## 2 NFTBT Finite Element Model Establishment  
### Finite Element Model Parameters  
This research used ABAQUS software to establish a finite element model of the NFTBT's structure (Figure 4). The ABAQUS software is developed by the Dassault Group in France, and the software version used in this study is ABAQUS 2021, which is released in 2020. The rail is simulated by a beam element, and the filling is simulated by a Cartesian spring element [29]. Other components such as track slabs, concrete adjustment layers, and subgrade support layers are all simulated by solid elements [30]. The rail is considered a long elastic beam, allowing for displacement and corners, and the displacement constraints are given at both ends of the rail; the fasteners have been simplified to point-supported linear springs. Each track slab is designed according to nine groups of fastener nodes, and the rail below the fastener node slab is coupled to a fastener node, then connected to the rail node with a vertical spring element (Figures 5 and 6). Frictional contact is applied between the track slab and the cast-in-situ concrete adjustment layer, the friction factor is 0.3, and the binding constraint is set between the adjustment layer and the support layer [31; 32]. The basic parameters of the track structure selected in the calculation are shown in Table 1.  
Figure 4: Three-dimensional view of the optimized design of the track slab. Figure 5: The finite element model of the NFTBT’s structure. Figure 6: The model of rail and fastener system: (**a**) Side view; (**b**) Main view. During the running stage, the NFTBT's structure mainly bears the load of the tram, and its own weight. In the design process, the additional load of the track structure caused by the uneven settlement of the subgrade should not be included in the design load of the track structure for structural calculation, but it should be regarded as an abnormal load with a small probability of occurrence in the track structure; thus, it can be considered to be a checking condition in the design of the track structure [33]. Considering the influence of the unevenness of the subgrade under the track, the wavelength of the uneven settlement of the track is 20 m, and the amplitude of the uneven settlement is 10 mm, and five track slabs have been selected for modeling and analysis. The rail is simulated by beam element B31, and the elastic modulus is shown in Table 1. The wrapping material is modeled with the linear spring element CARTESIAN. The track slab, the concrete adjustment layer, and the support layer are all simulated by the solid element C3D20R. The tram load is simulated by multiple concentrated forces and loaded onto the track, and the spacing is set to a fixed wheelbase of 1600 mm. Among them, since the vertical static wheel load is 62.5 kN and the dynamic coefficient is 1.67, the concentrated load value is set to 104.375 kN [34]. Since the uneven settlement of the subgrade is an uneven vertical deformation along the longitudinal direction of the track, there are many forms of settlement of the subgrade. The models for simulating the uneven settlement of the subgrade include cosine, staggered, and angled models [35]. When the ballastless track is located on the roadbed, the uneven settlement of the roadbed will cause the ballastless track to generate additional force. The sine (co)sine settlement curve mainly occurs on the roadbed. Therefore, considering the influence of an uneven settlement on the ballastless track, this study will mainly focus on the sine and cosine-type irregularities. Thus, the commonly used sine-type half-wave curve is selected for this study [36].