4.2 Form of Structure
The factors for which advice should be sought and should be considered when selecting the forms of structures include:
- geometry of the interchange elements
- limitations on grade line and span lengths
- foundation conditions
- environmental and aesthetic requirements
- facilitation of stage construction
- traffic management
- economy of structure.
The parapet must be designed to an appropriate performance level and have the ability to redirect errant vehicles (refer to relevant bridge design codes). Similarly, where a road or pedestrian bridge passes over a freeway it is important that the integrity of the structure is protected in the event of a vehicle crashing into piers. This is particularly the case where an existing freeway is to be widened and slender columns of an existing structure will remain relatively close to the road. In these cases it is suggested that a barrier capable of containing heavy vehicles should be provided (refer to the Guide to Road Design Part 6: Roadside Design, Safety and Barriers (Austroads 2010c)).
The conventional type of structure over divided roads is a two-span deck-beam type bridge, which may be continuous for economy. The bridge pier on the median may need protection by safety barriers, which will need to be sufficiently strong to prevent trucks from crashing into the pier. The form of structure adopted will depend on the circumstances at the particular site and the particular road conditions being serviced. This is a matter for determination by bridge designers for the site being designed.
Significant savings in cost can be made by standardising bridge types and spans as much as possible. However, in some cases the bridge form will need to be selected for architectural reasons, to enhance the historical character of the area or to identify a location.