## 4 Model Lightweight Algorithm  
### Choice of Optimization Parameters and Objective Function  
The double-layer steel truss bridge structure is complex and involves a large engineering volume. It is necessary to optimize the relevant parameters that affect the structural stress and engineering volume within a reasonable range while meeting the specifications. The improvement of the performance of steel is an important direction for the future development of steel bridges [21]. The elastic modulus of steel, an important parameter reflecting its mechanical properties, can be optimized within a reasonable range to improve the structural stress performance and deformation. The elastic modulus of various steels at room temperature is mostly between 1.9 \(\times\) 10\({}^{5}\) and 2.2 \(\times\) 10\({}^{5}\) MPa [22]. The optimization ranges of the elastic modulus of steel described in this article include those of the steel used in the upper and lower main girders of the steel truss in the project, as well as the steel used  
Figure 5: The verification of the calculation accuracy. (**a**) The maximum axial force of the upper chord under the design load. (**b**) The maximum axial force of the lower chord under the design load. (**c**) The maximum axial force of the web under the design load. (**d**) The maximum axial force of the upper stiffened chord under the design load. (**e**) The maximum axial force of the stiffened vertical bar under the design load. in the bridge deck, bracing chords, and vertical stiffening bars. The parameter optimization ranges are considered to be within \(\pm\)5%. Orthotropic bridge decks consist of bridge panels, transverse ribs, and longitudinal ribs. Studies have shown that appropriately increasing the thickness of the bridge panel can improve the stress state of the orthotropic bridge deck [23], but it will increase the engineering volume. Therefore, considering the bridge panel thickness as an optimization parameter has significant engineering significance, and the parameter optimization range is considered to be within \(\pm\)20%. In the double-layer steel truss bridge with upper bracing chords, the maximum length of the vertical stiffening bars is an important parameter for structural stress, and its optimization can provide guidance for practical engineering. The relative distance be-tween the double-layer main girders is mainly determined based on the structural stiff-ness and overhead clearance requirements. Its value affects the structural stress and steel consumption, so it needs to be optimized. The design parameters of similar structures are compared with the maximum length of the vertical stiffening bars and the relative distance between the double-layer main girders of the supported project [24; 25; 26], and the parameter optimization range is determined within \(\pm\)10%. Based on the preceding analysis, the final selection includes four structural parameters for optimization: the elastic modulus of steel, the bridge panel thickness, the max-mum length of the vertical stiffening bars, and the relative distance between the double-layer main girders. The parameters to be optimized and their ranges are listed in Table 1.  
To reflect the stress and deformation characteristics of the main load-bearing members in a top-stiffened steel truss bridge, several objective functions were selected for correction. These functions include the mid-span vertical deflection, the axial internal force of stiffened strings, the longitudinal internal force of the bridge deck bottom plate, and the axial internal force of the empty tube of the web bar under compression at the pier top.