A beam or "girder" bridge is the simplest kind of bridge. In the past they may have taken the form of a log across a stream but today they are more familiar to us large box steel girder bridges.
A beam bridge needs to be stiff. It needs to resist twisting and bending
In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers.
The weight of the beam pushes straight down on the piers.
Under load, the beam's top surface is pushed down or compressed while the
bottom edge is stretched or placed under tension. If we
imagine that there is an imaginary line running down the centre of the beam
this line remains
at its original length while the material above is compressed and the material
below is stretched.
This line is referred to as the neutral axis.
Take a two-by-four piece of wood and place it on top of two empty milk crates -- you've just created a crude beam bridge. Now place a 50-pound weight in the middle of it.
Notice how the wood bends. The top side is under compression and the bottom side is under tension. If you keep adding weight, eventually the two-by-four will break.
Actually, the top side will buckle and the bottom side will snap
The farther apart its supports, the weaker a beam bridge gets. As a result, beam bridges rarely span more than 250 feet. This doesn't mean beam bridges aren't used to cross great distances it only means that there may be a series of beam bridges joined together, creating what's known as a "continuous span."
Many beam bridges that you find on highway overpasses use concrete or steel beams to handle the load. The size of the beam, and in particular the height of the beam, controls the distance that the beam can span. By increasing the height of the beam, the beam has more material to dissipate the tension.
To create very tall beams, bridge designers add supporting lattice work, or a truss, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force is dissipated through the truss.
Despite the ingenious 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 the truss cannot support it.
Beam bridges come in dozens of different styles. The design, location and
composition of the truss is what determines the type. In the beginning of
the Industrial Revolution, beam-bridge construction in the United States
was developing rapidly. Designers were coming up with many different truss
designs and compositions. Wooden bridges were being replaced by all-iron
or wood-and-iron combinations. The different truss patterns also made great
strides during this period. One of the most popular early designs was the
Howe truss, a design patented by William Howe in 1840.
Large Box Girder Beam Bridge