Plywood Design, 3-Span vs. 4-Span?

[Archie264]

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.

 

[jayrod12]

a 3 span condition generally exists when the joists are at 24 and not at 16. and the reduction in moment coefficient is more than compensated by the extended spans the span increases by a ratio of roughly 1.5 from 4 span to 3 span and then that number is squared so it's more like a 2.25 increase due to span compared to a .934 decrease from the coefficients. Still results in a higher moment.

 

[jayrod12]

ignore that comment about the 24 and 16 argument. not sure what I was thinking when I came up with that.

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.

 

[SlideRuleEra]

Anyway, is there some reason for using 3-span assumption?

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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[Archie264]

jayrod12, I went through the same thing with the 3-span issue. I'm used to 3 x 16" = 48" for stud spacing and plywood has a 48" side so, well... (And at this point the non-US engineers are probably laughing at us.)

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!

 

[XR250]

I always thougth that the plywood span ratings were based on a 2-span minimum condition. That is one reason I try not to max out the span rating on sheathing. You can never be sure your are getting 2-span everywhere - especially when it comes to repairs. The resulting softness and sagging can be problematic.

 

[jayrod12]

My floor sheathing is always 3/4" T&G. there's never sagging issues. or diaphragm shortfalls.

And I'm from Canada I'm just fluent in imperial.

 

[XR250]

@Jayrod

I was talking more about roof sheathing. Pretty common around here to have 7/16" OSB spanning 24" (mainly on tract houses)

 

[jayrod12]

Fair enough. I've never come across a situation where my roof sheathing span was in jeopardy.