Composite Decks

Composite Construction in bridge decks usually refers to the interaction between insitu reinforced concrete and structural steel.
Three main economic advantages of composite construction are :

1. For a given span and loading system a smaller depth of beam can be used than for a concrete beam solution, which leads to economies in the approach embankments.

2. The cross-sectional area of the steel top flange can be reduced because the concrete can be considered as part of it.

3. Transverse stiffening for the top compression flange of the steel beam can be reduced because the restraint against buckling is provided by the concrete deck.

Typical Composite Deck

Construction Methods

It is possible to influence the load carried by a composite deck section in a number of ways during the erection of a bridge.
By propping the steel beams while the deck slab is cast and until it has gained strength, then the composite section can be considered to take the whole of the dead load. This method appears attractive but is seldom used since propping can be difficult and usually costly.
With continuous spans the concrete slab will crack in the hogging regions and only the steel reinforcement will be effective in the flexural resistance, unless the concrete is prestressed.
Generally the concrete deck is 220mm to 250mm thick with beams or plate girders between 2.5m and 3.5m spacing and depths between span/20 and span/30.
Composite action is developed by the transfer of horizontal shear forces between the concrete deck and steel via shear studs which are welded to the steel girder. Typical types of connectors are shown below, the stud connector being the most commonly used.