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

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Dennis:

Although I would not have worded my reply as strongly as Mark did, he 
does make a valid point.  (Please read my *long* response through 
before forming an opinion; you may find that we agree on important 
points.  I make general comments and comments on woodframe 
construction.)

In general:
While considering diaphragms to have either zero stiffness or 
infinite stiffness might result in easier calculations, it is not 
correct and the code does not indicate that it is appropriate.  For 
instance, Section 1630.6 of the 97 UBC says that "Vx shall be 
distributed to the various elements of the vertical 
lateral-force-resisting system in proportion to their rigidities, 
CONSIDERING THE RIGIDITY OF THE DIAPHRAGM."  It goes on to indicate 
a condition where it is acceptable to consider the diaphragm to have 
zero stiffness for the purposes of horizontal shear distribution.  In 
other words, it defines a case where CONSIDERING equals NEGLECTING.  
If your structure doesn't meet this condition (deemed to be 
"flexible"), you must "consider" the rigidity of the diaphragm just 
as you consider the rigidities of the vertical elements.  There 
certainly are cases where it is reasonable to consider a diaphragm to 
be infinitely rigid with respect to the vertical elements.  However, 
that is not true of every case where the diaphragm doesn't meet 
the "flexible" criterion.

You said:
> 1. If the diaprhagm deflection is less than 2 times the story drift
> the analyisis distribution of shear through the diaprhagm is
> calculated considering the contribution of shear from torsion in the
> diaprhagm.
> 
> 2. If the diaprhagm deflection is equal to or greater than 2 time
> story drift, the shear may be distributed through the diaphragm by
> tributary area.
> 
I agree.  However, as I noted above, the consideration you note in 
item 1 does not lead to your conclusion:
> If it equals 2 times the story drift it is considered not-flexible - which
> in my book means rigid.
While it is true that consideration of infinitely rigid and 
infinitely flexible behavior does bound the real condition (and is 
therefore an acceptable approach), such is not required or indicated 
by the code.

In my practice, it has been important for me to "consider" diaphragm 
stiffness somewhere between zero and infinity.  However, we should 
also keep in mind practical limitations as follows...

For woodframe construction:
Although residential, woodframe construction is very common,
rigorous consideration of all of the complexities is impractical 
(perhaps even impossible) and unnecessary.  I would suggest that 
noone in human history has rigorously considered all of the 
complexities (sloped intersecting roof diaphragms with 
discontinuities for dormers, etc.; stepped floor diaphragms with 
multiple openings; and dozens of structural and nonstructural walls 
and partitions; each item above composed of hundreds or thousands of 
little and not-so-little pieces of wood, gyp bd, and steel with 
highly variable, anisotropic behavior).  Even the most rigorous 
university research (both testing and analysis) ever conducted or 
contemplated (even for the CUREe-Caltech Woodframe Project) starts 
with dozens of simplifying (and arguably unrealistic) assumptions.  
The fact is that the exact behavior of a typical American home is 
more complicated than that of a high-rise office building, a 
long-span bridge, or a stadium.  While it is practical to spend many 
hundreds of thousands of dollars to rigorously consider the behavior 
of one of these "major" structures (that is simpler than a typical 
home), the same can't be done for a home that costs less than the 
analysis and testing would cost.  Our collective experience would 
indicate that such an approach is also unnecessary.

Like Bill, I believe that such buildings merit a special approach.  
The Simplified Analysis Procedure in the 97 UBC and 2000 IBC is a 
step in the right direction.  With the changes being made in the 2000 
NEHRP Provisions, it will be even more simple (no rho factor; no 
accidental torsion; no amplification of torsion; no calculations of 
period, drift, or P-delta; no consideration of deformation 
compatibility; and untopped metal deck, wood panel, or plywood 
sheathed diaphragms may be considered flexible without further 
calculations).

Your concerns have been heard, and it appears that the codewriters 
have responded.  Now we begin the waiting game until these revisions 
hit the street; it will probably take at least three years.

-Mike


> From:          Seaintonln(--nospam--at)aol.com
> Date:          Thu, 23 Sep 1999 22:01:52 EDT
> Subject:       Re: '97 UBC Lateral Design - Envelope Solutions???? Calmed down

> Sorry, I might have lost my temper. I considered this paragraph a personal 
> attack. With that said, I don't agree with you. The code only defines one 
> extreme or the other since it fails to instruct the designer what to do if a 
> condition occurs in between - which can not happen. There is no test in the 
> code that defines a point when the calculated plywood diaphragm deflection 
> equals 2 times story drift as between rigid and flexible. If it equals 2 
> times the story drift it is considered not-flexible - which in my book means 
> rigid. If it is EQUAL TO or greater than 2 times the story drift it is 
> non-rigid, which again in my book of definitions means flexible. There is no 
> in-between unless it is in-between the lines in hidden ink:>)
> ...
> There can be only one answer.
> 1. If the diaprhagm deflection is less than 2 times the story drift the 
> analyisis distribution of shear through the diaprhagm is calculated 
> considering the contribution of shear from torsion in the diaprhagm.
> 
> 2. If the diaprhagm deflection is equal to or greater than 2 time story 
> drift, the shear may be distributed through the diaphragm by tributary area.
> 
> 3. A combination (envelope) of the above.
> ...
> 
> In a message dated 9/23/1999 5:26:45 PM Pacific Daylight Time, 
> Mark.Swingle(--nospam--at)dgs.ca.gov writes:
> 
> << (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. >>

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201