## 3 Materials and Methods
### Preliminary Design
#### 3.3.1 Materials and Member Selection
There are four types of steel sections employed in this project, including universal beams (UBs), Universal Columns (UCs), Welded Beams (WBs), and Welded Columns (WCs). The specific material and section properties are based on InfraBuild's available products [96]. Part of the welded section properties refer to a previous version of the product file prepared by OneSteel (older name of InfraBuild) [97]. Moreover, Members of IABSE Working Group 6 [32] recommended the use of high-grade steel because the strength can be increased by around 30% with a minor rise in cost (around 5%) without any influence on fatigue design, and lighter self-weight is preferred for the BCE. Thus, welded steel sections with Grade 400 and universal steel sections with Grade 350 are considered in this project. For the transverse members in each structural component, though they are designed to act as trusses, the bending capacity is still considered for those structural members, given the horizontal arrangement of those segments. \begin{table}
\begin{tabular}{c c c c} \hline
**Component** & **Structural Member** & **Beam Type** & **Initial Member Size** \\ \hline \multirow{8}{*}{Secondary Girder (Sub-girder)} & Truss Top Chords & Truss & 410 UB 59.7 \\ \cline{2-4}  & Truss Bottom Chords & Truss & 410 UB 59.7 \\ \cline{2-4}  & Truss Bracings & Truss & 250 UB 37.3 \\ \cline{2-4}  & Truss Verticals & Truss & 250 UB 37.3 \\ \cline{2-4}  & Bracings & Truss & 410 UB 59.7 \\ \cline{2-4}  & Top Transverse Bracings & Truss & 250 UB 37.3 \\ \cline{2-4}  & Web Transverse Bracings & Truss & 250 UB 37.3 \\ \cline{2-4}  & Bottom Transverse Bracings & Truss & 250 UB 37.3 \\ \hline \multirow{8}{*}{Wing} & Truss Chords & Truss & 410 UB 59.7 \\ \cline{2-4}  & Truss Sracings & Truss & 250 UB 37.3 \\ \cline{2-4}  & Truss Verticals & Truss & 250 UB 37.3 \\ \cline{2-4}  & Bottom Transverse Bracings & Beam & 410 UB 59.7 \\ \hline \multirow{8}{*}{Substructure} & Hanger Truss Chords & Truss & 310 UB 46.2 \\ \cline{2-4}  & Hanger Truss Bracings & Truss & 250 UB 37.3 \\ \cline{2-4}  & Hanger Transverse Bracings & Cut-off Bar & D = 100 mm \\ \cline{2-4}  & Suspension Bars * & Cut-off Bar & D = 100 mm \\ \cline{2-4}  & Platform Primary Beams & Beam & 410 UB 59.7 \\ \cline{2-4}  & Platform Secondary Beams & Beam & 410 UB 59.7 \\ \cline{2-4}  & Sub-truss Chords & Truss & 250 UB 37.3 \\ \cline{2-4}  & Sub-truss Bracings & Truss & 250 UB 37.3 \\ \cline{2-4}  & Sub-truss Verticals & Truss & 250 UB 37.3 \\ \hline Modified Members & Girder Truss & Truss & 610 UB 125 \\ \hline \end{tabular}  
*Suspension bars employ AISI 4140 high-tensile-strength round bar with tensile strength range of 850 to 1000 MPa and available diameter from 8 mm to 690 mm.  
\end{table} 
Table 3: Cont. Figure 40: Excel spreadsheet for calculating member capacity under axial tensile forces complying with AS 4100: 2020. Figure 39: Data input page of Excel spreadsheet for calculating member capacity. Figure 41: Excel spreadsheet for calculating member capacity under axial compressive forces complying with AS 4100: 2020: (**a**) steel section properties; (**b**) calculation process. Apart from the results of the member capacity calculation, which can determine the use of universal or welded sections, a general selection strategy is derived considering the special form and structural behaviour of the OMSS. For normal building structures, UBs and WBs are often used as beam members, and UCs and WCs are used as column members. Beam sections can provide greater rigidity to resist deflections and have a stronger bending capacity about the principal axis. Column sections have a remarkable buckling resistance capacity and a more balanced bending capacity in both the principal and minor axis, making them ideal for axial forces and for large bending moments in both axes. When designing the member size for an OMSS, the choice among those steel sections depends on balancing structural requirements and practicality, which relies on the results of the forces and moments on each structural member from the FEA.