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Re: Gypsum ceiling as structural diaphragm

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Title: Re: Gypsum ceiling as structural diaphragm

Good point Mark. I should have qualified my statement. Table 3.15 of the 2001 WFCM only applies to ceiling diaphragms bracing gable endwalls for wind loads. The WFCM also does NOT assume contribution of gypsum for shearwall design for seismic.


From: Mark Gilligan <MarkKGilligan(--nospam--at)>
Subject: Re: Gypsum ceiling as structural diaphragm
To: seaint <seaint(--nospam--at)>

When using gypsum board for structural sheathing you should consider the
fact that the capacity degrades rapidly when subject to cyclic loading su=
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 th=
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)> 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 p=
when ceiling framing members are spaced 16" o.c., per ICBO Report No.

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 value=


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 = =

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
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)> =

Subject: Gypsum ceiling as structural diaphragm =

To: seaint(--nospam--at) =

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 diaphra=
to resist lateral wind loads? If so, what are the loading limits and othe=
assumptions and precautions that go along with it?

This is a single story gabled end wood structure. The out-of-plane load p=
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 =