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Re: WOOD - Load bearing studs in a party wall

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• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: WOOD - Load bearing studs in a party wall
• From: ErnieNSE(--nospam--at)aol.com
• Date: Sat, 25 Oct 1997 23:37:24 -0400 (EDT)

In my opinion, using sheathing just on one side of the studs is adequate
bracing to be able to use the strong axis for axial load capacity. Without
any test results or textbooks written about this special condition, I will
accept 2x4 and 2x6 stud studs, sheathed on one side only as fully braced in
the weak axis if it is a basic compression member with a very small bending
stress. For a bearing party wall, the 5 psf lateral load required for
partition will cause tension on the unbraced tension edge of the stud and a
seismic force of .3 x 6 = 1.8 psf lateral load will cause compression on the
unbraced edge of the stud. I'll consider the weak axis braced for axial
compression stress plus whichever governs on the bending stress due to
partition load with fully braced compresson edge or the seismic load with
unbraced compression edge.

In a pure axial concentric load condition with both ends restrained from
rotation (which a typical stud is actually restrained by 2 nails at the ends)
a stud fails in buckling in the weak axis when the full height starts
buckling due to non- restraint in the weak axis. A stud cannot buckle only on
one edge. If one edge is restrained by nails at 12 " oc from a typical wall
sheathing, the other edge is automatically restrained. I believe that only a
very small restraining force is required to keep a slender stud from buckling
and a solid 2x4 or 2x6 stud is stiff enough that one edge being restrained is
enough to keep the other edge from buckling. As I recall, lacings for a built
up steel column  made up of 4 angles are designed for 2% of axial force.
Based on my judgement, this equivalent full restraint is effective in a 2x4
and a 2x6. As the stud becomes deeper as in 2x8 or 2x10 the unrestrained edge
gets further away from the restrained edge and the effectiveness of full
restraint becomes questionable. Again, this is based on engineer's judgement
and other factors should be considered such as other loads (bending,
eccentric axial load, etc.), the possibility of the wall actually collapsing
if only one or two studs buckle(are loads redistributed if one stud fails?),
conservativeness of live loads, etc.

I would use a more conservative judgement if it's an isolated column.  A 4x8
or a 6x12 is OK with restraint on one edge only but I would require restraint
at both edges for anything deeper.

This is just my opionion. I'm also assuming that for some reason, we cannot
put  blocking, bridging, or any intermediate lateral restaraint in the weak
axis.

Ernie Natividad