Prestressed concrete is made by pouring concrete over stretched and anchored steel strands. After the concrete has set, the anchors are released. As the steel tries to return to its original length, it compresses the concrete, resulting in a relatively lightweight, extremely strong material.
B3 Foundations
All bridge piers rest on foundations that transfer loads from the bridge structure into the ground. The foundations support the bridge, and their design is critical. Difficult conditions, such as deep water or soft ground, can make foundation construction complicated and expensive. In such circumstances designers may choose to decrease the number of piers by increasing span length. Of course, greater span lengths often require a more expensive bridge type, and therefore the tradeoffs must be evaluated carefully.
If the ground is very strong at a bridge site, a foundation is formed by pouring a simple concrete mat beneath each of the piers. If the soil is weak, it may be excavated down to bedrock, and the piers can then be built directly on the solid rock. Alternatively, a group of vertical posts, or piles, can be driven through the soil to bedrock, and piers can be built on top of the piles. B4 Construction Methods
Bridges are erected using a variety of construction methods. Some techniques are associated with a particular bridge type, and care must be taken not to select a design that requires construction methods unsuitable for the site. Concrete and steel bridges are generally built using similar techniques, although concrete bridges are built in shorter sections than are steel bridges because of the greater weight of the material.
One of the simplest construction methods for bridges is to assemble a span away from the bridge site and then transport it to the site. The span can then be lifted into position as one piece. This method is most often suitable for small truss bridges or for the suspended span of a cantilever truss. Another approach is to use falsework, or temporary scaffolding, to support the incomplete parts of a bridge before they are joined and able to support themselves. The use of falsework is not always possible, owing to strong river currents, interference with river traffic, or great distances to the ground. If falsework is impractical, bridges can be constructed by the cantilever method. With this technique, a bridge is built piece by piece, with the entire structure supported from the section previously completed. Thus, the structure is self-supported throughout the entire construction process.