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Re: Gypsum board for bracing in 03 IBC Seismic Design Category D

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A 0.5% drift is twice of the code requirement (0.25%).  It is a lot - 1/2" for a 8' wall, imagine a 1"-amplitude movement of your floor.
During the Northridge earthquake, my house was moving with great amplitude. I remember waking up at 4:30 AM, analyzing - for few seconds - the magnitude of movement, and, based upon what I knew - expecting the whole thing to come down pretty quickly.  In the morning, when I checked, there were no cracks in the drywall and stucco.
I went to the SF valley for inspection the same afternoon, and was on those for several years.  I very rarely saw any signs of failure (cracking, etc.) in the drywall-only finishes on solid portions of the walls with reasonable proportions (L/H>0.75).
Based upon my own observations after several good-size quakes, drywall appears to actually be a pretty good structural stuff.  I mean, it is good to have it's resistance mostly neglected in our structural analysis :).
Steve Gordin SE
Irvine CA
----- Original Message -----
Sent: Thursday, October 06, 2005 7:52 PM
Subject: Re: Gypsum board for bracing in 03 IBC Seismic Design Category D

Having tested a great number GWB "shear walls", I can tell you they have a fair amount of strength (SLS of about 150 plf) BUT if the drift goes beyond  ~.5% the lateral strength falls off VERY quickly.

SO as long as the GWB shear walls are not "over deflected" they'll be fine.  It's when they're expected to work w/ & see the same drift as plywood shearwalls that problems develop.

IMO an all GWB system can work if one designs for proper drift limits.


On 10/6/05, S. Gordin <mailbox(--nospam--at)> wrote:
I have a suspicion that at least some steel-framed buildings in the LA area are being supported laterally by the unintended (but quite welcomed) action of their "non-structural" drywall finishes.
Steve Gordin SE
Irvine CA  
----- Original Message -----
Sent: Thursday, October 06, 2005 2:09 PM
Subject: RE: Gypsum board for bracing in 03 IBC Seismic Design Category D

Dan Morrow said
...I personally don't believe in gypboard shearwalls for seismic (cyclic)
loads as it seems like after 1 or 2 cycles, the gyp at the nails or
screws is crumbled and offers no resistance.
That is like saying I don't believe in wood because steel is stronger.
Design loads and capacities are adjusted for gypsum wall board behavior. A resulting design may be better than a plywood design. (you are not being gypped).
The gypsum board is more resistant than that allowed for cyclic loading and therefore opined as a weak wall. It is overlooked as an UNINTENDED SHEAR WALL. End bearing, due to prejudicially unintended overturning can over load supports. ie joists
Using gypsum wall board may be a little "green" alternative.
David Merrick, SE
Sacramento, CA