Trusses are generally used for bridge spans between 30m and 150m where the construction depth (deck soffit to road level) is limited. The small construction depth reduces the length and height of the approach embankments that would be required for other deck forms. This can have a significant effect on the overall cost of the structure, particularly where the approach gradients cannot be steep as for railway bridges.

High fabrication and maintenance costs has made the truss type deck less popular in the UK; labour costs being relatively high compared to material costs. Where material costs are relatively high then the truss is still an economical solution. The form of construction also allows the bridge to be fabricated in small sections off site which also makes transportation easier, particularly in remote areas.

Choice of Truss

The underslung truss is the most economical as the deck provides support for the live load and also braces the compression chord. There is however the problem of the headroom clearance required under the deck which generally renders this truss only suitable for unnavigable rivers or over flood planes.
Where underslung trusses are not possible, and the span is short, it may be economical to use a half-through truss. Restraint to the compression flange is achieved by U frame action.
When the span is large, and the underslung truss cannot be used, then the through girder provides the most economic solution. Restraint to the compression flange is provided by bracing between the two top chords; this is more efficient than U frame support. The bracing therefore has to be above the headroom requirement for traffic on the deck.

Design Standards  |  Bridge Components | Choice of Deck