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 aisle between the second 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 a ConstructionCalc seminar quiz question:
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 were 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 related more to comfort and cosmetics. 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 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.
Tim K. Garrison P.E. of ConstructionCalc.com has authored books and short courses and lectures on topics relevant to builders.
Got a technical or management issue? Email firstname.lastname@example.org. Tim reads every one.
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