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Re: Wood Design - 'G' shearing Modulus & 'E' elastic Modulus

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At 03:42 PM 7/7/99 EDT, you wrote:
>Can someone provide me with the Shearing Modulus 'G' for OSB panels?
>
>The design problem in the SEAOSC Seminar notes uses an Elastic Modulus for 
>the Chord of 1,700,000. However, if the chords are constructed from the same 
>grade of lumber as the walls (stud grade) then the 'E' value for Douglas fir 
>stud grade should be 1,400,000. 
>
>Is this true, or am I missing something in the example?
>
>Dennis Wish PE
______________________

There has already been a reply that informs us how to ask the OSB people
about their 'G' value.

You didn't say if the example's E value was for a horizontal diaphragm or a
shear wall, but the example's author had to decide on some value to use;
surely it wasn't the purpose of the example to advocate a particular 'E'
value in these chords. 

If you are using stud grade lumber for horiz diaphragm chords, I would
expect the same faction that loves common nails to crucify you for it. If
you are using stud grade lumber for chord studs, they will be tempted to
build your cross out of the same, but they couldn't rely on the stuff to
hold common nails, and they wouldn't profane themselves to use box nails.
Small comfort, I suppose.

Whatever, I bet there isn't much difference in shear wall deflection for any
of those E values, because there can't possibly be much PL/AE stretch in the
vert chords before they break. No ductility in this feature.

Likewise, the sheathing G shouldn't amount to a very big contribution to the
deflection that matters, namely when the earthquake really clobbers the
building and the fasteners get plastically tweaked.

Think: 'E' and 'G' are ELASTIC properties for elements that cannot go
plastic. The chord studs either stay elastic or fail; OSB either holds or it
rips. But three other features go wildly beyond the elastic. Hold-down
devices deform and stretch, and their resistance to excesses of plastic
stretch are critical to the sheathing not tearing apart the sill plate in a
brittle way. Jamb "hold-up" bearings crunch and compress. The sheathing
nails are subject to lots of post-yield bending while giving good service
and contributing to high values of damping. See the SEAOC Convention
Proceedings for each of the last four years for useful papers on these
matters.  

Now to adapt this thread to the recent ones on rigid diaphragms and common
vs box nails: 

There's an ominous implication: since the deformation stiffness of wooden
shear diaphragms and shear walls is so grossly non-linear when the
earthquake takes them toward their limits, what difference does it make to
do a tidy little elastic flexibility comparison analysis for elastic loading
that won't hold true during the event that counts, and then declare solemnly
that the diaphragm is flexible, stiff, or rigid?  Realistically, nothing can
fail at the loadings for which such an elastic flexibility analysis is
plausibly valid. Instead, the failures that aren't acceptable occur at
drifts that don't have usable, predictable load-drift relationships. And not
all elements will be at the same degree of extremis. Shear walls likely
limit-out far sooner.

Hence the flexible vs rigid diaphragm issue is just as Joe Grill said last
Friday, to wit:   "Once again, my opinion is if you can't determine the wall
deflection, hence its rigidity, THE ARGUMENT BETWEEN FLEXIBLE VS RIGID IS
BOGUS."   Yep, folks, it's bogus.

That means we have to do these analyses, not because the buildings need it,
but because certain people who don't like to think (and are unaware of it)
put it in the code that we have to do it. And if you don't do it, they and
others who don't like to think will testify that the buildings need it
because it's in the code. 

I say Wal-Mart doesn't call their janitors "engineers" primarily because it
doesn't want to insult them. 

Charles O. Greenlaw  SE    Sacramento CA