Over Truss Bridges

over truss steel bridge

Over Truss Bridges

Composite’ means that the steel structure of a bridge is fixed to the concrete structure of the deck so that the steel and concrete act together, so reducing deflections and increasing strength.

This is done using ‘shear connectors’ fixed to the steel beams and then embedded in the concrete.

Shear connectors can be welded on, perhaps using a ‘stud welder’, or better still on export work, by fixing nuts and bolts.

Over Truss Bridge Diagram



Carriageways are supported by pairs of trusses at 1.7m centres. A carriageway may therefore be 5.1m overall wide, with 1.2m walkways both sides. The carriageways may be wider in increments of 3.4m and may always be extended width ways in increments of 3.4m.

There is an element of choice in the marked carriageway widths and the widths of the walkways. The minimum bridge will be single lane with a 4m marked roadway. dge.


The decks have local reinforced concrete 0.25m thick, placed on our lost formwork decking with no propping needed. They can have 0.05m of surfacing. The concrete will become composite with the steel via shear connectors.

They have steel crash barriers on both sides of the carriageways. The walkways will be outside the crash barriers and will have handrails on the outside. The bridge decks are carried by two or more pairs of trusses below deck level.

There is no steel above the deck other than barriers. Please note that the roadway will have to be at a height well above the flood level: as a guide the depth of the trusses is about 8% of the span, and the roadway is above this truss.

The bridge decks are cambered from side to side using pre-cambered steel transoms; and slightly cambered from end to end using the built in camber of the trusses. All spans are simply supported.


There are two walkways, both outside the main trusses, 1.2m wide, with handrails outside. The pedestrian, handcart and cycle traffic using them is protected from the vehicle traffic by the crash rails.

The walkway decks are local reinforced concrete 0.125m thick placed on our lost formwork decking.


The bridges can be erected in-situ on a temporary causeway or on temporary jackable props, or may be built on the ‘home bank’ and cantilever launched across the gap.

For the cantilever launch, a ‘Launch Kit’ is needed, consisting of sets of rollers, a steel ‘launching nose’ fitted to the leading edge of the bridge (and removed for re-use after launch), and come-along cable jacks.

The bridges will sit on our elastomeric bearings on your abutments. Expansion joints for the roadway are provided at both ends.


The bridges may be combined with other bridges to make multi-span crossings. Again the bridges can be built in-situ, or they can be cantilever launched.

For multi-span bridges which are to be cantilever launched it is necessary to use a ‘Link Kit’ which consists of further sets of rollers, and further jacks, and a set of link steelwork which joins adjacent bridges during the launch and roll-out.

As with the Launch Kits, the Link Kits can be used again and again. You will need one Link Kit for a two span bridge and two Link Kits for a three span bridge and so on.