Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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.......


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
 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