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Re: Effective Length of Wall Studs

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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