Valid comments and I thank you. Here is where I think the reasoning breaks
down. The seminar notes are not the legal documents. Although they might
begin to clarify an intent, the sections of code do not appear to bear this
out. Neither do the notes provided in the 1998 SEAOSC Wood Seminar, the
ICBO/SEAOC Seismic Design Problems or the 1998 ICBO seminar in preparation
for the 97 UBC.
The intention is clear that engineers must check the diaphragm and compare
them to a measure used to determine rigidity. All those who work in smaller
wood structures with closely spaced shear walls will confirm that the
majority of these buildings will prove - based upon this criteria - to be
Putting into proper perspective, the designer of residental homes must comply
with these changes in the last few years:
1. 25% reduction in allowable shear capacity of plywood diaphgams
2. Increases in base shear calculations depending upon the proximity of the
structure to an active fault.
3. Increases to the lateral load demand based upon the use of embedded
columns anywhere within the structure to resist shear (including if used in a
portion of a structure that is discontinuous from the rest of the building by
nature of the framing - ie, California framed roofs).
4. Increases in shear due to rotation from a diaphgram considered by
comparison to code test proceedures to be designed as rigid.
Considering the majority of damage is attributed to construction defect, this
seems extraordinarily restrictive and does next to nothing to improve
buildings constructed by Conventional Framing Prescriptive measures.
I agree that ICBO should be willing to address these issues, but didn't they
begin the the SEA committees that presented them to ICBO? I think this is
where the justification or ability to make changes should occur.
I fear that we (the List and any who write with dissention about these code
changes) may be disregarded or considered trouble makers rather than members
who deserve better cooperation. I also fear that constructive critism is
considered rebellious and an insult to those who monitor our posts. I would
hope that they have the stength of character to understand the issues and
present us with appropriate justifcation or protection from liability in the
Dennis S. Wish PE
In a message dated 5/9/99 8:41:44 AM Pacific Daylight Time, FEMCCLURE(--nospam--at)aol.com
<< Subj: Re: Wood Diaphragms
Date: 5/9/99 8:41:44 AM Pacific Daylight Time
Reply-to: <A HREF="mailto:seaint(--nospam--at)seaint.org">seaint(--nospam--at)seaint.org</A>
CC: FEMCCLURE(--nospam--at)aol.com, cgreenlaw(--nospam--at)speedlink.com, MESSINGERD(--nospam--at)aol.com,
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
"Accidental Torsion - Where diaphragms are not flexible, story shear shall
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
diaphragm is considered as "flexible" according to the seminar notes, and
horizontal torsion considerations apply only to "rigid" diaphragms, then
not this wording mitigate the need to consider the relative rigidities of
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