity, speedy construction, and light self-weight [1]. During fabrication, a considerable amount of welding works is applied to make up the complicated structural details in OSDs. As a result, fatigue cracking of welded joints becomes a very prominent issue in OSDs, especially when the truck traffic volume is high [2, 3]. Among the different types of welded joints, fatigue cracks are frequently observed in the rib-to-deck (RD) joint located between the U-rib and deck plate [4]. Two major factors can accounted for the phenomenon--the large amount of RD joints and the vehicle loads directly applied on the joints. As a result, cracking in RD joints will cause serious consequences, i.e., the damage to the pavement and even the failure of the deck system. Obviously, the cracking issue will hinder the OSD from the continuous service over a design life of 100 years [5]. Generally, two types of fatigue failure are frequently observed in RD joints, i.e., the weld root-to-deck cracking and the weld toe-to-deck cracking. In the case of weld root-to-deck cracking, the fatigue crack first initiates at the weld root on the deck side and then propagates into the deck plate. This type of cracking is often observed in the early-built OSD bridges, especially those erected in the 1990s and early 2000s. According to the studies [1; 6], the root cracking is mainly controlled by the penetration depth of U-ribs and the fit-up gap between the deck plate and U-rib before welding. When the fit-up gap is well controlled under 0.5 mm during the fabrication, root cracking can be prevented [7; 8]. Thus, the weld toe-to-deck cracking is now of particular concern, in which the crack initiates at the deck toe and then propagates into the deck plate. In recent years, extensive research efforts have been performed to solve the fatigue issue of RD joints, including the post weld treatment and innovations of structural details. The post weld treatment can be classified by the target of improvement, i.e., the weld profile and residual stress [9]. In terms of the weld profile, the grinding and TIG-dressing can be employed to generate a smooth transition between the weld and the base metal at the weld toe. As a result, the stress concentration can be ameliorated at the weld toe, which can in turn lead to the improvement in the fatigue strength. Another type of treatment is to improve the residual stress condition through peening methods, including hammer-peening, needle-peening, brush-peening and the ultrasonic impact treatment (UIT). As suggested by [PERSON] and [PERSON] [9], the above treatments can result in a significant enhancement in the fatigue strength. For structural steel with yield strengths less than 355 MPa, which is commonly used in steel bridges, the fatigue strength of the treated joint can be improved by 30% when compared with the as-welded joint. It is also worth stating that the application of post weld treatments is not against the application of novel structural details. As a result, further improvement can be expected in the fatigue strength of rib-to-deck joints. In the case of detail innovation, there are three representative novel technologies, including the thickened edge U-rib (TEU) [10], both-side fillet welding [11], and steel-concrete composite deck [12]. The concept of TEUs is to increase the thickness of rib walls at the edge through the continuous rolling method, as shown in Figure 1. The fabrication and labor costs will be just slightly increased in TEUs, since a standardized steel sheet is used with the special operation on the edge only. At the same time, the thickness available for welding in rib walls can be notably improved with the thickened edge.