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Re: '97 UBC Lateral Design - Envelope Solutions???? Calmed down reply

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Mark, see my comments below:

In a message dated 9/24/1999 12:01:55 PM Pacific Daylight Time, 
Mark.Swingle(--nospam--at) writes:
<<1.  Within item 1 above is a whole range of possibilities, not just one
answer.  The range includes 100% rigid at one extreme and 100% flexible at
the other extreme.>>

(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 
In the fourth paragraph of this section it states "Diaphragms shall be 
considered flexible for the purposes of distribution of story shear and 
torsional moment when the maximum lateral deformation of the diaphragm is 
more than two times the average story drift of the associated stories. This 
may be determined by comparing the computed midpoint in-plane deflection of 
the diaprhagm itself under lateral load with the story drift of adjoining 
vertical-resisting elements under equivalent tributary load."

This, in my opinion states that if the diaphragm passes this test, forces 
shall be distributed by tributary load. 

Theoretically, we argue the possiblity of something in between, but in my 
opinion there is a definate difference between the rigid and flexible until 
someone defines both the condition and the means to resolve the comprimise.

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 

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 analysis
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 done 
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.

So the answer, in my opinon, was to design the "U" shaped structure using an 
enveloped solution.

Mark, these are practical questions since another engineer will, ultimately 
face the same problem and not know how to address it.

<< What I hope is that we can move the discussion beyond what the code says 
 toward developing the analytical tools required to comply with what the code
 says.  (I also think that we should continue to lobby to have this portion
 of the code changed, with whatever means there are available to us.  That
 will be a different post.)>>

(DW) How can we discuss simplified analytical tools when we analyze the 
results of the present code provisions and recommend to the user just what 
the authors intention was in the publication of a methodology. 
I may agree with you that the performance of wood diaprhagms is somewhere in 
between, but it is the responsiblity of the policy makers to address these 
problems and create policy to resolve them and not simply to leave an 
interpretation that creates more revenue for the legal profession.
If the tools are not adequate for a project - replacement.

(DW) Where in the code is their a proceedure that explains what to do if the 
 I know you have spent considerable time in using the sw deflection formula
 for wood walls.  I am interested in your thoughts and/or experience with the
 use of these formulas.  The most important piece in this puzzle is the
 stiffness of the WALLS, not the stiffness of the diaphragms.  I still don't
 know how one can determine the forces to the walls assuming 100% rigid
 diaphragm when the shear wall deflection equations are so poorly defined,
 limited in scope, subject to interpretation, and non-linear.  I would be
 interested in your methodolgy with regard to determining wood sw

(DW) I agree and am not arguing the issue of resisting wall stiffness but 
there is obviously significant discrepensies in the distribution of shear 
through the diaprhagm when one wall calculates as needing to resist 1,500 
pounds while the same wall, using another appropach is calculated at 21,000 

If we follow your strict interpretation of the the code method so that if the 
diaphragm is "not-flexible" we distribute shear by torsional analysis - how 
many of us would feel justified in ignoring the flexible resulsts when the 
discrepancy between the two is 18 times larger than what we chose to resist. 
Remember, it is not a clear cut case of one analysis yielding more 
restrictive answers - this depends on the wall stiffness and the location of 
the applied loads. In my example there is sufficient different on some walls 
to warrant compliance with the flexible analysis while other walls require 
compliance with rigid.
Following the code, which has not addressed these possiblities may make us 
liable for damages, loss of live or the simple headache of being accused of