Centrings and beams
- The centring supports the bridge deck during construction
Centrings and pre-stressed concrete beams are used for the construction of bridges and viaducts. These support both scaffolding and equipment for lifting and transport of heavy loads.
A bridge is mainly composed of:
a deck supporting the traffic lanes,
abutments serving as support points between the two extremities,
and piers supporting the deck.
The technique for the construction of a deck is called "launching" and consists of moving forwards whilst building the pre-fabricated deck elements one after the other. The centring is the propping structure of the deck under construction.
The classic centring is a propping system of reinforced scaffolding which supports the formwork thus enabling the casting of the deck to take place. This is the type of construction most widely used for small-scale bridges.
The self-launching centring (or self-supporting centring) is not supported from the ground but instead it is supported on the deck being constructed. This makes it possible to install the deck elements by using its metal structure as a support. This type of centring allows the construction of very high bridges and viaducts, where centring supported at ground level would not be possible.
- (Both photos by D.R. IS Béton)
The pre-stressed concrete beam was invented by Eugène Freyssinet so that bridges could be constructed without either centrings or supports between piers. The beam is supported on two piers and the deck elements are installed and joined together with the aid of an overhead travelling crane. Once a bay between two piers is completed, a beam is installed on the next two piers and so on.
This technique was used for the construction of the Ile de Ré bridge, the longest in France, which was built in less than 23 months.