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

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I haven't been following this thread, but the answer to your question on G and E
for OSB is in APA Tech Note N375.  EI values for various span ratings are
tabulated, and I values for various thicknesses are also tabulated; therefore,  E
= EI / I.  Same is true for Gt; G = Gt / t.
John Rose/APA

Seaintonln(--nospam--at) wrote:

> Chuck
> I couldn't agree with you more - however, I am not ready to close up my
> office an until this gets resolved I have no choice but to try and digest
> this c*!p.  As to my question:
> 1. The diaprhagm deflection analysis requires the user to input the area of
> the chord. I assume (as I have for most wood structures) that the chord is
> the double 2x plate at the top of the wall. Conviently, the design example
> uses a 2x4 as the area of the chord (not a double, but a single plate).
> I don't remember specifically specifying the grade of lumber used for the top
> plate of a stud wall, but I know that it would not be more than #2 Lumber
> (Kiln dried) here in the desert. More likely it would be the same material
> used in the walls - either stud or construction grade. At any rate, the E
> value used in the example is not realistic for a stud wall construction. I
> don't really care if the example is not intended to suggest an E value to
> use, but I would appreciate it if the author of the example does not make us
> wonder why he is using a #1 or better grade lumber in a stud wall. This is
> not typical in residential construction.
> 2) The examples pay particular attention to determining the effective
> thickness 't' of the plywood panel - which varies in Structural (5-ply) and
> non-structural panels as it does in Sanded an Unsanded finishes. I would also
> assume that to be accurate (and to avoid liability) it is not unreasonable to
> assume that the majority of diaphragm materials used will be OSB and that
> they will not have the same effective thickness even though they are rated
> the same in shear.
> So, Chuck, my friend, don't pull my chain at 11:45PM as I struggle with the
> poor examples that are suppose to make us all digest this garbage. I have
> five custom homes to complete, clients who don't give a rat's butt if there
> was a code change and aren't going to pay more just because "engineers are
> trying to line their pockets" by creating codes that nobody else can contend
> with (not even the engineers).
> I'm going to bed.......
> Dennis
> In a message dated 7/7/99 5:10:27 PM Pacific Daylight Time,
> cgreenlaw(--nospam--at) writes:
> << 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
>   >>