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Re: Gypsum ceiling as structural diaphragm

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I find that in residential remodeling, anything that isn't in the vertical load path or explicitly braced is considered to be a non-loadbearing wall. This is my biggest reason for not counting drywall as a lateral stiffening element, as it is likely that one or more will be removed during the life of the building. If I have to use interior partitions, I will usually indicate 1x let-in braces or (in severe cases) have one side of the wall sheathed.

While this is less of a concern for ceilings, I'm finding it a popular idea to remove the ceilings from old houses (1900-1960 era) in order to get a cathedral ceiling in an existing space. This causes several problems in addition to the ceiling sheathing problem. Obviously, this isn't an issue where there are trusses above the space.

At 02:55 AM 12/7/2004 -0500, you wrote:



When using gypsum board for structural sheathing you should consider the
fact that the capacity degrades rapidly when subject to cyclic loading such
as earthquakes.  The nail crushes the gypsum board leaving you with a big
hole with nothing for the nail to push against.  In addition these values
assume a level of workmanship that may not be achievable.  If they hit the
nails too hard the gypsum adjacent to the nail will be pulverized before
there is any loading.

Thus my advise is not to use gypsum board sheathing to resist earthquake
loads.  It is interesting to note, that at one time I was engineering a
house for a drywall contractor and when I asked him his opinion regarding
using this product to resist seismic loads, he indicated that he wasn't
interested in persuing this option.

Mark Gilligan


+++++++++++++++++++++++++++++++++++++++++++++++++++++++
Thank you!  I must have flipped through too quickly.

I also found it in the 2003 IBC section 2508, but that sections stops you
at 70/90 plf.  Now its time to crunch numbers to hopefully get it to work
for 150 plf...

Jim Wilson

AWC Info <AWCInfo(--nospam--at)afandpa.org> wrote:
AF&PA's Wood Frame Construction Manual (WFCM) for One- and Two-Family
Dwellings, 2001 Edition outlines a procedure for calculating diaphragm
capacity to brace gable endwalls against wind loads. The tabulated gypsum
diaphragm capacity is 70plf, with a note that it can be increased to 90 plf
when ceiling framing members are spaced 16" o.c., per ICBO Report No.
1874-89.

Table 3.15 of the WFCM actually tabulates minimum attic floor/ceiling
lengths for various wind loads and building geometries. WFCM Table 2.6
outlines lateral diaphragm loads that are used in Table 3.15. The WFCM
Commentary provides background calculations for all these tabulated values.

HTH

Buddy

John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
P: 202-463-2769
F: 202-463-2791
http://www.awc.org

The American Wood Council (AWC) is the wood products division of the
American Forest & Paper Association (AF&PA). AWC develops internationally
recognized standards for wood design and construction. Its efforts with
building codes and standards, engineering and research, and technology
transfer ensure proper application for engineered and traditional wood
products.

*********************
>The guidance provided herein is not a formal interpretation of any AF&PA
standard.  Interpretations of AF&PA standards are only available through a
formal process outlined in AF&PA's standards development procedures.

*********************



From: Jim Wilson <wilsonengineers(--nospam--at)yahoo.com>
Subject: Gypsum ceiling as structural diaphragm
To: seaint(--nospam--at)seaint.org

I thought this was discussed a few months ago, but I can't find it in the
archives -
Can a ceiling covered with gypsum drywall be used as a structural diaphragm
to resist lateral wind loads? If so, what are the loading limits and other
assumptions and precautions that go along with it?

This is a single story gabled end wood structure. The out-of-plane load per
foot at the base of the end truss is about 150plf. At their peak, the
trusses will be 16ft tall (piggy-backed) practically eliminating the
possibility of bracing with kickers up to the roof diaphragm.

Thanks,
Jim Wilson, PE
Stroudsburg, PA


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