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RE: Stucco or GWB as shear wall in seismic zone 4 (1997 UBC/2001 CBC)

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Bill,
 
If you are looking for good reasons NOT to use stucco as a sheathing material, there is some evidence to suggest two biggies:
 
1.  Drip screed -- Dave already mentioned this.  There is a report by Ben Schmid that was presented at the SEAOC 71st Annual Convention in 2002 in Santa Barbara, in which he states that "The introduction of a metal drip screed at the sill plate in the 1967 UBC ... prevented suitable attachment of stucco to sill by staples or furring nails." The shear value, he states, is then defined by the end or toe nailing of the studs to the sill plate.  However, in areas of high seismic activity (MMI 8 or even 9), Schmid does not suggest a discontinued use of stucco, but recommends the use of special double-headed nails along the sill in combination with furring nails and a 26-gage drip screed. The raised head is embedded within the 7/8" plaster (reinforced with mesh), providing a solid connection.
 
2.  Brittle behavior -- For seismic activity, we know how important ductility is.  Testing by CUREE and others have proven the brittle behavior of stucco which is a temptation right off the bat to exclude it's use as a sheathing material for seismic resistance.  However, CUREE suggests that the Code-prescribed allowable shear loads are so low that drift limitations will not be exceeded, thus justifying the use of this material at least until further testing can be completed.  the CUREE website has a HUGE report on sheathing material testing ... FREE (the best part)!
 
Similar results and recommendations have been issued for the use -- or disuse of GWB -- by a variety of sources, much testing of which can be found on the internet ... FREE (the best part)!  The researchers acknowledge that further testing is needed under a variety of conditions and there is evidence leaning in BOTH directions that you could use to justify your decision, whatever it may be.  Obviously, the regularity of the structure must be taken into consideration when deciding on sheathing materials.  Remember that the Code prescribes a low level of performance:  Lots of structural damage may occur after a major seismic or wind event that needs to be fixed, but the occupants should be able to get out safely -- another reason that can certainly be considered in specifying a more robust lateral force resisting system.
 
HTH,
Dave K. Adams, S.E.
Lane Engineers, Inc.
Tulare, CA
 
 
 
-----Original Message-----
From: Dave Gaines [mailto:gainesengr(--nospam--at)earthlink.net]
Sent: Friday, June 23, 2006 8:32 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Stucco or GWB as shear wall in seismic zone 4 (1997 UBC/2001 CBC)

Bill,
I've seen illustrations and architectural details of the exterior stucco drip screed at the bottom of the wall that suggest that it may reduce or eliminate the connection of the stucco to the bottom plate and/or the foundation wall, thereby eliminating shear transfer of the stucco at the foundation.
If GWB is used, then in the event of an earthquake or wind storm there will be architectural damage, such as to wall finishes at window corners.  Riverside is a windy place and without checking the seismic map I know it's fairly close to the San Andreas fault.  One large event or several smaller ones could cost the owner more than a few wood shear panels.  If the loads are low the structure could be designed with a few sections of WSW, reducing the materials and labor cost.
What is the deflection of GWB at maximum lateral load?  Is the drift acceptable?
Given the code and design issues raised here by others, I'd guess that most houses with windows would not satisfy design requirements for GWB as SWs.
I would recommend to the owner that he design a more durable building.
Dave Gaines,
Pasadena, CA
----- Original Message -----
From: Bill Allen
Sent: Thursday, June 22, 2006 6:17 PM
Subject: Stucco or GWB as shear wall in seismic zone 4 (1997 UBC/2001 CBC)

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?

 

Thanks,

 

T. William (Bill) Allen, S.E.

ALLEN DESIGNS <http://www.AllenDesigns.com>

Consulting Structural Engineers
 V (949) 248-8588 * F(949) 209-2509