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# Re: Rigid plywood diaphragms

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Rigid plywood diaphragms
• From: ErnieNSE(--nospam--at)AOL.COM
• Date: Sat, 21 Nov 1998 01:33:33 EST

```Sasha,

Do a rigidity analysis if you believe it is the right thing to do. Not just
because you are afraid of a lawsuit.

And if you do believe that a rigidity analysis is appropriate, study the
formula for relative rigidity calculations and do not just accept and use
something that "somebody" said is OK. Unless there is a textbook accepted and
recommended by the engineering community or by a known authority in this
concept that is available. If anybody knows of such publications, please let
us know.

The only book I know is the Army Manual TM 5-809-10 Chapter 5, which talks
about wood diaphragm rigidity but there are no examples of relative rigidity
calculations. You have to do plywood diaphragm and shear wall calculations
using the UBC formula. Just use your judgement on those that are not as
specified in the formula such as unblocked diaphragm or if nail spacing is not
the same throughout, nail slip at chord splice, shear wall deflection due to
wood shrinkage and/or crushing, hold-down slip, etc. And then calculate
rigidity = 1/deflection.

I haven't done this type of analysis but I know of one engineer who said he
has been doing it for a while on his church buildings.

Has anybody had a plan check correction that says .......comply with 1994 UBC
Sec. 1630.6 .......the design story shear ........shall be distributed to the
various elements of the lateral force resisting system in proportion to their
rigidities, considering the rigidity of the diaphragm............Diaphragms
shall be considered flexible for the distribution of story shear ......when
the maximum lateral deformation of the diaphragm is more than two times the
average story drift of the associated story.......? How should we respond to
this correction? Can we say "not applicable to residential wood diaphragms"?

I personally believe that it is applicable to certain wood diaphragms.