Is it true or is it a myth that a column of marching soldiers has to break their rhythm while crossing a bridge to prevent structural damage to the bridge?

zbeckabee

This common public belief is actually rooted in a historic bridge failure that occurred in France in the mid-1800s. Reportedly, the Angers suspension bridge failed when hundreds of troops were marching in step across the bridge. Today it is impossible to know if the Angers Bridge had a design or construction flaw leaving it vulnerable to failure, but it does appears that the troops contributed to the disaster. When troops march in cadence, they are essentially impacting the bridge at a fixed rate, or frequency. Every structure has a "natural" frequency; if the loading frequency (whether from troops or other sources) matches the natural frequency of the structure, a phenomenon known as resonance can occur. The Tacoma Narrows Bridge is widely cited as a modern day example of resonance-induced collapse. In this case, the frequency of the wind matched the natural frequency of the bridge deck resulting in higher than normal response amplitudes and subsequent failure. In modern day bridge design, resonance is accounted for in the design process. Therefore, if troops were to cross a modern U.S. bridge in step at a frequency equal to the natural frequency of the structure, higher bridge amplitudes would be encountered consistent with physics. However, such responses would likely remain within the safe operational envelope of the bridge, resulting in no damage to the structure or cause for concern. http://www.pbs.org/wgbh/nova/sciencenow/0304/04-ask.html Aug 21 08, 5:18 PM

queproblema

Physics teachers, which I am not, love this one! Six days ago I rode over the 1950 replacement of the Tacoma Narrows "Galloping Gertie" and was reassured both it and the eastbound 2007 structure I'll cross tomorrow are perfectly safe. One paragraph below Zb's quote the Millennium Bridge in London is mentioned as a recently-built bridge retrofitted to avoid the oscillations caused by the resonance of pedestrians' steps. And they weren't even marching! This link adds more details and names three other bridges with a similar problem. http://en.wikipedia.org/wiki/Millennium_Bridge_(London)#Resonance Aug 21 08, 6:10 PM