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Civil / Structural PE
Trade:
Engineering & Software
Join Date: Oct 2009
Location: Mount Vernon, WA
Posts: 26
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Bouncy floors. Dangerous?
(I wrote this a few years ago for Nation's Building News OnLine. It goes very well with the current thread on TJIs)
Not long ago I got the call every engineer dreads. It was from a past client regarding something I’d engineered. In the world of structural engineering, this can be very serious for a couple reasons. First, if one of my structures fails, someone could be seriously injured or killed. Second, even if no one is hurt, the failure would likely be very expensive to repair – an expense I’d have to bear if it was my fault.
The call came from a good, repeat client, Wayne.
“Tim,” he said. “Remember the 20,000 square foot SR20 project you did last year? Well, the floors….”
My mind raced with visions of catastrophic failure, people pinned under tons of rubble. “Yes,” I replied cautiously.
“… the floors,” he continued. “They’re a little bouncy.”
Relief surged through me as the sweat spigot that had suddenly sprung from my forehead closed. “Really? I said, masking my relief. “How so?”
“Well, when a heavy person walks down the isle between the 2nd floor cubicles, people in the cubicles can really feel it. They’re complaining.”
“Sounds like a deflection issue,” I said. “I’ll recheck my calcs and get back to you.”
In doing this, it brought to mind one of my seminar quiz questions:
Bouncy Floors…
a) Are always an extreme hazard, and must be replaced.
b) Happen because the framer forgot to put the glue under the subfloor.
c) Are much more likely with a floor system that has not been designed by a qualified person.
d) May be perfectly fine from a strength point of view, but sure are annoying to the people who live there.
e) Occur because the deflection criteria of the floor system was not adhered to.
f) Should be reserved for trampolines, not buildings.
Answer a) is not true. Bounciness is due to deflection, not lack of strength. Deflection and strength do not depend on one another, though a lot of deflection (or bounciness if we’re considering a floor) may well indicate a strength problem.
Answer b) is not true. Glue helps strength and deflection a little, but generally less than 5%.
Answer c) is generally true.
Answer d) is true, and is the applicable answer to my client’s complaint.
Answer e) is generally true, but not always. The above scenario is an excellent case in point.
Answer f), is true enough, and could have also included big-time wrestling rings. But I digress.
When sizing beams and joists, there are three criteria that must be examined: bending strength, shear strength, and deflection. Deflection is not a strength issue, it is more comfort and cosmetic related. With floor joists, too much deflection can result in bounciness, cracked drywall ceilings, or sagging. It is possible that a poorly designed floor system could have both deflection and strength problems, but they don’t always go together.
Back to my SR20 project, I rechecked my calcs and found that I had designed everything correctly, and in fact used a more stringent criteria for deflection than code required. Still, the floor was a little bouncy. The primary reason was that the joist span was in excess of 20-feet; quite long.
Lesson learned: long floor joists, say 14-feet or more in span, are prone to bounciness, even if they are designed correctly. The best remedy is to use shorter spans. If that isn’t possible, be sure the designer knows what he’s doing, and doesn’t skimp on the deflection criteria.
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