Web stiffeners for compression flange bracing? 1

[teleBob]

We are designing a general aircraft hangar building with 50 foot simple span steel header beams that support prefab wood trusses spaced at 24 inches. I am concerned about lateral support of the compression flanges. Using ASD AISC 9th Edition, for W27x94 (Fy=50) beams with Fb=.66Fy, lateral bracing supports would need to occur at maximum 8.9 ft (Lc) spacings and would be designed for capacity of 4.9 kips horizontal force (using 2% of the maximum compression flange force).

Question 1: Could lateral bracing be provided by wood trusses? If so, is it reasonable to use a design horizontal force of 4.9*2.0/8.9=1.1 kips per truss and provide adequate connectors from the truss to the nailer plate and from the nailer plate to the beam flange? In other words, can a designer distribute the lateral bracing force along the length of the beam?

Question 2: In lieu of bracing by the trusses, is it permissible to use full fitted web stiffeners, spaced at Lc or less, to brace the top flange by connecting it to the relatively laterally-stable bottom tension flange?

Any code or text references would be appreciated!

 

[teleBob]

It is a TORSIONAL brace to keep the section from twisting, differentiated from a LATERAL brace that would keep the compression flange in a straight horizontal position.

So, for the torsional brace, I will add steel tabs on the underside of the upper flange and also on the upper side of the lower flange, bolt wood struts at each location, then connect the far ends together through a truss node, say 8 feet away. This will result in a torsional brace with a moment arm of 8 feet, with low forces (in this specific case, only about 400 pounds) that can be easily carried by the wood truss.

Again, the torsional bracing is provided because AISC 13th Ed Appendix 6 allows a calculation to determine the required bracing moment per foot for continuous torsional bracing, but not for continuous lateral bracing.