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Steel Beam Lateral Bracing

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DTS419

Structural
Jun 21, 2006
180
I'm a "structural engineer" whose experience is 99% concrete design. All I know about steel design is what I learned in school from a lack-luster professor and text book, and that was many brain cells ago. I'm doing a small mechanical building that is part of a bigger concrete job, and the building will have a simple steel beam roof system. Steel designed always seemed pretty straight forward, but I'm confused with some of this lateral bracing criteria...

Besides other framing members that are orthogonal to a beam, what else constitutes lateral bracing? This roof will be a non-ballasted membrane, so I am assuming that metal decking does not have sufficient stiffness to act as bracing...or does it?

Do web stiffeners accomplish lateral bracing? If not, what do they do?

Assuming that there's more than one way to acheive lateral bracing, what's the best way to go about determining the most efficient method to use?

Thanks in advance for your help, although these should be some really easy questions for a lot of you. ;-)

 
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Stiffeners will typically only act as lateral bracing in stacked construction where you have a beam over a column (or girder) and the stiffeners in the beam are in line with the supporting members web. The bottom flange must also be attached to the supporting member to make the stiffeners effective in bracing the member. Stiffeners are used more often than not to help with web crippling and web yielding in beam over column or beam over girder conditions.

Roof deck typically will serve to act as a lateral brace provided the flutes are perpendicular to the member and attached to the top flange.

The most effective way to achieve lateral bracing is to use the other members in your framing system.

Hope that helps somewhat.
 
I wasn't planning to use any purlins. If I use C-channels placed perpendicular to my roof beams for lateral support, do they have to be designed as bending members (purlins), or can they be ignored as bending members?
 
DTS-
With regard to your last post....... it depends which way your roof system (presumably metal deck) is spanning.
 
Alright, now if all my beams are braced to each other, and they are all fully loaded simultaneously, what makes the bracing effective? I can see how the bracing is effective in the case that one beam is heavily loaded but the beams on either side or not. Here you have something nearby, not undergoing the same deformation and therefore able to resist the heavily loaded member's tendancy to buckle. But in the case where all beams are loaded equally like my roof system, aren't the lateral braces just along for the ride if all the beams have the same tendancy to buckle at the same time?
 
In regard csd72's post, when do you need bottom chord bracing? I thought that was only for negative bending moment over column.
 
JrstructuralEng,

Whenever you have compression in your bottom chord then you may need bottom chord bracing.

Most typical case is for roof beams with wind uplift, but it can apply elsewhere.

DTS419,

You are 100% correct, if you only have pinned connections to the top chord then all beams can roll at once. You need some sort of membrane action to take these loads back to the ends.

Hard to comment further without knowing the exact details.
 
DTS-
the deck (acting as a diaphragm) takes those buckling loads to the lateral force resisting system.
 
csd, what do you mean by the "ends"? The diaphragm makes perfect sense because it behaves like a giant deep beam, but how then could any lateral bracing in the form of purlins, c-channels, diagonals, or similar methods be effective?

My question is more of general theory and curiosity than this specific little roof. Although, a senior associate has recommended the use of c-channels placed orthogonally between the beams at whatever Lc is for the chosen beam members, but didn't offer an explanation as to why or how this acheives adequate bracing force/stiffness.
 
PS - In other words, what is the load path of a lateral bracing system that is not a diaphragm?
 
By ends I mean the columns/supports that are connected to the lateral bracing system.

As long as you have 2 bolts this will offer some resistance to twisting. This twisting then gets taken out with a small vertical reaction at the other end.
 
DTS419

The goal of lateral bracing for lateral torsional buckling is to stop the beams from rotating about thier longitudanl axis.

Channels connected to web stiffeners on adjacent beams by sufficient welds or bolts will go into bending if the beams attempt to rotate. Z or X bracing will like wise prevent the beams from rotating, the braces resisting in tension or compression.
 
DTS419-

The bracing has to be connected to the LFRS. If you do not have a diaphragm, then the purlins will not act as bracing, unless you have a space frame.

Think of the load path. To serve as a brace, the purlins must be capable of resisting axial load. The axial load is quite small, but the resistance must be there nonetheless. Typically, the axial load is transfered to the diaphragm which, in turn, transfers it to the LFRS and on to the ground. If no diaphragm is present, then your only option (others- please correct me if I'm wrong) would be to have that axial force transfered into a space frame (essentially everything welded to everything else in order to resist loads by bending). If you do this without a diaphragm, then you must account for the fact that your bracing elements are now themselves unbraced and are, therefore, susceptible to buckling.
 
frv,

I disagree, and if what you say is correct, none of the buildings we design in Australia are adequate, as we do not as a rule use diaphragms. The required bracing force is small, and a purlin system capable of resisting the design wind loads has been shown to be adequate for bracing the rafters. Instead of a diaphragm, we use horizontal bracing trusses in the roof plane. The main reason we don't consider diaphragm action is that we generally use the deck, which is screwed to the purlins through the deck crowns, as the roofing membrane.
 
But if no diaphragm is present and the purlins (or equivalent orthogonal members) are to prevent lateral buckling by carrying axial force, where does this axial force go? Isn't the purlin just pushing into the next beam? Is this next beam's weak axis providing the stiffness needed to resist the small axial load? If each beam is responsible for preventing lateral buckling in the beams next to it, what prevents all of them from buckling when they are all equally loaded?
 
hokie66..

whether or not you consider it as acting as such, a deck screwed into your purlins WILL act as a diaphragm. Please correct me if I misunderstand.

DST49-

Exactly. But then that beam's weak axis bending creates a reaction at the top of the column in the direction of the purlins which then must be resolved to the ground. From a practical standpoint, the capacity of the column as a cantilever (even if designed as pinned) will probably suffice, but the axial load must be resolved somewhere.
 
hokie66,

In the US they use diaphragms where we would use plan bracing - different ways of achieving the same thing.

In order to have it act like a diaphragm, welds are used to connect the ends of the decking and to connect it to the structure. Site welds are more commonly used in the US than Australia as there are are better quality control procedures in place for this.
 
I've read "Fundamentals of Beam Bracing" by Joseph Yura, and it sounds like the sufficiency of a bracing system is defined by both the strength and the stiffness of the braces. The strength is easy, usually just a small percentage of the compression flange force of the beam. But how do you quantify the stiffness of the bracing system?

It seems like this is a subject that is largely taken for granted...
 
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