The following cheat sheet is designed for footbridge superstructure selection.
Different superstructure options are compared in terms suitable span ranges, key constraints and span to depth ratios.
Common constraints in the exam include:
- Limited structural depth due to headroom clearance: trusses and half-through girders are often useful as the deck is slim and most of the structural depth is above the walkway surface.
- Aesthetic constraints such as the maximum height of parapets / structural parts above the deck. This is somewhat open to interpretation - you could take a strict view and say that no structural parts are permitted in this zone, but you could also interpret it to mean that the client doesn't want to block the view i.e. solid girders would not be permitted but slim truss elements or cables would be ok.
- Poor ground conditions -> minimise the weight of the structure. Structures with an all-steel construction tend to be lighter, especially if the deck plate is designed to participate in global bending.
- Limited access for transportation of materials to site / no suitable locations for a crane during construction.
Due to the lightweight nature of most of the forms of construction, dynamic analysis is often more important for pedestrian footbridges than for other types of bridge. Most steel decks are susceptible to excitation by pedestrians, and cables can also be susceptible to vibration due to wind loading. If using a susceptible structure type, it is generally a good idea to make a note that a dynamic analysis would be required during detailed design.
The following sources of information were used to prepare the graphs:
- Jensen, C (2008) 'ICE Manual of Bridge Engineering'
- https://www.steelconstruction.info/Design_of_steel_footbridges
- Advice from senior colleagues!
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