Beam bridges are subject to the two basic forces that are in effect for all bridges. These forces are compression and tension. These forces are at work in a beam bridge in an arguably more direct manner than in other bridges. With a beam bridge, with its single deck, the deck itself is subject to both compression and tension. There are modifications to the deck which we’ll discuss in the future, but at its simplest, the deck is left to “deal” with both compressional and torsional stresses.
As you can see in the diagram above, the load puts weight on the deck, or beam. The abutments support the weight of the load. So where is the tension and compression in the beam itself?
In this diagram you can see that when the load adds weight to the bridge, the bridge flexes slightly. This flex is caused by the weight causing a compressional stress on the top surface of the bridge. As the top surface compresses, the bottom surface is forced to stretch, or be subject to tensional stress. If the compressive force is too great, the bridge will buckle. If the tensional force is too great, the bridge will snap. If the forces can be balanced, the bridge will stand and function as it was designed to function.
Let’s look at it in a less scientific way. These elephants are crossing a beam bridge. In this instance, the rope is the beam. The stakes are the abutments. It’s easy to see that if the abutments cannot support the weight on the beam, they will fail. It’s also clear to see that if the rope is not strong enough to support the weight of the elephants, it will fail. All in all, the beam bridge must be able to support the live load.