wood headers

[bjb]

When designing a wood header in a wood framed building, and the header is below the double top plate with cripple studs sitting on it, should you assume that the compression edge is adequately braced so that the beam stability factor doesn't need to be applied? It seems that since headers are usually multiple pieces nailed together with sheathing on each face, that applying the stability factor is over-conservative.

Also, in the case where the header is under the top plate in exterior walls, should the design should account for weak axis bending of the header from wind loads?

 

[boo1]

Beam Stability, CL is controled by lateral support, not bearing. Without lateral support or proper design considerations, the compression side of a beam may buckle prematurely at a load much smaller than what was intended in a normal bending mode. But, the height to thickness ratio of typical wood headers (2-2"x12" or 2"x10&quot, 3:1 or 4:1 ratios requires ends of members by blocking, bridging, hangers to have CL=1.0. Typically the number of trimmer and cripple studs are selected by the span and provide this support. Additionally the interior and exterior sheathing provides additional support.


Cheers

 

[bjb]

boo1, I realize that stability is provided by bearing support, not bearing. Another way of stating my question is, are cripple studs capable of providing lateral support to a low header? It seems from your reply that you are saying that when properly considered, low headers may be designed on the assumption that they are laterally supported. I agree that sheathing provides additional support. I think another source of lateral support could be a continuous top plate nailed to the top of the header, and adequately connected to the full height studs at ea. end. This might not be a major source of support in a 2x4 wall, but in a 2x6 wall it probably is, depending on span.

 

[boo1]

Yes the top plates provide the compression support as does the end (trimmer) studs do. It is not the span ( beam slenderness ratio) it is the height to depth ratio that is more important. The beam slenderness ratio is a square root function. My comment was, for normal wood beams the Beam Stability, CL=1.0

With the 6" wall the ratios are even lower. Rule of thumb for wood beams.
For Height:Thickness to assume CL=1.0
2:1 and less case, no supports are required
3:1 or 4:1 case, At ends of members by blocking, bridging, hangers.
5:1 case, then the lateral stability requirements are:
a. The compression edge is held in-line the entire beam length
b. The member ends are held in position with full depth blocking, hangers, or bridging.

Beam Stability Factor Equation:
CL=((1+Q/1.9)-((1+Q/1.9)**2-Q/0.95)**1/2)</= 1.0
G=Fbe/Fb*
Fbe=(Kbe/Ey'/Rb)**2 Kbe= 0.438 Lumber, graded
0.610 Glulamms
Ey'=Ey(Cm)Cl

I hope this is helpfull