## 4 Results and Discussion  
### Parametric Modelling  
In this project, the parametric modelling of the OMSS was carried out using Rhino 8 with Grasshopper to provide a line model for numerical simulation, which is assumed to be capable of constructing a 110 m span cast-in-place box girder bridge. The overall design philosophy was borrowed from a BCE precedent, MSS M1-90-S [4]. Due to the special structural requirements of long-span bridge construction, efficient and flexible model construction was achieved through Grasshopper's parametric design approach, allowing complex structural components to be quickly defined and adjusted. Initial structural optimisation was carried out in Karamba3D to give the model an initial mechanical \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Ultimate Limit State (ULS)** & **Serviceability Limit State (SLS)** \\ \hline \hline \multirow{3}{*}{Load Combination} & 1.35\(G\)\({}^{*}\) & \(G+Q+0.8W_{s}\) \\  & 1.2\(G\)\({}^{*}+1.5Q\) & \\  & 1.2\(G\)\({}^{*}+Q+W_{u}\) & \\ \hline \end{tabular} \({}^{*}\) For considering the superimposed dead loads, a 30–40\(\%\) increase is applied.  
\end{table} 
Table 11: Determined load combinations for the structural design of MSS. adjustment to the geometry of the primary load-bearing truss, and the simplified linear model was then imported into Strand7 for in-depth finite element analysis. One of the significant advantages of this modelling approach is that the structural parameters can be adapted to different construction requirements rapidly, while ensuring a clear workflow for convenient component repositioning and design improvement. The model is divided into six structural modules, including the main arch, secondary arch, main girder, secondary girder, wing, and substructure. The primary and secondary girders form the horizontal load-bearing framework, sitting on the foundation and connecting the upper and lower structures to maintain the structural integrity. The primary and secondary arches act as the main vertical supports, similarly to a trussed arch bridge, providing the overall structural stiffness and stability to withstand the structural self-weight and other vertical loads. The wing can further assist the overall load-carrying capacity by redistributing the loads transferred from the bottom of the substructure. Hanger trusses and suspension bars, parts of the substructure component, can hold up the platform and create a stable environment for bridge deck construction. Through this modular division and parametric design method, the dimensions and positions of the components can be quickly adjusted according to the specific construction requirements, thus accurately and efficiently meeting the complex requirements of engineering practice in the design of the OMSS and enhancing the adaptability and operability of the model. Such a design strategy also allows the model to be prepared for subsequent structural optimisation and rigorous numerical analysis. The limitations of the model in practical engineering applications are mainly in two aspects. Firstly, the parametric model does not include the OPS technology, which is capable of actively adjusting the mid-span deflection of the structure to reduce deformations during the construction process [24]. Due to the neglect of the OPS design, the current model is unable to achieve the real-time control of deflections, thus limiting the simulation accuracy of the real force and deformation state of the whole structure. This is particularly important in long-span bridge construction, where insufficient deflection control may lead to stress concentration in the mid-span structure, influencing the moment redistribution and increasing member sizes. Secondly, although the model can be flexibly adjusted parametrically in Grasshopper and imported into Strand7 with prepared beam properties and model groups, there is still a lack of compatibility between the two software in setting up the model's mechanical properties.