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A beam
bridge is easily the oldest type of bridge around. It is simply a
piece of wood that goes from one side of a valley to the other.
On our beam bridge (which is on our tester), you can see that we
have placed a pressure plate, a five pound weight and a family of
animals. The pressure plate and the five pound weight are causing
our bridge to sag. When the bridge sags, the pressure plate only
touches the bridge in two places: on the far left side and on the
far right.
The fact that the pressure plate only touches the wood in two places
is very important, and we will use this fact both when we build our
bridge, and when Sabrina analyses it in the "Advanced Topics"
section.
Unfortunately, a beam bridge uses wood inefficiently. A truss
bridge can hold much more load with less wood. I’ll show you
that type of bridge next.
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