Cantilever bridges are subject to the two basic forces that are in effect for all bridges. These forces are compression and tension. In a cantilever bridge, with the space between the cantilevered portion spanned by a suspended deck, the compression and tension are straightforward.
As you can see in this diagram above, from Merriam-Webster, a load will put weight on the deck. The cantilever arms support the weight of the load. So where is the tension and compression in the beam itself?
Here you can see that when the load adds weight to the bridge, the top supports are subject to tension (<– –>), the pulling force. The cantilever arms and bottom supports are in compression (–> <–), the pushing force.
As with any type of bridge, if the bridge is subject to too much tension, the bridge will snap. Too much compression and the bridge will buckle. As long as the forces are balanced, the bridge will stand and function as it was designed to function.
The Forth Railway Bridge