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RE: Wood on Steel Studs

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On Monday, July 28, 1997 10:10 AM, FredT5(--nospam--at)aol.com 
[SMTP:FredT5(--nospam--at)aol.com] wrote:
| Now here's a subject that has been troubling me. Steel stud
| shear walls.
| Check out Chapter 22, Div VIII, Lateral Resistance for Steel
| Stud Wall
| Systems in the 97 UBC. Also see the recent tests from Santa
| Clara Univ. If
| you need copies, contact Hank Martin at AISI, 916-887-8335.
| Wood structural
| panels on steel studs behave quite a bit differently than on
| wood studs.
| Screws are essentially pinned and rotate at the face of steel
| studs after
| first yield.
|
| After additional cycles of loading, the screws have very 
little
| or no initial
| stiffness as reloading for the next cycle begins. It takes
| quite a bit of
| drift before the screws are again engaged and then the lateral
| load
| resistance picks up quite rapidly only after a considerable
| drift.
|
| Despite having very little initial stiffness after the first
| cycle,  design
| values for panels on steel studs can end up being quite a bit
| higher than
| comparable test assemblages on wood studs. The flexural
| capacity of screws is
| not really being tapped. Screws essentially rock at the face 
of
| studs and
| then are engaged in a tension/prying action after considerable
| drift. Since
| tension in screws is higher than bending, the design values 
can
| be higher. Be
| sure to specify screws with the largest/deepest threads to
| reduce the chances
| that screws will simply strip and walk out of their holes. I
| wonder if steel
| studs are taking advantage of a loophole in the way design
| values are
| established.
|
|
| I think this means that after future earthquakes steel stud
| buildings will be
| swaying around and replumbed with very little lateral force.
| Its not a matter
| of instability so much, but it could be unsettling to owners
| who prefer that
| their walls not creak in the wind and not move when leaned
| against. To repair
| damaged steel stud walls, owners may have to plan to remove 
and
| replace the
| screws into newly drilled holes in the studs to restore the
| initial stiffness
| that was lost in the first or few cycles. So the life cycle
| costs for steel
| stud shear walls in high seismic areas may be higher than wood
| studs.
|
| In contrast, nails into wood studs are essentially fixed just
| below the face
| of the stud and take advantage of the flexural capacity of
| nails to dissipate
| energy. After many cycles, wood stud walls with wood 
structural
| panels still
| have considerable initial stiffness for the next cycle. For
| owners, that
| means damaged wall will not creak or drift as easily when
| pushed against
| compared to comparably damaged steel stud walls. In theory,
| wood stud walls
| dissipate much more energy because their hysteretic curves are
| not as pinched
| as steel stud walls.
|
| Have others out there thought about this concern?
|

Inasmuch as cyclic tests were not available when I started to 
design with light gauge steel studs, my main concern was that 
the plywood panel would not seat flat against the stud. This was 
due to the action of the screw working it's way through the 
panel which tightens down before penetration is complete in the 
stud.
To compensate for this, I found a screw called a 
Ply-Metal_Driller.  I believe that it has a smooth shank that is 
slightly smaller than the threads and less than the thickness of 
a 3/8" plywood panel. The shank will allow additional tightening 
which pulls the panel tight to the stud.
Most tests that were done on metal stud shear walls used 20 
gauge studs for their values. The weak link is the stud which 
tends to buckle before damage occurs at the connection. For this 
reason, the highest allowable shear values in the APA reports 
(#157) is 395 plf.
We are now doing a lot of metals stud "starter (low income) 
homes" out in the desert. None of them exceed about 1500 square 
feet or one story. The allowable shear is developed from wind in 
most cases, so cyclic testing may be conservative at best for 
this type of use. Although APA uses 20 gauge studs in their 
tests, their values are based upon a minimum 16 gauge stud shear 
assembly.
There were some interesting posts that I'm sure are one the new 
Web archive locations. These include:
Shear Wall Tests - Steel Framing	Aug. 15, 1996	John Rose (APA)
					Aug. 15, 1996	John Rose
Steel Stud Shear Panels		Aug. 13, 1996  	John Rose / Dennis Wish
Plywood Shear Walls on Metal Studs..	Aug. 08, 1996	John Rose
 NOTE: this one is great because it covers the SEAOSC Cyclic 
load test procedure and the work done in a joint US - Japanese 
conference (TCCMAR) - informative
Re: A new question for APA		Aug 03, 1996	John Rose

Another good source for metal framing is HUD - which you search 
for their web address. There is one web site devoted to light 
gauge which has a document called "A Primer In Light Gage Metal 
Construction" by Robert Schraff. I'm not sure where I found this 
on the web, but if you search for the document you should be 
able to find it.

American Iron and Steel Institute publication No. RG-934a dated 
June 1993 and is titled "Residential Construction Guidelines" 
also contains details for typical residential construction and 
excerpts from the APA report number 154 by John R. Tissel.

I most certainly agree that this is still a very new field and 
that we need to establish much better design criteria for the 
design and detailing of metal stud structures. My main concern 
has been how we are going to identify and repair damaged studs 
once an earthquake hits when the panels don't appear damaged but 
the damaged studs are hidden in the wall.
Inasmuch as many steel stud homes are used for low income 
housing, we need design standards that are agreeable in all 
communities - unfortunately we do not seem to have agreement on 
this as yet.
Finally, one very major problem is the connection of 
conventionally framed wood roofs on metal studs. Conventional 
metal connectors must be properly detailed so that the 
equivalent capacity is provided through screws as is reported 
developed in nails in wood. This is a weak link in the detailing 
stage that I have been working to clear up with my clients. 
Others may have similar problems to share.

Dennis Wish PE