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Re: Wood Diaphragms

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Frank, 
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 
rigid.

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 
mean time.

Respectfully,
Dennis S. Wish PE

In a message dated 5/9/99 8:41:44 AM Pacific Daylight Time, FEMCCLURE(--nospam--at)aol.com 
writes:

<< Subj:	 Re: Wood Diaphragms
 Date:	5/9/99 8:41:44 AM Pacific Daylight Time
 From:	FEMCCLURE(--nospam--at)aol.com
 Reply-to:	<A HREF="mailto:seaint(--nospam--at)seaint.org">seaint(--nospam--at)seaint.org</A>
 To:	seaint(--nospam--at)seaint.org
 CC:	FEMCCLURE(--nospam--at)aol.com, cgreenlaw(--nospam--at)speedlink.com, MESSINGERD(--nospam--at)aol.com, 
hblla(--nospam--at)earthlink.net
 
 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 
 "flexible" diaphragm. 
 
 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
   >>