The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet. Truss bridges are commonly made from a series of straight, steel bars. The Firth of Forth Bridge in Scotland is a cantilever bridge, a complex version of the truss bridge.
Rigid arms extend from both sides of two piers.
Diagonal steel tubes, projecting from the top and bottom of each pier , hold the arms in place. The arms that project toward the middle are only supported on one side, like really strong diving boards. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete , and they can span up to 800 feet. Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.