## 1 Introduction  
Orthotropic steel decks (OSDs) have been extensively applied in steel bridges since the reconstruction efforts following the Second World War due to their excellent performance such as light self-weight, short construction period, high loading capacity, and large redundancy etc. [1]. The application of OSDs in China could serve as an illustrative example. Since the 1990s, OSDs have been successfully applied in long-span bridges to reduce the self-weight [2]. Meanwhile, OSDs are also widely employed in urban bridges to speed up the construction process and minimize the disruption of daily traffic. For many cases, the OSD has become a crucial symbol of modern steel bridges. Further, the OSD is also believed as the only deck system that can provide an engineering service life of more than 100 years [3]. In the OSD system, the deck plate is stiffened by the longitudinal ribs and transverse floor beams in orthogonal directions. Welding joints are extensively used to connect the components, which in turns result in the fatigue issue. Under the persistent loading-and-unloading process by vehicles, premature fatigue cracks are frequently observed in the rib-to-deck (RD) joints in OSDs after several decades of operation [4]. In some extreme cases in China, the deck could be poorly designed and fabricated, while the truck traffic can be heavy and of dynamic conditions [5; 6]. Under this situation, fatigue cracks could even begin to initiate in the RD joints within only a few decades after construction [7; 8]. Thus, the fatigue issue is now of particular concern, and may hinder the further application of OSDs. Many attempts were undertaken by various researchers in dealing with this problem [9; 10; 11; 12; 13]. Among them, the thickened edge U-rib (TEU) has been proposed as a feasible way to enhance the fatigue strength of RD joints [13]. In TEUs, the thickness of the rib wall is remarkably increased at the edge, without a significant increase in the self-weight and fabrication costs. As a result, the allowable welding depth of RD joints could be improved a lot, which may in turns enhance the fatigue strength. A series of experimental studies [14; 15] has been carried out on the OSDs with TEUs, including the tests of rib-to-deck specimens and OSD specimens. The results suggested that the fatigue strength of RD joints could be significantly improved using TEUs, which in turns may prolong the fatigue life of the joints. However, more investigations are now urgently required on the exact fatigue performance of the RD joints in OSDs with TEUs, by considering the situation of real engineering applications. Generally, fatigue evaluation includes both fatigue strength (resistance) and fatigue action (loading effects) [16; 17; 18; 19]. In OSDs, the vehicle load is the dominated factor inducing the fatigue damage in welding joints [15]. In most of the design codes and engineering specifications, vehicle loads are represented by the standard truck model with deterministic configurations [20; 21; 22]. The stress history in the welding joints could be solved using the truck model. Through the cycle-counting method [23], the stress history can be transformed into the equivalent stress range, which is then used in fatigue evaluation to reflect the fatigue action. Although the above approach is concise in engineering practices, it is not comprehensive enough to describe the highly complicated fatigue action. Thus, measurement-based traffic data have recently been employed in fatigue evaluation. [PERSON] et al. [24] proposed a traffic model based on the measured data from a cable-stayed bridge in China. In the model, both the uncertainties in the vehicle and lane occupancy were considered. [PERSON] et al. [25] employed the measured traffic data to calculate the stress response of welding joints in OSDs. In the study, the in-lane lateral position of vehicles was considered, but the differences in lane occupancy and vehicle type were ignored. [PERSON] et al.