Just like electrical circuits oscillate, structures do also. Wind and 
earthquakes can cause big time vibrations, but floors can bounce up and 
down from a person walking. Today the phenomenon is better understood 
and certain floor weights/spans are avoided. For tall buildings the 
latest is mass tuned dampers. Heavy weights near the top, suspended like 
a pendulum or on low friction bearings with hydraulic dampers, just like 
on your car, except much larger.

>
> In London (UK) there is Battersea Bridge, which is basically made of 
> wood - whilst cars streem over it into and out of London (used to be 
> time of day directional) the horseguards (their barracks used to be 
> just down the road in Chelsea), would have to dismount to cross the 
> bridge. The vibrations caused by a 20 odd horses trotting over the 
> bridge managed to be at the right natural oscillation frequency and 
> the bridge was in danger of falling down, but a a 10 ton truck roaring 
> over no problem.
>
>
>   
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