It is with considerable fear and trepidation (nervous apprehension) that I
would even try to contribute to the discussion concerning "rigid" and
"flexible" wood diaphragms with so many more knowledgeable persons having
posted their thoughts concerning the subject matter.
I sincerely appreciate the time and effort that Lynn Howard, Dennis Wish,
Neil Moore, Bob Bossi, Charles Greenlaw and others have taken to discuss the
issue of "rigid" and "flexible" wood diaphragms.
The following thoughts are offered for whatever they might be worth and
perhaps some will consider that they are not relevant to the issues at hand.
On November 16, 1995, I attended an International Conference of Building
Officials, Seminar 102 on the 1994 Uniform Building Code Update, Structural
Provisions. On page 29 of the notes for this seminar, under the heading,
"Section 1628.5, Horizontal Distribution of Shear" was the following wording
"Accidental Torsion - Where diaphragms are not flexible, story shear shall be
assumed to act at an eccentricity of 5% of building dimension. Horizontal
torsion and accidental torsion apply for rigid diaphragms ONLY. Rigid -
Concrete floor and roof slabs, steel deck systems with concrete fill.
Flexible - Plywood floor and roof systems, steel deck systems (w/o concrete
fill)." (Emphasis added.)
Now, I realize there are other sections in the 1994 Uniform Building Code
that might modify the above wording in the seminar notes. However, if a wood
diaphragm is considered as "flexible" according to the seminar notes, and
horizontal torsion considerations apply only to "rigid" diaphragms, then does
not this wording mitigate the need to consider the relative rigidities of the
vertical resisting elements in the lateral force analysis when you have a
Can we calculate with any reasonable degree of accuracy the "average story
drift of the associated story" in a wood frame structure to make a
determination whether a diaphragm is "flexible" or "rigid" according to 1994
UBC, page 2-18, Section 1628.5, Horizontal Distribution of Shear?
Has anyone considered contacting ICBO, ( Telephone 310-699-0541) and
discussing the problems related to whether a wood diaphragm is to be
considered "flexible" or "rigid" and under what conditions?
Jerry Neville and Jim Hodges at ICBO were the excellent lecturers at the
above seminar and they might be able to help. I am not sure if Rick Okawa,
P.E., Vice President, Codes, Engineering and Education Services is still at
ICBO, but you might contact him for help. Feel free to use my name if you
believe it might help to get a response from ICBO. I went to my first ICBO
Annual Meeting in 1947.
Frank E. McClure FEMCCLURE(--nospam--at)aol.com May 8, 1999