....................................................................................................................................................
Soldiers Marching In Step On A Bridge
Why
Do Soldiers Break Stride On A Bridge?
By Elizabeth Howell - Live
Science Contributor
Marching
soldiers are cautioned to break stride on a bridge, lest they match the
bridge's frequency of vibration.
In April 1831, a
brigade of soldiers marched in step across England's Broughton Suspension
Bridge.
According to accounts
of the time, the bridge broke apart beneath the soldiers, throwing dozens of
men into the water.
After this happened,
the British Army reportedly sent new orders: Soldiers crossing a long bridge must "break stride," or not
march in unison, to stop such a situation from occurring again.
Structures like
bridges and buildings, although they appear to be solid and immovable, have a
natural frequency of vibration within them.
A force that's
applied to an object at the same frequency as the object's natural frequency
will amplify the vibration of the object in an occurrence called mechanical
resonance.
Sometimes your car
shakes hard when you hit a certain speed, and a girl on a swing can go higher
with little effort just by swinging her legs.
The same principle of
mechanical resonance that makes these incidents happen also works when people
walk in lockstep across a bridge.
If soldiers march in unison across the structure,
they apply a force at the frequency of their step.
If their frequency is
closely matched to the bridge's frequency, the soldiers' rhythmic marching will
amplify the vibrational frequency of the bridge.
If the mechanical
resonance is strong enough, the bridge can vibrate until it collapses from the
movement.
A potent reminder of
this was seen in June 2000, when London's Millennium Bridge opened to great
fanfare.
As crowds packed the
bridge, their footfalls made the bridge vibrate slightly.
"Many
pedestrians fell spontaneously into step with the bridge's vibrations,
inadvertently amplifying them," according to a 2005 report in Nature.
Though engineers insist the Millennium Bridge was never in danger of collapse, the bridge was closed for about a year while construction crews installed energy-dissipating dampers to minimize the vibration caused by pedestrians.
Elizabeth
Howell
is a regular contributor to Live Science and Space.com, along with several
other science publications. She is one of a handful of Canadian reporters who
specializes in space reporting. Elizabeth has a Bachelor of Journalism, Science
Concentration at Carleton University (Canada) and an M.Sc. Space Studies
(distance) at the University of North Dakota. Elizabeth became a full-time
freelancer after earning her M.Sc. in 2012. She reported on three space shuttle
launches in person and once spent two weeks in an isolated Utah facility
pretending to be a Martian.
https://www.livescience.com/34608-break-stride-frequency-of-vibration.html
No comments:
Post a Comment