Additional Bridge Forces

We have so far touched on the two biggest forces in bridge design. There are dozens of other forces that also must be taken into consideration when designing a bridge. These forces are usually specific to a particular location or bridge design.

Torsion, which is a rotational or twisting force, is one which has been effectively eliminated in all but the largest suspension bridges. The natural shape of the arch and the additional truss structure of the beam bridge have eliminated the destructive effects of torsion on these bridges. Suspension bridges, however, because of the very fact that they are suspended (hanging from a pair of cables), are somewhat more susceptible to torsion, especially in high winds.

All suspension bridges have deck-stiffening trusses which, as in the case of beam bridges, effectively eliminate the effects of torsion; but in suspension bridges of extreme length, the deck truss alone is not enough. Wind-tunnel tests are generally conducted on models to determine the bridge's resistance to torsional movements. Aerodynamic truss structures, diagonal suspender cables, and an exaggerated ratio between the depth of the stiffening truss to the length of the span are some of the methods employed to mitigate the effects of torsion.