Web stiffeners for compression flange bracing?
Web stiffeners for compression flange bracing?
(OP)
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!
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!






RE: Web stiffeners for compression flange bracing?
That doesn't work. You can't brace a top flange with a vertical stiffener.
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
In your case, if you assume the beam is braced 2' oc, I think you should design for the 2 percent force every 2' oc. If you assume the beam is braced 6' oc, you should design for the 2 percent force every 6' oc.
DaveAtkins
RE: Web stiffeners for compression flange bracing?
It is unnecessarily expensive to design a beam spanning 50 ft to be unbraced. The load will always brace the beam if adequate connections exist.
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
The flange brace is intended to prevent lateral torsional buckling of the section - twisting. The stiffeners are useless in the prevention of LTB.
RE: Web stiffeners for compression flange bracing?
For any sort of element to resist LTB, you must have some external entity to brace the beam against to reduce the unbraced length of the compression flange and reduce Lb.
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
Can anyone point to any code language or research that indicates that it is proper to distribute the required brace strength by the ratio of Lb/Lq, when Lb is less than Lq?
RE: Web stiffeners for compression flange bracing?
That's not what the quoted language is saying (although not sure if that's what you're implying). Lq will certainly be larger than Lb, often by a very wide margin. Plugging Lq into the equations results in much less severe requirements. The crappy part is coming up with Lq (say Section F4 applies, for example).
RE: Web stiffeners for compression flange bracing?
If I understand the op, he/she wishes to distribute the required brace force by the ratio of Lb/Lq. This comes from Question 1:
While it seems reasonable, I don't think the Appendix 6 allows it. However, from some of the responses, it appears that others believe it is correct to ratio the required brace strength, or, put another way, to distribute the required force to adjacent braces. So, I'm asking if anyone can back up that position.
I quoted Appendix 6.3.1b to show that the code had addressed short unbraced lengths for the stiffness requirement, but was mum on the strength requirement.
RE: Web stiffeners for compression flange bracing?
Appendix 6 of the 13th Edition is quite helpful in describing design parameters for bracing, but does not specifically allow LATERAL bracing forces to be distributed along the beam length, as my Question 1 proposes. However, there are provisions for continuous TORSIONAL bracing on a per-foot basis (Section 6.4.2b). I think we can discretize that for every truss bearing condition (2' o.c.) and provide connections on both the top and bottom flanges to resist the design torsional bracing moment. We will ask the wood truss designer to accommodate those forces.
RE: Web stiffeners for compression flange bracing?
RE: Web stiffeners for compression flange bracing?
You are on safe ground.
csd72,
What moment? I think we are talking about an axial force.
RE: Web stiffeners for compression flange bracing?
The bracing force is required at the top flange, the brace is at the bottom flange, the bracing now also needs to take the moment from this eccentricity otherwise it will twist.
RE: Web stiffeners for compression flange bracing?
Telebob's last post says he is providing connections at both the top and bottom flange.
RE: Web stiffeners for compression flange bracing?
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.