During the discussion in the previous sections concerning the different types of bridges, I only mentioned compression and tension forces that acton bridges. However, there are other forces caused by dynamic movements, resonance and nature that must be considered in the design and construction of bridges. Torsional resistance is rotational or twisting forces that act on all bridges at times when lateral or unbalanced loads are applied some bridges. It is a major factor on the larger suspension bridges. However, the roadbed tends to be thinner in the section hanging from the cables. This section is very susceptible to torsion during high winds due to unequal wind pressure. Most suspension bridges have a supporting trusses system on the deck to eliminate the effects of torsion. But suspension bridges with extremely long spans need more than just the supporting truss to protect them form torsion. Some of the methods employed to mitigate the effects of torsion include; diagonal suspender cables, aerodynamic truss structures or an exaggerated ratio between the depth of the supporting truss to the length of the span.
If not considered when designing a bridge resonance can also have a detrimental effect on a bridge. Vibrations can travel through a bridge in the form of waves. In order to prevent a resonant magnification of the vibration, the potentially destructive resonance is controlled. Methods to dampen such vibrations must be built into the bridge design in order to avoid any resonance frequencies. Interrupting the waves prevents the harmonics from growing in length and becoming destructive. The dampening technique used to prevent resonance waves from growing usually involves variable inertias. For example, if a bridge roadbed is made up in different sections of overlapping plates the movement of one section is transferred to the next section via the slightly varied weights and sizes. This simple technique will create enough friction to counteract the frequency of the resonant wave. This change in frequency will prevent the wave from building and create two different waves, thus eliminating the chance of the resonant wave strength to become destructive. An excellent example of a resonance wave destroying a bridge occurred in 1940 when the Tacoma Narrows Bridge was destroyed by a forty miles per hour wind.
