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Rigid plywood diaphragms

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Mr. Ed Dickman was the speaker at the "Performance of Wood Structures"
seminar in San Francisco.  The topic of his presentation was "Litigation and
Code Issues in the design of wood frame buildings".

The following is an excerpts from his presentation:

" Consideration of torsion also requires the rigidity of the resisting
elements be taken into account.  This is easy to say, but hard to do because
the engineering profession does not have proper data regarding the rigidity
of wood frame walls..... Lacking data the engineer is left to judgement and
the injunction that whatever is done must be rational and conform to
principles of mechanics.  It appears that the rigidity may be approximated
by utilizing the design unit shear for the wall sheathing multiplied by the
length of the wall."

His last statement caused some confusion.  I he talking about the  rated
capacity of the wall times the length or the unit shear force in the wall
times the length.  If it is the unit shear then how is it derived? If it is
based on the flexible diaphragm analysis then it just does not make much

He also said:

"Since most residential buildings typically have deep, short span
diaphragms, it is virtually certain that these diaphragms can not be
considered flexible under [1994 Uniform Building] code definition and that
force distribution to the shearwalls must be made on the basis of wall
rigidities.  Engineers should adjust their design practices to conform to
these 1994 and 1997 UBC provisions in order to avoid lawsuits at the later

Your opinions would be appreciated.

Sasha Itsekson, PE
Ingraham/ DeJesee Associates