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.
Resonance (a vibration in something caused by an external force that is in harmony with the natural vibration of the original thing) is a force which, unchecked, can be fatal to a bridge. Resonant vibrations will travel through a bridge in the form of waves. A very famous example of resonance waves destroying a bridge is the Tacoma Narrows bridge, which fell apart in 1940 in a 40-mph (64-kph) wind. Close examination of the situation suggested that the bridge's deck-stiffening truss was insufficient for the span, but that alone was not the cause of the bridge's demise. The wind that day was at just the right speed, and hitting the bridge at just the right angle, to start it vibrating. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart.
When an army marches across a bridge, the soldiers are often told to "break step." This is to avoid the possibility that their rhythmic marching will start resonating throughout the bridge. An army that is large enough and marching at the right cadence could start a bridge swaying and undulating until it broke apart.
In order to mitigate the resonance effect in a bridge, it is important to build dampeners into the bridge design in order to interrupt the resonant waves. Interrupting them is an effective way to prevent the growth of the waves regardless of the duration or source of the vibrations. Dampening techniques generally involve inertia. If a bridge has, for example, a solid roadway, then a resonant wave can easily travel the length of the bridge. If a bridge roadway is made up of different sections that have overlapping plates, then the movement of one section is transferred to another via the plates, which, since they are overlapping, create a certain amount of friction. The trick is to create enough friction to change the frequency of the resonant wave. Changing the frequency prevents the wave from building. Changing the wave effectively creates two different waves, neither of which can build off the other into a destructive force.
The force of nature, specifically weather, is by far the hardest to combat. Rain, ice, wind and salt can each bring down a bridge on its own, and in combination they most certainly will. Bridge designers have learned their craft by studying the failures of the past. Iron has replaced wood and steel has replaced iron. Pre-stressed concrete is used in many highway bridges. Each new material or design technique builds off the lessons of the past. Torsion, resonance and aerodynamics (after several spectacular collapses) have been addressed in better designs. The problems of weather, however, have yet to be completely conquered. Cases of weather-related failure far outnumber those of design-related failures. This can only suggest that we have yet to come up with an effective solution. To this day, there is no specific construction material nor bridge design that will eliminate or even mitigate these forces. The only deterrent is preventive maintenance.
For more information on bridges and related topics, check out the links on the next page.
Lots More Information!
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More Great Links!
- The Tay bridge disaster
It was a poor choice of design for the area it was in -- an early failure from which future bridges have benefited (see the Forth Bridge link below). Notice the beam bridge design; it's a variation of the Warren truss.
- A Brief History of the Forth Bridges
The original Forth Bridge design was abandoned after the Tay bridge collapsed and the resulting inquiry found the design to be inadequate.
- Tacoma Narrows Bridge Failure
There are dozens of sites related to this famous collapse. I include this one because it is concise and offers good photos.
- I-5 Arroyo Pasajero Twin Bridges
Not a particularly interesting or influential failure, just one that underlines the need for preventive maintenance. This bridge collapsed due to 'scour' (the effect on the bridge piers of rapidly moving water, and it's associated sediment). This case does include a pretty clever solution for a short-term interim bridge.
About the Author
Michael Morrissey is a full-time software engineer at a research firm supplying online financial information and a part-time design engineer at Imaginary Industries, a model engineering firm located in north western Connecticut.