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Re: '97 UBC Lateral Design - Envelope Solutions???? - Part 1

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This message is too long to be posted to this list.

Well, I guess I have to do it in two parts then.....

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


Now that you have volunteered to be on the panel this weekend, you have to
be prepared to stand by your statements more than ever before.  You are now
in a similar position to that of the committee members of the past who have
been criticized by you (and others) for not being rational.


Dennis Wish (DW) is quoted from his Friday, 24 Sep 1999 posting.
Mark Swingle (MS) responds:

Dennis said:
(DW)  "I don't see any provision in the code that explains alternatives to 
what you describe as the "gray area".  In 1630.1 second paragraph, the code 
explains that if the diaphragm is not flexible "the mass at each level shall
be assumed to be displaced from the calculated center of mass in each 
direction a distance equal to 5% of the building dimension at that level 
perpendicular to the direction of the force under consideration.." This is 
the method to use for a 100% rigid diaphragm since no other comprimise is 

Mark responds:
(MS)  I disagree with your conclusion that "This is the method to use for a
100% rigid diaphragm since no other compromise is suggested."  The
compromise is not only suggested, it is clearly stated in the code.  I was
discussing the paragraph in the code ABOVE this, which says that the forces
shall be distributed CONSIDERING THE RIGIDITY OF THE DIAPHRAGM.  Perhaps the
code should say "considering the rigidity of the diaphragm with respect to
the rigidity of the various elements of the vertical lateral-load-resisting
elements."  In my opinion, that is what the code implies.  So, if your
diaphragm is only PARTIALLY stiff (less than 100%), then the forces will be
only PARTIALLY redistributed, so the force to each wall will be somewhere
between the 100% rigid (infinite stiffness) and the 100% flexible (tributary
forces only) diaphragm cases.  Moving the center of mass is one thing, but
it has nothing to do with what I was talking about.

Dennis continues:
(DW)  "Yes, the stiffness of the wall is the ultimate concern, but the load 
distributed to these walls can vary a great deal on a non-rectangular 
building. This is what prompted my initial concern. If chosing one solution,
we may be ignoring the possiblity of the other. If this "gray area" were 
adequately defined, then the recommended solution might be an envelope 
Mark responds:
(MS)  It sounds to me as if you have become an advocate of the necessity of
rigid diaphragm analysis for wood buildings.  In the past, I thought that
you were saying there is no need for rigid diaphragm analysis in the "real
world" of actual buildings, that tributary force distribution was adequate
for all wood buildings.  That is what I believe.

(For you careful readers, my previous postings on this subject always had to
do with complying with the CODE, but I think the code should be changed.  I
think the code should ALLOW tributary analysis for ALL wood buildings with
wood shear walls and diaphragms.  Some allowance can be made for assuming
closely-spaced walls to be in the same line, if desired.)

Dennis continues:
(DW)  "We simply can not assume the adoption of one method when the
discrepency in 
results is great - and I man hundreds of percentage differnce not a small 
degree. These are the results I am getting in my analysis."

 "Hard facts: in one wall, the flexible analysis yielded 21,371 pounds of 
shear into this lline of resistance. The same grid line, under torson 
analysis 1,523 pounds of shear."
 "Wall 13 yielded 4,536 pounds from tributary distribution while  only 598 
pounds from torsion."
 "Wall 8 yeilded 10,656 # of shear from flexible, but 15,025 from rigid 
 "Wall 24 1,758 from flexible 12,476 from rigid"
 "Wall 25 1,670 from flexible 12,305 from rigid"
 "The building was a "U" shaped structure - 9500 square feet - with one 
portion (the large garage) skewed from the orthagonal."
 "The flexible results were confirmed. The torsion or rigid analysis was
using Enercalc software. The center of Mass determined within Autocad."
 "Results when the shape was rectangular yielded differences between rigid
and flexible that was less than 30% at the max. This is reasonable."
Mark responds:
(MS) I have a hard time believing these numbers.  There must be a mistake
somewhere.  I think you need to check this.  The numbers do not pass the
"sanity check".  Simply the fact that the building is of an unusual shape
should not cause such a large change.

Also, as far as I know, Enercalc's program has no module to calculate the
stiffness of plywood-sheathed walls, so I don't see how it could be used for
such an analysis.  I could be wrong, but I think you need to check your
numbers.  "Fancy" analysis programs are notorious for providing wrong
answers for several reasons: the input may be flawed, the program is not
well-documented, the results are misinterpreted, or some combination of

Let's do a SANITY CHECK on one of your examples.  Since I don't know all the
details of your building, I will have to make some (many) assumptions.
Please bear with me, and perhaps I'll make some mistakes that you can
correct to show where I have gone wrong.  Even so, I think the conclusions
will nevertheless be the same.

To be continued....


Mark Swingle, SE
Oakland, CA

Disclaimer: These are my own opinions and are subject to change.