[Strawbale] plwood vs. osb for arch gussets

[Rob Tom]

On Mon, 26 Feb 2007 22:33:00 -0500, Chris Green <pojeros at telus.net> wrote:


> It is my understanding that any lumber used in load-bearing construction
> in Canada has to be inspected and grade-stamped.

Well, sort of. For residential construction in Code-regulated  
jurisdictions in Canada, it definitely has to be or failing that,  
inspected by and signed off by a structural engineer.

> The minimum thickness of ply for plywood gussets is 3/4", by the way,

Not really.
In some instances, even 5/16" plywood will do the job.

The thickness of the gusset material is, in part, a function of density of  
the gusset material and the wire gauge of the nails used.

The most common gusset material is 1/2" Fir plywood and the nails  
specified will be 10d, and the nails long enough to be clinched over.

I just had a look at the plans that Chris mentioned and noted that the  
nails specified for the 3/4 plywood gussets mentioned were concrete nails  
which are significantly more robust than 10d nails so would require  
thicker gusset material to develop full bearing strength of the nails.  
Anyone who has whacked on concrete nails will know that trying to clinch  
them over in wood when there's only about 1/2" protruding, is an exercise  
in futility so the designer of those trusses would not specify a length of  
nail that requires clinching.

The truss designer(s) had their reasons for specifying concrete nails (and  
hence thicker 3/4" gusset material) but obviously, it's not a universal  
requirement.

Which is better ?  Some thought on the matter will tell you.

ie All lumber used for trusses (and light structural framing for that  
matter) should be kiln dried as required by Code here in Canada.  Concrete  
nails are fatter than common nails. Whack in a bazillion closely-spaced,  
big fat concrete nails near the ends of pieces kiln-dried lumber and what  
is likely to happen ? And then, with what degree of difficulty could those  
un-clinched nails be withdrawn ?

Then do the same with narrower gauge nails, which are clinched.

So whaddya theen ?

Trusses, rigid arches, box beams etc are all engineered items with  
definitive specs for all of the materials used combined in an optimal  
manner to perform in a specific manner for a specific set of  
circumstances. Changing any of the specs changes the design will of  
course, change their performance. That is to say, if you don't know what  
you're doing, don't make changes to engineered specs.

=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply