## 4 Master Digital Model for a Suspension Bridge
### Creation of the Initial Master Digital Model  
#### 4.2.1 Bridge Alignment  
The bridge alignment is one of the main factors in the design process. It is also the most important task for bridge engineers in the early stage of the bridge project. A suspension bridge consists of thousand members and their structural complexities. At the starting time of the design task, only the most needed information is selected; therefore, the proportions and forms of the bridge alignment can be better appreciated. The bridge alignment not only gives a clearer feel for what the target configuration of the bridge model will be but, on the other hand, should also create a premise for the flexibly use of the design analysis model and also construction geometric control later. The simpler the bridge alignment, the more help reducing the resource of derivative information in further steps. The local alignment of each structural member should follow the entire bridge alignment directly or in a certain way. For example, a stiffening girder uses the segmental alignment which coincides with the horizontal alignment, while its fences are using the same segmental alignment but in appropriated offset distances. Bridge alignment should be well maintained during the exchange of data even if each stakeholder is using a different BIM platform. The IFC bridge schema can be expanded to cover all types of bridge structures. In this study, it pertains to suspension bridges. Similar to the concepts of the IFC bridge model, this study proposes bridge alignment where the alignment is well established to handle both horizontal and vertical alignment separately. The suspension bridge mostly using the fabrication method; therefore, for a more intuitive understanding, all structural members can be considered as the knot-to-knot structure. The horizontal alignment defines the distance between each knot, while the horizontal alignment is focused on the profile of that knot-system. Data for the bridge alignment can be filed in the general spreadsheet format such as \".xls\" or \".cvs\", which includes three-dimensional coordinates. This piece of the database can be used for both design and analysis purposes. Figure 10 shows an example of the bridge alignment of a suspension bridge. 1. Horizontal alignment: For cable-supported bridges in general and especially for suspension bridges. the horizontal alignment is defined by \"Station\". The information for a \"Station\" consists of three-dimensional coordinates, but when mentioning this term, it is suggested that only the horizontal coordinate is the most important. The station is defined by the location of suspended structures. Due to the suspended structures are a complex assembly of main cables, hangers, and stiffening girders. In this regard, the exact location of a station is defined through the exact location of the centroid line of the hanger cables distribution. As a long-span bridge is a kind of horizontal-development facility, therefore this will be the main direction that must pay more and more attention in order to drive the bridge design to meet the multiple requirements including the structural, transportation service, or even aesthetic issues. The fact that the horizontal alignment refers to the direction in general is noteworthy. That means that this kind of alignment still includes the change of elevation (as the eccentricity) and longitudinal alignment (as the lateral curvature) for each station. Therefore, the longitudinal alignment itself is a three-dimensional interpolate curve, where each interpolated point is a station. 2. Vertical alignment: The main vertical alignment for the pylon or pier is defined as absolutely vertical. However, due to the aesthetic of design requirement, usually the pylon and pier are designed not a rectangular block. Depend on the certain case, the vertical alignment might be changed from a main absolute vertical line or a combined of inclined-centroid-lines but symmetrical across the main one.