Bridges are required for roads, railways, and pedestrian to cross over rivers, valleys, and other obstructions. Most bridges are constructed as load-bearing structures and can be of the beam, suspension or arch bridges.
A simple beam bridge will have steel or concrete beams that span between piers or abutments that are built in the river or valley floor. In most cases, nowadays, the beams are prestressed to give them a better strength to bear the load imposed on them. A slab or other support arrangments are made to support the road surface or railway lines. Bridges may also be built to convey large water mains over undulating terrain. Piers are built into river beds and will require caissons that are sunk into the river bed, or through the use of coffer dams that keep out the water, while the foundations are laid. Construction of beam bridges requires specialized techniques and often innovative methods for both the pier construction and the placing of the beams on the pier foundations.
Arch bridges and suspension bridges are often used to span across the obstruction while creating foundations on both banks of the divide. Arches can be supported by trestles or launched in cantilevered sections till the arch is completed from both sides. The arch will then support the bridge deck through required structural arrangments. Suspension bridges will have cables made of high tensile wire on which the bridge deck and structure are suspended. This requires tall towers to be constructed on either bank that take the suspension cables in an inverted arch form. They are aesthetically very pleasing structures that allow spans that can be quite large, and help in maintaining sea or river traffic under them, without any obstructions.
In beam bridges, it is very important to cater for the expansion of long beams, and this is ensured by providing expansion joints between parts of the deck that abut each other, and the movement is also facilitated by having bearing plates on which the beams rest over the piers. These bridge bearings can be made of rubber or other flexible materials, that absorb the stresses caused by the movement of the beams.
Construction of bridges calls for the use of heavy lifting equipment and techniques that have to be developed specially for each bridge, depending on its particular design and requirements. Many companies specialize in their construction. Bridges are also built to cover long distances that cross the seas.