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Re: WOOD - Load bearing studs in a party wall

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     I had a similar question which I asked awhile ago. Certainly, typical 
     construction of many houses is gyp board on interior and Celotex-like 
     material outside plus say vinyl siding. It would be reasonable to 
     assume that these outside materials offer little bracing benefit, and 
     so the gyp board on one side only is available to brace the stud 
     against weak-axis buckling under axial load. So the question is how 
     effective is gyp board in bracing the stud against weak-axis buckilng 
     so that it forces the buckling mode into strong-axis buckling. I 
     contacted the Gypsum Association and National Gypsum to see if any 
     testing had **ever** been done to answer this question. In short, the 
     answer apears to be "no": although, testing has been done on gyp 
     panels under in-plane (racking) loads. But that addresses a different 
     problem. The only thing which could eventually find are "soothing" 
     statements in NDS (National Design Spec for Wood Construction).  See 
     Code section A.11.3 and Commentary to Code section 3.6.7.1. 
     
     A.11.3 says when wall studs in light frame construction are adequately 
     sheathed on at least one side, the depth, rather than the breadth, 
     shall be permitted to be used as the least dimension in calculating 
     L/D. "The sheathing shall be shown by experience to provide lateral 
     support and shall be adequately fastened".
     
     Commentary to 3.6.7.1 says use of the depth as the least dimension in 
     determining axial load carrying capacity of "normally sheathed or clad 
     light frame wall systems is long standing practice.  Experience has 
     shown that any code allowed thickness of gypsum board, hardwood 
     plywood, or other interior finish adequately fastened directly to 
     studs will provide adequate lateral support of the studs across its 
     thickness irrespective of the type or thickness of exterior sheathing 
     and/or finish used." 
     
     Personally, I would like to see this tested. When one calculates the 
     axial load capacity based on weak-axis buckling and then strong-axis 
     buckling, the large difference in these two capacities indicates the 
     large axial load increase the gyp board has to undergo to reach the 
     axial load capacity based on strong-axis (depth) buckling. Now 
     certainly the issue of the gyp board goes away if you put wood 
     blocking between the studs, but this creates problems with running 
     insulation and is not normal construction practice from what I've 
     seen.  


______________________________ Reply Separator _________________________________
Subject: WOOD - Load bearing studs in a party wall
Author:  seaoc(--nospam--at)seaoc.org at Internet
Date:    10/22/97 3:54 PM


I am checking the load bearing capacity of a wood framed wall that is 
intended to be used in a party wall. This party wall consists of 2x4 studs @ 
16" mounted on 2x6 top and bottom plates. These studs exist on each side of 
the wall. In other words, there are 2x4 studs @ 16" framed flush to one edge 
of the sill plate and 2x4 studs @ 16" framed flush with the other edge of 
the sill plate. These two "rows" of studs are offset so that there actually 
exists 2x4 studs @ 8".
     
In a "normal" wall, these studs are laterally braced on both faces. For an 
interior wall, there is gypsum wallboard on both sides. For exterior walls, 
there is some kind of exterior sheathing (plywood or stucco) with gypsum 
wallboard on the interior. Therefore, these studs are laterally braced 
continuously about the minor axis. To determine the axial capacity based on 
l/d ratios, I have used the distance between the sill plate and top plate 
for my "l" and the depth of the stud for my "d".
     
In this party wall case, there is gypsum wallboard on only one face of the 
studs. True, it is the compression face for the Code mandated 5 psf 
partition load, but one edge is still laterally unsupported. My gut feeling 
is that the capacity of the stud is somewhat less than if there was gypsum 
wallboard on both faces but a lot more than if there was no gypsum wallboard 
at all.
     
Since I realize that I'm not the first structural engineer to be asked to 
use a party wall as a load bearing wall, I am curious what others have done 
in the past to determine the capacity of studs laterally braced on one face 
only.
     
Regards,
Bill Allen
     
     
     
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From: "Bill Allen" <BAllenSE(--nospam--at)mail-gw3.pacbell.net>
To: <seaoc(--nospam--at)seaoc.org>
Subject: WOOD - Load bearing studs in a party wall
Date: Wed, 22 Oct 1997 15:54:26 -0700
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