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Re: Ron .....Rigid Diaphragm Analysis

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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 1988 
If I remember correctly, the 1985 UBC had the same or similar language (I do 
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.