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!

 

[JAE]

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?

That doesn't work. You can't brace a top flange with a vertical stiffener.

 

[MarcbSE]

If the wood trusses bear directly on the steel beam they will provide the lateral bracing for the top flange.

 

[miecz]

I don't see anything in the code that allows you to distribute the required strength by the ratio of the brace spacing over L[sub]c[/sub]. In addition to the required strength, Appendix 6 of the new code has a stiffness requirement, something you should pay attention to, seeing you're bracing steel with wood.

 

[UcfSE]

If you want the trusses to provide bracing, you'll have to provide the truss engineer with bracing forces and design connections to handle the brace force. This is unusual so I typically won't use trusses as bracing per se. Instead I feel comfortable saying that with trusses, my unbraced length, say L/2 instead of at each truss location. By the time you go through everything it may prove easier to just provide more beam weight and leave it at that.

 

[271828]

Read the 13th Ed. Manual, Spec. Appendix 6.

 

[DaveAtkins]

I have dealt with this same issue myself--on a project with a steel truss top chord (a WT) braced by wood roof trusses. Basically, I did what you are proposing--I figured out what my maximum unbraced length could be, and provided enough connections IN THAT LENGTH for the 2 percent force. So I did not distribute the 2 percent force across the entire truss span, but only across the assumed unbraced length.

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

 

[hokie66]

I concur with the way you propose to provide bracing. But don't forget the bottom flange. You will probably have net uplift under some wind conditions, so you have to brace the bottom flange accordingly.

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.

 

[slickdeals]

Would anyone explain why a full fitted stiffener can't be used? I am trying to understand the rationale behind it. Would it not prevent local buckling?

 

[MarcbSE]

slickdeals:

The flange brace is intended to prevent lateral torsional buckling of the section - twisting. The stiffeners are useless in the prevention of LTB.

 

[JAE]

Yes, I should have been more explicit. The vertical stiffeners, as MarcbSE says, DO help to brace against local buckling (web or flange) but the original post was definitely talking about lateral torsional buckling of the overall member.

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.

 

[271828]

Stiffeners used in that manner are not always useless. If the top flange is in compression and something's providing torsional restraint at the bottom flange, then stiffeners might provide enough web distortional stiffness to make a torsional brace point. Doesn't work for continuously braced situations, though.

 

[miecz]

Appendix 6.3.1b includes the following:

When L[sub]b[/sub] is less than L[sub]q[/sub], the maximum unbraced length for M[sub]r[/sub], then L[sub]b[/sub] in Equation A-6-8 shall be permitted to be taken equal to L[sub]q[/sub].

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 L[sub]b[/sub]/L[sub]q[/sub], when L[sub]b[/sub] is less than L[sub]q[/sub]?

 

[271828]

jmiec, I've never heard of such a thing. Doesn't sound correct to me.

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).

 

[miecz]

271828-

If I understand the op, he/she wishes to distribute the required brace force by the ratio of L[sub]b[/sub]/L[sub]q[/sub]. This comes from Question 1:

...is it reasonable to use a design horizontal force of 4.9*2.0/8.9=1.1 kips per truss...

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.

 

[teleBob]

Thanks, everyone, for your comments. I now understand that web stiffening is not a proper substitute for external bracing, although it could serve to brace the bottom flange of a beam over a torsionally connected column if the top flange were laterally supported (for instance at a continuous or cantilevered beam support). In that case, it would serve as a type of weak-axis moment connection through the bottom flange.

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.

 

[csd72]

It only works if your bracing member can take the moment.

 

[hokie66]

telebob,

You are on safe ground.

csd72,

What moment? I think we are talking about an axial force.

 

[csd72]

hokie66,

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.

 

[hokie66]

csd72,

Telebob's last post says he is providing connections at both the top and bottom flange.