Resource: Firth of Forth Cantilever Bridge

Despite its unusual appearance, the bridge that spans Scotland's Firth of Forth has a lot in common with the simplest and oldest bridge type: the post and beam bridge. In a post and beam bridge, solid ground or columns, called piers, on opposite banks of a river or gap support long horizontal beams, often with support from supplemental columns at regular intervals along the way. Similarly, the Forth Railway Bridge spans the waterway called the Firth of Forth, stretching between columns near opposite banks of the Firth to a column on a small island in the center.

In one important and innovative way, however, the Forth Bridge, designed in the late 1800's by structural engineers Benjamin Baker and Sir John Fowler, is very different from a simple beam bridge. Whereas a beam bridge relies on the strength of the beam spanning the distance between columns to carry its own weight, as well as the loads generated by cars and trains that travel over it, the Forth Bridge uses a cantilever system to counteract the downward force generated between columns. The two main beams that make up a cantilever bridge rest almost perfectly balanced on piers located part of the way across the span. One end of each beam is also anchored to the nearest bank, while the other end stretches toward the center of the span, like a diving board over a pool. These suspended ends are joined by a relatively lightweight connecting span.

The Forth Bridge's shape -- tall and busy with many angled supports in some areas, almost dainty in others -- results from the need to be both as strong as necessary and as light as possible. The midsection of each beam that attaches to the piers and carries most of the beam's weight and its load must be very strong, so engineers increased the depth (or height) of the bridge in this location, making it resistant to bending even under extreme loads. In contrast, the connecting spans, which are supported entirely by the main beams they connect, are built minimally to be as light as possible. Such a design allows the Forth Bridge to span a much greater distance with many fewer vertical piers than a typical post and beam bridge would require.

Examine the Forth Cantilever Bridge. What are the forces at work? How do you think they are distributed so that they hold up the roadway in the middle? Make a sketch to explain.

Could we build a bridge like this with our bodies, as they did? Why or why not?

Act out the forces at work on the bridge, although not necessarily with a human load, and feel the tension in your arms. Try it with and without the compression pieces to see how important these are.

Host David Macaulay says, "But very few [bridges] were built like this." Why do you think this is so?