The simplest form and the most inexpensive form of a bridge is the beam or "girder'. It takes on the natural basic form of a log fallen across a stream (Dupre, 12). It consists of a horizontal beam which has at its end two support called piers or abutments. The beam must be able to bear its own weight plus the weight of the traffic without bending. The beam experiences the forces of the dead and dynamic loads which produce a compression force which forces it together, while the bottom edge is pushed apart- the tension force.
A good example is that of taking a thick piece of sponge which is about two inches thick and cutting a strip about three inches in width and a length of about six to eight inches. Get two cans of the same size to provide the support and place them about four inches apart. Use the sponge as a plank across the two supports. Cut a notch in the middle on either side. Slightly press on the middle section, then observe the notches. You will notice that the notch closes on the topside which is the evidence of compression force while on the under side, the notch tends to widen-- evidence of the tension force.
An ideal material used in beam bridge construction is pre-stressed concrete. It withstands very well the forces of compression and the steel rods imbedded in it resist the forces of tension-- pre-stressed concrete is one of the least expensive materials used in construction. However efficient the material, there is no compensation for beam bridges greatest limitation, its length. The further apart the supports are placed, it's the weaker the beam bridge. Considering this factor, beam bridges seldom span more than 250 feet, which does not imply that it cannot be designed to span great distances. In situation where it spans a great distance, the spans are daisy-chained together a connection known in the bridge world as "continuous span'. The world's longest bridge, which is 24 miles long, is a continuous span beam bridge, Lake Ponchartrain Causeway. It consists of two, two- way lane sections running parallel to each other. The Southbound Lane, which was completed in 1956, consists of 2243 separate spans, while the Northbound Lane, completed in 1969, consists of 1500 longer spans.
One big drawback of continuous spans: they are not suitable for places where there is need for the unobstructed clearance below the span given the fact that a great number of piers (support) are required.
Arch bridges pattern the natural rock formation. This type of bridge is considered one of the oldest and does posses great natural strength. In this design, the load of the bridge instead of pushing straight down, it distributes the load outward along the curve of the arch to the support at the ends of the bridge, the abutments. They support the load and prevent the ends of the bridge from spreading out.
How do the abutments support an arc bridge?
Get a 2- inch by 12- inch strip of cardboard. Bend the strip gently forming an arch. Place both ends on the table in the fashion of an inverted U. Press down gently on the top of the arch and notice the effect on the ends. The likely outcome is the spreading outward of the ends of the arch.
Place two stacks of books about six inches apart ( the stacks must be lower than the height of the arch). Place the arched strip between the stacks of book in the inverted U fashion. Apply a gentle force to the top of the arch and notice how the stack of books act as abutments, preventing the ends of the arch from spreading apart. Whenever the arch bridge is supporting its dead load, its own weight, and the load of the traffic crossing the bridge, very part of the arch is under compression. Given this situation, arch bridges must be constructed from materials whish can withstand much compression force.
The Romans, who are known for their great engineering genius at building arch bridges, used stones as their primary construction material. The upper portion of the bridge is held together by mortar while the rest of the structure is held together by its own weight.
In present time, the availability of steel and pre-stressed concrete makes it possible to construct longer and more elegant arches. Generally, modern arches span a distance of 200-800 feet. In New River Gorge, West Virginia, there is a spectacular arch bridge that has a span of 1700feet.
Constructing arch bridges can be somewhat tricky due to the fact that the structure is unstable until the both spans meet in the middle. To overcome this problem, sufficient scaffolding is used to provide support for the spans until they meet in the middle, a technique known as centering. Another method of support is the use of cables that are attached to the spans at one end while the other ends anchored to the ground on the other side of the bridge. This method allows for the use of the waterway or road below the structure while it is still under construction.
The Natchez Trace Bridge in Franklin, Tennessee, which was opened in 1994, is the first American arch bridge to be constructed from pre-cast concrete. Two arches are used to support the roadway above. Conventionally arch bridges of comparable size require vertical supports called 'spandrels" to distribute the load of the roadway to the arches. This bridge is designed without the use of spandrels for aesthetic reasons. Instead, most of the live load is resting on the crown of the two arches that are slightly flattened to provide better support.(Super Bridges)