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Re: Ron .....Rigid Diaphragm Analysis

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I don't believe the rigid diaphragm language was in the 1985 UBC.  But
anyway, my point was no new requirement had been created by the 97 UBC,
which was the earlier suggestion by this thread of discussion.

Now back to the real question - Do I think that houses should be designed
as if they have rigid diaphragms? Absolutely not!  Have I ever done that? -
again, NO!    Does the code require this ?- probably.

The '97 UBC has a Simplified Procedure - intended for buildings like
houses, that eliminates the need to do some of the base shear calculation
and also eliminates the need to check deflections.  It may be possible to
modify this to take out a need to do rigid diaphragm analysis.  I will have
the Seismology Committee look into it.


SDGSE(--nospam--at) on 05/08/99 08:29:46 PM

Please respond to seaint(--nospam--at)
 To:      seaint(--nospam--at)                                   
 cc:      (bcc: Ron O. Hamburger/EQE)                         
 Subject: Re: Ron .....Rigid Diaphragm Analysis               

In a message dated 5/8/99 6:45:29 AM Pacific Daylight Time, ROH(--nospam--at)

<< Gentlemen-

 Here's the facts with regard to the rigid diaphragm requirement,

 Refer to your 1988 UBC (yes that's right 1988!!)

 Section 2312 sub para 5.
 The desgin story shear Vx, in any story is the sumof the forces Ft and Fx
 above that story.  Vx shall be distributed ot he various elements of the
 vertical lateral force resisting system in proportion to their rigidities,
 considering the rigidity of the diaphragm....

 Section 2312 sub para 6.
 Provision shall be made for the increased shears resulting from horizontal
 torsion where diaphragms are not flexible.    Diagphrams shall be
 considered flexible for the purpose of this paragraph when the maximum
 lateral deformation of the diaphragm is more than two times the average
 story drift of the associated story....

 Seem familiary????

 Also Ron Gallagher is not the Seismology chair.  So - I don't think he was
 agrandizing himself.


If I understand the above reply correctly, you are implying that wood
diaphragms should have been analized/designed based on rigidity per the
If I remember correctly, the 1985 UBC had the same or similar language (I
not have the 1985 UBC handy). Regardless, it's been widely accepted and
presumed that wood diaphragms were to be regarded as flexible up until the
rigid vs. flexible wood diaphragm issue came about (I don't know how it

My question to you is if section 2312 sub para 5 & 6 in the 1988 UBC
triggered the wood diaphragm rigid/flexible analysis requirement, have you
done such an analysis since then for a single family residence? I have not,
and I know quite few engineers who haven't. Were we wrong?


Oshin Tosounian, S.E.