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RE: '97 UBC Lateral Design - Envelope Solutions????

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Dennis,

"....What are your opinions?...."

(MS) You asked for it this time, Dennis :)

"What are your opinions? Do you interpret the code as requiring the
distribution by enveloped methods (worst case from each design), or from the
test applied to the stiffness of the diaprhagm?"

(MS) Neither one, at least not in all cases.  Read on.

You state that the "....analysis should be based on the determination if the
diaprhagm is rigid or flexible."

(MS) This is NOT what the code says.  The code only asks you to determine if
it is flexible or not.  The shear distribution is based upon the actual
stiffness, not on whether it passes the "flexibility test."

"Inasmuch as the majority of wood diaprhagms in residential construction are
determined to be rigid...."

(MS) Sorry, Dennis, but if you are going to rant like this, READ THE CODE.
The code does not use the phrase "rigid diaphragm".  It is not an either/or
situation, it is a continuum, with the only exception being that all
diaphragms that meet the test are deemed to be 100% flexible.  However, most
diaphragms in residential construction are neither 100% rigid nor 100%
flexible, they fall somewhere between.  We have been through this before
several times, Dennis.

"The idea of using the diaphragm deflection test as a means to determine
which method governs is not acceptable to me - ...."

(MS) I agree with you since it is a poorly-defined test at best, but
unfortunately, that IS in the code, so we have to do our best with it.  This
test (UBC 1630.6) is only to determine if the diaphragm is flexible per the
code; if it fails this test THAT DOES NOT MEAN THE DIAPHRAGM IS 100% RIGID.
Suppose the diaphragm just misses the test (say 1.5 instead of 2), in this
case it will still behave MORE like a flexible diaphragm than a 100% rigid
one, and shear REdistribution (from tributary) will be minimal.

As I said above, most diaphragms are neither 100% rigid nor 100% flexible,
they fall somewhere between.  Therefore the horizontal distribution of shear
to the various walls will fall somewhere BETWEEN the two extremes of the
flexible case and the rigid case for each individual wall.  An envelope
solution is ONE way to assign shears, but it is not the only way AND AN
ENVELOPE SOLUTION IS NOT PRESCRIBED IN THE CODE, regardless of the SEAOSC
seminar notes.

In some cases an envelope solution may be the easiest way to finish the
design (with some conservatism), and is not always a major penalty on the
final shear wall designs (edge nailing and holdowns).  But sometimes this is
too conservative, I agree with you.

The final design forces to each wall are the result of judgement, based upon
experience and/or calculation as to the relative rigidity of the diaphragm
with respect to the rigidities of the supporting shear walls.  This is the
"considering the rigidity of the diaphragm" that the code requires.

Bill Allen's reponse to your other post of the same day <<Subject: RE: '97
UBC Design - Are you too old to change your ways???>> is also a fine answer
to this post.  I quote in part:

(BA)"....O.K., now do a vertical redistribution of forces in some rational
manner. Either inverted triangular distribution or rectangular. Doesn't
really matter that much.  O.K., now do a horizontal distribution of forces
in some rational manner. It doesn't really matter if it's flexible, rigid,
or rigid-flexible. Just something rational. Now, stand back and look at the
structure. Make sure you have a load path...."

Mark Swingle, SE
Oakland, CA

Disclaimer: these are my own opinions
and they are subject to change.
__________________________________

Dennis Wish wrote on 22 Sep 99:

From: Seaintonln(--nospam--at)aol.com
Subject: '97 UBC Lateral Design - Envelope Solutions????
To: seaint(--nospam--at)seaint.org

I have assumed the design menthodology of the '97 UBC for wood framed 
structures to be an envelope proceedure - where the results of the flexible 
and rigid diaprhagm shear distribution are forces are distributed based upon

the worst case condition.

I had a discussion with another engineer who indicated that this is not 
clearly defined in the code and that the analysis should be based on the 
determination if the diaprhagm is rigid or flexible. Inasmuch as the
majority 
of wood diaprhagms in residential construction are determined to be rigid, I

would need to resort to distribution by torsion and may be allowed to
neglect 
proportional distribution (flexible) analysis.

The design examples from the 1998 Wood Seminar by SEAOSC uses an envelope 
solution - which I have followed on the designs I have completed. The
results 
are, in my opinion, overly conservative at best, based on the knowledge I 
have of good performance based on proper detailing and construction.

The idea of using the diaphragm deflection test as a means to determine
which 
method governs is not acceptable to me - especially when the distribution of

forces may result in a reaction that is less than I historically determined 
from flexible methods. It is not so much a question of policy as it is that
I 
have no devloped sense of how the materials will behave torsionally as I
have 
from my history of flexible design and the knowledge of the good performance

of the buildings I have designed.

What are your opinions? Do you interpret the code as requiring the 
distribution by enveloped methods (worst case from each design), or from the

test applied to the stiffness of the diaprhagm? 

Dennis S. Wish PE