In a message dated 5/8/99 6:45:29 AM Pacific Daylight Time, ROH(--nospam--at)eqe.com
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....
Also Ron Gallagher is not the Seismology chair. So - I don't think he was
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.