## 7 Conclusions  
In this paper, a fatigue assessment has been performed for the first time on a novel orthotropic steel deck (OSD) with thickened edge U-ribs (TEUs) based on a stochastic traffic model. Through fatigue tests, the effectiveness of TEUs has been validated in enhancing the fatigue strength of rib-to-deck (RD) joints in OSDs. However, more investigations are still urgently required on the exact fatigue life of the joints in order to further validate the application of TEUs in engineering practices. On this end, comparative studies are conducted to determine the fatigue life of the RD joints in the OSDs with and without TEUs. A typical OSD bridge in China is selected as the prototype for the investigation. Since the vehicle load is the dominated factor inducing fatigue damage in OSDs, a stochastic traffic model is proposed and employed to highlight the uncertainties in vehicles. The framework of the stochastic model is idealised by considering both the vehicle properties and their lateral distribution. Then, the model is first instantiated through the standard truck models in the design codes, including the JTG (China), Eurocode 1 (Europe) and AASHTO (America). Further, measurement-based observation data have also been applied to construct the stochastic model, which can provide a better insight into the differences between, and within vehicle groups. At the same time, the multi-scale finite element (FE) model is established for the prototype bridge to transform the traffic model into the stress spectra required in fatigue evaluation. Influence surface method is used along with the FE analysis, which could improve the computational efficiency by almost 50,000 times. Monte Carlo simulations are then carried out to derive the stress spectra of RD joints at different lateral positions. Using the generated spectra, the influence of lateral positions has been illustrated in detail, including the lane type (fast, middle and slow) and in-lane position. Finally, based on the test data and derived spectra, a fatigue life assessment has been made for the RD joints in the OSDs with and without TEUs. According to this innovative study, the following conclusions could be drawn:  
1. The four vehicle models are compared in terms of the critical stress history, including the standard trucks in the three codes and the six-axle critical truck in the observation model. The result shows that the standard truck in AASHTO will lead to the most conservative outcome, followed by the critical truck in the observation model, and then the standard trucks in the JTG, and at last the Eurocode 1. It is worth noting that, when using the AASHTO model and observation model, a notable stress range can be induced by the front axle, which is not negligible in fatigue evaluation. 2. According to the solved spectra, the vehicle-induced stress ranges are widely distributed, illustrating it is oversimplified to describe the fatigue action by the equivalent stress range calculated with the deterministic truck model. Meanwhile, through a comparison between the spectra, it is proven that the observation model can bring an in-depth insight into the randomness in traffic since the model takes into consideration not only the randomness in lateral position but also the difference within and between vehicle groups. 3. The RD joints at various positions have been compared in terms of the stress spectra. Due to the lane preference, the proportion of heavy trucks is higher in the middle lane and the slow lane than in the fast lane. As a result, the proportion of high-level stress ranges is also higher in the spectra of the RD joints within the former two lanes. Meanwhile, comparisons are also made between the RD joints within the same lane. The result exhibits a remarkable difference in the stress spectra of the RD joints within the same lane, which is induced by the in-lane distribution of vehicles. 4. Based on the test data and the derived spectra, the fatigue life of RD joints at various locations can be determined.