Don_P, Deflection Expert

For those of you thinking "jeez, I wish I knew more about deflection in floor joists" - you're in luck: Don_P offered this treatise over at CountryPlans.Com:

"Floors are sort of unique in that we typically don't design them by strength but by stiffness. The limiting factor in most floor design is not bending strength or shear but deflection. The original deflection specification that appears as the code minimum is L/360. If a floor spanned 360" it would be allowed to sag 1" under full design load. This standard was initially to prevent plaster cracks in ceilings under a second floor. The use of 30 psf for "sleeping rooms" dates back at least 40 years. This predates our current lifestyles, water beds, mountains of stuff in a home, thin set tile, etc. Length/360 is a comfortably stiff floor in shorter spans, as they get longer it can become annoying. A sure recipe for disappointment is to design for 30 psf, L/360 on a span greater than about 15'. One simple design rule of thumb that came out of this research was that using 40 psf Live Load and designing for L/480 (25% less deflection than L/360) on the joists almost always works. The extreme example given in class was a 2X10 SPF 30psf floor framed 12" oc and spanning the maximum allowable 19'. The floor passes code but the vibration frequency would be 9 Hz, right in the middle of the most annoying range.

The floor frequency check is a way of looking at how a floor performs. In testing researchers found that people are very sensitive to vibrations in the 8-10 Hz range (I've read that this is the range of frequencies for functions in our bodies). They found that most people were comfortable if the floor vibrations were above 15 Hz. I'm not that finely tuned, what drives me nuts is to hear stuff rattling when someone walks across the floor. I'm not really happy with our 2x10 SPF floors that have roughly the same span as yours, my wife doesn't notice them. They check out right at 15 Hz so the average person, whatever that is, should be happy with them.

I've built long spans of 24'+ in a single lightweight engineered span , deflection of L/480+ a few times. In one case the customer complained, I hated the floor. In that case we had a very high vibration frequency but very little mass, it was too easy to get that snare drum vibrating. Having studied this some we added another layer of subfloor and screwed it on 8" centers in every direction. One of two things happened either the floor was made stiffer or we added enough mass to make it harder to excite.

To me that all gets pretty deep, the easy solution is to quit trying to span so far and don't push the limits with longer spans. Under a building its easy to break long spans in half, 1st to second floor requires some thoughtful placement of load bearing walls rather than a clear span building that can be cut up many ways. A stiff floor will still not feel like concrete underfoot until you get way out there. "

Wow. There's a man who knows from deflection.