Beam Bridge Facts, History and Design

Beam Bridge - Wooden Footbridge

Beam bridge or girder bridge is the simplest and oldest bridge type. It generally consists of one or more spans which are supported by an abutment or pier at each end.

The first beam bridges built by humans appeared as an imitation of nature – prehistoric humans saw a tree that had fallen across a stream and used the same technique nut in places where it was convenient for them. Herodotus, historian from the ancient Greece, was the first to leave the written document about the bridge in 484 BC. Bridge he wrote about had been built across the Euphrates River in 8th century BC and was made from wood and stone. Ancient Romans are famous for their arch bridges made of stone and concrete but they also begun with simpler bridge types. The earliest known ancient Roman beam bridge was built across the Tiber River in 7th century BC and was known as “Pons Sublicius” which literally means “bridge made of wooden beams.” Romans were also the first to use cofferdams to construct columns. They would drive a circular construction, made of wood and lined with clay, into the riverbed and empty it of water with a pump. That would leave the place to pour the concrete in and would also serve as a mold. When French engineer Hubert Gautier wrote the first book about building bridges in 18th century, bridge building became more of an exact science. Next book, “A Work on Bridge Building” written by American Squire Whipple improved this further by being the first text on analytical methods for calculating the stresses and strains in a bridge.

To build a simple bridge over a creek all you need is a stone slab or a wood plank. Modern beam bridges that can carry road traffic and railroad are made of steel or reinforced concrete which can also be prestressed or post-tensioned.

Beam Bridge - The Footbridge

First step in building of a beam bridge is placing of cofferdams which are constructed around each column location in the riverbed. Water is pumped from inside of them and shafts are drilled into the riverbed until they reach bedrock (which can be more than 25 m in depth.) Cylindrical cage of reinforcing steel is lowered into the shaft and concrete is poured. This forms foundation. Columns can be cast onto the foundation or precast and then placed there. Abutments are constructed on the riverbank where the bridge end will rest by pouring concrete between the top of the bank and the riverbed. It will hold the deck of the bridge and prevent dirt to be washed down into the river. Steel or prestressed concrete girders are placed by crane onto columns and bolted to the column caps. To complete bridge superstructure, steel plates or precast concrete slabs are placed across the girders which forms a solid platform. After that, hot-applied polymer-modified asphalt is placed on the platform. Its purpose is to be a moisture barrier. On the top of asphalt is placed a grid of reinforcing steel bars and then encased in a concrete slab. This grid has two layers. As a final layer of the deck a concrete pavement is poured in a layer between 20 and 30 cm. Concrete can be poured into stay-in-place forms if they are used. If not, concrete pours a paving machine that spreads, consolidates, and smooth the concrete – all at once. Before concrete stiffens, a skid-resistant texture is placed on it by manually or mechanically scoring the surface. Concrete also gets joints to prevent cracking every 5 m before it is poured or after. These joints are the sealed with flexible sealant.