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RE: WOOD RIGID DIAPHRAGM RESIDENTIAL UBC 1628.5

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Lots of questions here. The gist of the requirement is:
1. In most cases, the diaphragm deflection will be less than 2 times the
story drift and the diaphragm is to be treated like a rigid diaphragm.
2. The code intended for diaphragms in rigid structures to be determined by
analysis as blocked. However, this is not the case in all instances and
diaphragms will be designed well within the unblocked requirements as is
typical in residential construction. Again, the code does not omit unblocked
diaphragms from rigid analysis since the deflection is still expected to be
less than 2 times story drift.
3. I need to find the spreadsheet that was given to me by James Lord in
Northern California - but this can be used for rigid diaphragm analysis and
I will place it on the website as soon as I locate it (I have not done very
much with rigid analysis in the past and this accounts for why I don't have
it handy).
4. You still should compare the results of rigid and flexible diaphragms and
design the line of shear for the worst case (also compare to wind). I'm not
sure it is stated this way in the code, but the design examples from the
SEAOSC Wood Seminar from February 1998 uses this approach. You might contact
Bill Nelson at SEAOSC for his opinion (he is chair of the wood committee).

Hope this helps
Dennis Wish PE

-----Original Message-----
From: merrick group [mailto:merrickgroup(--nospam--at)compuserve.com]
Sent: Friday, December 18, 1998 1:29 PM
To: SEAonc seaint
Subject: WOOD RIGID DIAPHRAGM RESIDENTIAL UBC 1628.5


RIGID WOOD DIAPHRAGM ANALYSIS FOR
RESIDENTIAL.
I have the following inquiry
Comments please, SEND TO
MERRICKGROUP(--nospam--at)COMPUSERVE.COM

UBC 1997 section 1630.6 and
UBC 1994 section 1628.5 requires a diaphragm to
be analyzed as flexible when the deformation of the
diaphragm is more than 2 times the average story
drift of the associated story.

If deformation was less than 2 times..., could it be
analyzed as flexible?

There is no wood house exclusion directly made in
the code.  Most residential wood frames have rigid
diaphragms per code.

For a wood diaphragm that is not blocked, the
deflection equation is missing in the Standards.
Does this mean that the rigid diaphragm rule
excludes unblocked diaphragms?

Many code sections require the use of equations
that are not given.  An adjustment factor could be
made to the equations for the blocked diaphragm
equations found in the Standards.

Possible factors are..
******Increase the deflection six times for loads
parallel to unblocked edges.
***Increase the deflection three times for loads
parallel to the joists.

Code sections are given with deflection equations
for some plywood diaphragm cases and not others.
Does an excluded unblocked diaphragm equation
of a family of included equations exclude the
unblocked diaphragm case from the code section
1628.5?

Does a building official have the power to add the
exclusion of unblocked diaphragms for residential
construction to the code section 1628.5?

Does a popular vote of engineers create a Standard
of Care over the legally accepted Standard of Care,
the code?  Legal cases exist where the answer is
no.

Are there wood analysis programs using the 1994
code with a rigid diaphragm option?

Wood diaphragm force distributions have been
treated as being flexible in shear and rigid in
flexure. Another method uses the envelope of two
combinations of shear or flexure and rigid or
flexible.

1. Flexible shear, rigid flexure: This is
the tributary area method

2. Rigid shear, flexible flexure: This is
where interior shear walls are
increased per the continuous beam
model

3. The enveloping of case 1 and case 2:
Enveloping case 1 and 2 is found in
ATC-4

4. Rigid shear, rigid flexure: By demand
of the code section of 1628.5.  This is
the method used for simple concrete
structures, and seems to be the
recommended approach per code.

5. Is an enveloping of all cases, or cases
1 and 4, reasonable?