How Bridges Work

By: Robert Lamb, Michael Morrissey & Patrick J. Kiger  | 

The Beam Bridge

The Forth Bridge across the Firth of Forth in Edinburgh, Scotland is a beam bridge. Martin Berry/Loop Images/Universal Images Group/Getty Images

Bridge building doesn't get any simpler than this. In order to build a beam bridge (also known as a girder bridge), all you need is a rigid horizontal structure (a beam) and two supports, one at each end, to rest it on. These components directly support the downward weight of the bridge and any traffic traveling over it.

However, in supporting weight, the bream bridge endures both compressional and tensional stress. In order to understand these forces, let's use a simple model.


If you were to take a two-by-four and lay it across two empty milk crates, you'd have yourself a crude beam bridge. Now if you were to place a heavy weight in the middle of it, the two-by-four would bend. The top side would bend in under the force of compression, and the bottom side would bend out under the force of tension. Add enough weight and the two-by-four would eventually break. The top side would buckle and the bottom side would snap.

Many beam bridges 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 latticework, 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 spreads through the truss.

Yet even with a truss, a beam bridge is only good for a limited distance. To reach across a greater length, you have to build a bigger truss until you eventually reach the point at which the truss can't support the bridge's own weight. Brace yourself for some serious stats on truss bridges on the next page.