Beam Bridges also known as “Girder Bridge” are the most simple of structural forms being supported by pillar at each end of the deck. No moments are transferred through the support and that is why their structural type is known as simply supported. The simplest beam bridge could be a slab of stone, or a plank of wood laid across a stream. Bridges designed for modern infrastructure will usually be constructed of steel or reinforced concrete or a combination of both. The concrete used can either be reinforced, pre stressed or post-tensioned.
Because of its simple design, the Beam Bridge is the most common bridge construction across the world. It is usually used to span shorter distances and is commonly found along local roads. A beam bridge is made with large supports on the edges and heavy beam crossing over water, a ditch or other open space. Over the sturdy beam there are boards, metal or most commonly a layer of concrete. The bridge road surface is level with the road, making it very strong. Bean bridges are used to span no more than 250 feet.
A Beam bridge works on the principles of compression and tension, so a strong beam is needed to resist twisting and bending under the weight it must support. When a load, like a group of traveling cars pushes down on a beam, the weight of the beam pushes down on the piers. The beam’s top edge is pushed together as a result of compression and tension causes the bottom edge to stretch and lengthen. This works in the same way that a wooden plank supported by blocks on each end can only hold a certain amount of weight before it buckles. The top reaches the maximum compression and bottom snaps under too much tension.
Ancient beam bridges were constructed in a simple manner which consisted of a tree placed across a stream or a river. With the advancement in civilization, more useful methods were discovered in bridge building with the use of rock, stone and mortar. Roman techniques of bridge building included the driving of wooden poles at the intended location of the bridge columns, and then filling the column space with the construction material. Roman bridges were strong and uniform. After the industrial revolution, material science developed rapidly with enhanced physical properties and wrought iron was replaced with steel because of its greater tensile strength.
Many beam bridges used in road construction are made from concrete and steel beam, because these materials are strong enough to withstand the forces of compression and tension. The distance a beam can span is directly related to the height of the beam, since higher beams offer more material to dissipate tension. To create taller beams, trusses may be added for reinforcement. A truss (e.g., Warren Truss) is a construction of lattice work that supports a beam creating rigidity and increasing the beam’s ability to dissipate the compression and tension. This technique only works to a certain degree, because eventually the weight of the bridge and trusses will be too heavy to be supported.
In spite of the creative addition of a truss, the beam bridge is still limited in the distance it can span. As the distance increases the size of the truss must also increase, until it reaches a point where the bridge’s own weight is so large that a truss could not support it. Longer distances can be reached by daisy-chaining bridge sections together to create a continuous span. One of the world’s largest bridges is the continuous span beam bridge that was created this way which is located in Louisiana Pontchartrain Causeway. It is a pair of parallel bridges that measure almost 24 miles long and are supported by 9,500 concrete pilings.