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Re[2]: Effective Length of Wall Studs

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I agree that the studs must be adequately braced against weak axis buckling 
in order to have sufficient axial strength to resist even minimum specified 
code live load on floors, snow load, and dead loads. And these loads do not 
have to act together to easily exceed the allowable axial force based on weak 
axis buckling. In section A.11.3 of Appendix A of ANSI/NFoPA NDS-1991 
"National Design Specification for Wood Construction", it states that in 
light frame construction when stud walls are sheathed on at least one side, 
the depth rather than thickness of the stud can be used in the l/d ratio, 
with the proviso that "the sheathing be shown by experience to provide 
lateral support and shall be adequately fastened." In most residential 
construction I have seen, there is no bridging between stud walls. Therefore, 
the exterior and interior sheathing is all that is available to brace the 
studs in the weak direction. In the case of a house like mine with Celotex 
and aluminum siding on the exterior, the 1/2 "gyp board interior is all that 
is available as bracing. This construction is common, and I wonder what 
testing has been done to demonstrate that the gyp board on one face of a stud 
adequately braces the stud to allow it to reach its calculated allowable load 
capacity of 2100 lbs in the case of a S-P-F 2x4 Stud, as limited by strong 
axis buckling. I am picturing a test setup with an 8 ft. long 2x4 stud, 
toe-nailed into top and bottom 2x4 plates, with a 16" wide strip of 1/2" gyp 
board (both long edges restrained to prevent in-plane lateral dispacement) 
intermittently nailed to the 1 1/2" face of the stud over the 8 ft. length. 
Its hard for me to believe that the axial load carrying capacity of this is 
even the calculated **allowable** value of 2100 lbs, much less 2100 lbs 
multiplied by a safety factor, which would be its implied calculated 
capacity. Is there a gyp board industry group that has done any testing on 
this ?


______________________________ Reply Separator _________________________________
Subject: Re: Effective Length of Wall Studs
Author:  seaoc(--nospam--at)seaoc.org at Internet
Date:    1/23/97 11:13 AM


At 11:55 AM 1/23/97 -0500, Jim Fulton  wrote:
>In the design check of 2x4 studs (STUD GRADE) in a load bearing wall in a 
>residence, how effective is the lateral support provided by say a Celotex 
>exterior wall sheathing (generic designation?) and 1/2 " drywall (gypsum 
>board) interior towards preventing stud buckling under axial loads? I can 
see 
>that for a plywood or OSB exterior seathing and gypsum baord interior, 
>perhaps the stud is effectively braced continuously along its length against 
>column buckling. Therefore, its allowable axial load is based on compressive 
>strength parallel to the grain. But what about a lesser sheathing material 
>such as Celotex? I am looking at the loads produced by some bookcases in my 
>house located against an exterior wall on the second floor. Conservatively 
>assuming no bracing of the studs, each stud is good for 510# axial force and 
>this is exceeded by the load produced from the only dead load (roof+attic 
>floor+second floor wall + second floor) and bookcase weight. As I am not 
>experienced in residental design, I am hoping to get some feedback from those 
>that are. Thanks. Any specific references from industry associations on this 
>question ?  
>

Ummmm! An interesting question. I have some comments (of course).
1. The bookcase load may be considered a live load rather than a dead load 
inasmuch as the load is not necessarily permanent. Therefore the capacity 
of the bearing wall would include the floor live load from above. Since the 
live load is almost never achieved throughout the entire floor, the load 
created by the bookcase, in my opinion, should be well within the walls 
load capacity.
2. My guess is that the studs are braced in their weak direction by stud 
blocking (bridging). Therefore this is not much concern for buckeling in 
the weak direction. Additional strength is achieved by the use of OSB - 
however, I would not allow for values from the Celotex (plaster and gypsum 
is another matter as yet unresolved).
3. Unless the wall is free-standing, I would assume that the studs are 
braced by the roof framing and therefore the potential for buckeling is 
reduce compared to a free standing stud unbraced at the top. The capacity 
of the stud for buckeling is dependent upon the stud height - which you 
don't mention in your post.
4. Celotex (or Powerwall) is used out here in the desert for adding 
insulation to 2x4 framing. There is no value given to it for shear, but 
this is not your problem. There is a concern about nail bending due to the 
weakness of the Celotext and the 1/2" (approximate) distance between the 
face of the Celotex and the face of stud. I would be more concerned with 
nail/screw bending failures than I would with Stud Buckeling.

Interesting,
Dennis