When troops march across bridges....

[Hermann_Cheruscan]

What is the term that they use for the vibrations that are caused when troops are marched across bridges? I understand that troops are ordered to route step across bridges, because marching could cause some sort of harmonic disturbance and cause the bridge to collapse. Does anyone know anything about this?

[Ringo]

You used the term - if the troops march in unison they can create an acoustic energy wave that may be, in its wavelength, a harmonic of the bridge, by virtue of the bridge’s length.

[Ice_Wolf]

Resonance.

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.

[Gairloch]

“What is the term that they use for the vibrations that are caused when troops are marched across bridges?”

Darn. Resonance (or any equivalent term) is the right answer, but I wanted to say ‘Fear.’

Reason is only the companion of emotion, after all . . .
Gairloch

[UncleBill]

Resonance is related to Constructive Interference, if my memory serves.