Forth Railway Bridge

Technically the bridge is a way of taking forces from up in the air down to the ground. So imagine the flow of those forces through the structure. Think of a truck standing on the bridge and how its weight is transmitted through the bridge to the ground.

Read about the principle of the cantilever and suspended span for trusses…..

Benjamin Baker’s very famous illustration of the principle of the cantilever and suspended span is shown below.

Notice the heavy weights hanging at each end. In order to keep the balance in the actual bridge the equivalent of those weights are provided in a large box at the ends of the outer cantilever girders. The central suspended span (where the man in the middle of the photograh is sitting) is balanced. However if two trains were to cross the bridge at the same time and meet in the central suspended girder at either side of the central tower then the tower would become unbalanced.

That is why, looking along the length of the bridge, the columns of the central tower are further apart (79.3 m.) than the columns on the other two towers (44.2 m.). Simply to avoid the central tower tipping under that loading.