Plywood Design, 3-Span vs. 4-Span?
Plywood Design, 3-Span vs. 4-Span?
(OP)
I've noticed that in design examples that accompany the NDS Wood specifications the moment they apply to plywood is based on a 3-span condition, even when the joists are spaced at 24". Is there a reason for this? I also notice that, a bit counter-intuitively, a 4-span condition yields a higher negative moment then a 3-span condition (.107wl^2 vs .10wl^2), though perhaps that's splitting hairs.
If they're trying to account for a misplaced panel with a cantilever, well, shouldn't the cantilever portion be used?
Anyway, is there some reason for using 3-span assumption?
Thanks.
If they're trying to account for a misplaced panel with a cantilever, well, shouldn't the cantilever portion be used?
Anyway, is there some reason for using 3-span assumption?
Thanks.






RE: Plywood Design, 3-Span vs. 4-Span?
RE: Plywood Design, 3-Span vs. 4-Span?
However the second part of the comment can stand alone in a sense. Going from 3 span to 4 span will result generally in a shorter actual span and will offset the increased moment coefficient.
RE: Plywood Design, 3-Span vs. 4-Span?
In a word... History... at least that has been my conclusion. I thought about this same thing for many years and settled on this reason:
As long as the spans are equal, consider that a 2-span shear & moment diagram is very different from a single-span diagram.
A 3-span diagram is very different from a 2-span diagram.
However, a 4-span, or more, diagram is basically similar in shape and magnitude to a 3-span diagram. Before computers, 3-span was "good enough", for preliminary work anyway.
This same 3-span limit was used for other applications, too. An example is steel decking.
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RE: Plywood Design, 3-Span vs. 4-Span?
SlideRuleEra, thanks. I had assumed the 3-span assumption for steel decking was due to longer spans but, of course, that makes no sense since steel joists can be spaced at 24" and steel decking is longer. I guess if a cutoff of 3 spans isn't used then that would lead to investigating 5-span and 6-span conditions as well, and for what gain? It's a reminder to me that good enough is good enough.
Thanks!
RE: Plywood Design, 3-Span vs. 4-Span?
RE: Plywood Design, 3-Span vs. 4-Span?
And I'm from Canada
RE: Plywood Design, 3-Span vs. 4-Span?
I was talking more about roof sheathing. Pretty common around here to have 7/16" OSB spanning 24" (mainly on tract houses)
RE: Plywood Design, 3-Span vs. 4-Span?