Bounce on the Movie Set

was called once to a movie set. It was in an old house, out in the country. There was considerable `bounce’ in the upper story floor, and they wondered if it was `safe’ (… to do their filming). I did some `investigating’. The floor was framed with 2 x 8 wood joists. Did I mention the house was `old’? There were no construction drawings. There were no grade stamps. But I did examine the joists themselves, and measured the span, spacing, and floor materials, and did some simple calculations. Yeah, for the use (loads) they intended, the floor was `safe’. But what about the `bounce’? (I agreed there was considerable bounce … experienced while up there `investigating’.)

(not the actual house … but close!)

Woeste and Dolan did a paper on floor bounce … wait, no, on floor vibration (copy here). They provided an equation for determining the fundamental frequency of a floor joist, as follows:

f = 1.57 √ (386 EI/WL^3).

Further, their paper says that occupants are the most sensitive to vibrations in the 7 to 10 hz range.

Plugging in the old-house joist dimensions, and span, presumed modulus of elasticity, number of joists involved at any one time, and various weights of movie-set `occupants’ and stuff (for mass) … yeah, it was pretty easy to `land’ in the 7-10 range.

I advised that the floor was indeed safe, despite the bounce … “as long as you don’t put a crazy huge amount of stuff up there … if you want to shoot your movie up there, you’ll have some bounce.”

I tried to reconcile the bounce I felt due to the sheer effect of walking across the floor. I `calculated’ that my walking was, say, 1 to 2 hz (steps per second) … not exactly in the 7 to 10 range … but maybe a harmonic of my stepping. Or, each one of my steps engaged more than one joist!

To date, that’s been the extent of my movie-set engineering, except for some quick advice on what factor of safety to use for stage rigging.

Over the years I never did see very many 2 x 8 floors. Maybe `bounce’ is why.

I you have a movie for me (engineer, cast, or crew) … let me know!