Does gypsum board provide lateral support to a wall stud assembly - to prevent it from buckling about the weak axis due to axial loads, and also to ensure that it is fully braced for bending when subjected to lateral loads?
The following references may be helpful in this regard:
RE: AXIAL LOADS
From the 1997 NDS® Commentary (Clause 3.6.7 - Lateral Support of Arches, Studs and Compression Chords of Trusses). Last paragraph (page 37) states:
"Use of the depth of the stud as the least dimension in calculating the slenderness ratio in determining the axial load-carrying capacity of normally sheathed or clad light frame wall systems also 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 stud across its thickness irrespective of the type or thickness of exterior sheathing and/or finish used."
From the 2001 NDS (Appendix A Clause A.11.3):
"When stud walls in light frame construction are adequately sheathed on at least one side, the depth, rather than breadth of the stud, shall be permitted to be taken as the least dimension in calculating the l_e/d ratio. The sheathing shall be shown by experience to provide lateral support and shall be adequately fastened."
RE: LATERAL (BENDING) LOADS
From the 2001 NDS (Clause 4.4.1 Stability of Bending Members):
Clause 184.108.40.206 provides the following limits on the nominal depth-to-breadth ratios (d/b) for sawn lumber in order to use C_L = 1.0.:
"(a) d/b less than or equal to 2; no lateral support shall be required.
(b) 2 < d/b and less than or equal to 4; the ends shall be held in position, as by full depth solid blocking, bridging, hangers, nailing, or bolting to other framing members, or other acceptable means.
(c) 4 < d/b and less than or equal to 5; the compression edge of the member shall be held in line for its entire length to prevent lateral displacement as by adequate sheathing or subflooring, and ends at points of bearing shall be held in position to prevent rotation and/or lateral displacement."
More requirements are also specified for greater d/b ratios, however for wood studs, the range noted above should be adequate. For example, the d/b ratio for a 2x4 stud is (4/2 = 2), for a 2x6 stud is (6/2 = 3). Given these ratios, for pure bending, all that is required is that the ends be held in position.
RE: COMBINED BENDING AND AXIAL LOADS
Clause 220.127.116.11 also states:
"If a bending member is subjected to both flexural and axial compression, the depth to breadth ratio shall be no more than 5 to 1 if one edge is firmly held in line...."
So if the interior stud wall is sheathed with gypsum on both sides and is subjected to combined axial and lateral loads, for typical stud dimensions such as a 2x4 and 2x6 - which results in relatively low d/b ratios (lower than 5), Clause 18.104.22.168 would suggest that a C_L = 1.0 could be used as well.