The hard numbers are:
Per LA amendments to the CBC:
Stucco = 90 lbs/ft allowed in top story only
G.B. = 30 lbs/ft allowed in top story only
Per CBC Chapter 25 (in Vol. 1):
Stucco = 0.5 x 180 = 90 lbs/ft for sesimic zones 3 & 4
G.B = 0.5 x 125 = 62.5 lbs/ft for seismic zones 3 & 4 with EN & intermediate nailing @ 4" o.c.
= 0.5 x 100 = 50 lbs/ft for seismic zones 3 & 4 with EN & intermediate nailing @ 7" o.c.
Not allowed to support loads imposed by masonry or concrete construction.
By state law, City of Riverside can not allow more capacity but could do what LA has done in reducing it.
With these values, you would need a lot of wall length which may be your "hard" numbers.
I concur with others' comments about the low residual strengths but understand your attempts to give your client what he wants. The couple of times I've had a client push on this issue, I've written them a letter identifying the drawbacks of the brittle finishes. In each case they "saw the light".
Good luck with your discussions!
Bill Cain, S.E.
From: Bill Allen <T.W.Allen(--nospam--at)cox.net>
Sent: Fri, 23 Jun 2006 05:31:48 -0700
Subject: RE: Stucco or GWB as shear wall in seismic zone 4 (1997 UBC/2001 CBC)
The client is not as interested in residual strength as I am. He is interested in meeting code. From that perspective, how do you think he would respond to the suggestion that the building be designed for a lower R value and lower SW capacity than the current building code prescribes as well as story and roofing system limitations not specified in the building code? Don’t get me wrong, I agree with all of your points that the structure you describe would perform much better. But if my customer is looking to meet building code minimums, I need more data that this (like LARUCP has banned such materials).
BTW, the project is in the city of Riverside.
T. William (Bill) Allen, S.E.
Consulting Structural Engineers
V (949) 248-8588 •
From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com]
Sent: Friday, June 23, 2006 12:03 AM
Subject: Re: Stucco or GWB as shear wall in seismic zone 4 (1997 UBC/2001 CBC)
I would try and convince him that plywood/OSB provides a lot more residual strength to resist seismic forces, especially racking action in a house..its just to brittle. Like Charley said, I am concerned about using GWB because of its lack of post peak strengthen beyond peak strength.
If I was to design with GWB it would have to be limited to a single story house (no tile roof), lots of walls so the seismic forces in the walls were less than 30 pounds/ft ..not the higher values shown in the UBC (also shear wall lengths no less than 8 feet long) and I would probably look at using an R value much less than 4.5 (97 UBC), probably closer to 2.0 (I believe R = 2.0 in the current IBC). At this point, I don't think GWB would work for the design loads unless you had a lot of long walls.
I think LA City limits GWB to 30 pounds a foot.
In a message dated 6/22/2006 6:17:59 P.M. Pacific Standard Time, T.W.Allen(--nospam--at)cox.net writes:
I haven't used stucco or GWB since Northridge, but a new customer is asking
me to look into it. If the loads are less than the capacity (even the
reduced capacity of GWB), I might have trouble justifying WSP just because
it "feels good".
Does anyone have evidence which would allow me to not use stucco or GWB as
SWs and still feel like I've tried to accommodate a customer's request?