Steel Beam Lateral Bracing
Steel Beam Lateral Bracing
(OP)
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.
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.






RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
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RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
With regard to your last post....... it depends which way your roof system (presumably metal deck) is spanning.
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
the deck (acting as a diaphragm) takes those buckling loads to the lateral force resisting system.
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
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.
RE: Steel Beam Lateral Bracing
It seems like this is a subject that is largely taken for granted...
RE: Steel Beam Lateral Bracing
You are right. You must have a certain stiffness for a brace to function. The required stiffness depends on whether the brace is relative or discrete (for columns). I can't recall what the requirements for beams are.
Stiffness is defined as force/distance. i.e. what force is required for a unit displacement. I know the Yura/Helwig notes cover this in depth.
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
say you have a beam horizontal at an elevation of ten feet. There is a wind column running by it (and positively attached) with ends at elevation 0' and 20' and occurs at midspan of the beam. The wind column is providing a brace for the beam via flexure.
Now to determine the adequacy, apply a unit load (1 kip) at midspan - which is the location of the brace force on the bracing member- of the wind column and determine the displacement in the direction of buckling. It would be horizontal in this case because the beam wants to buckle laterally. Say the Istrong of the wind column is 500in^4, the lateral deflection of the column for a 1 kip load is 0.019862" (assuming a pinned-pinned column) from the equation (PL^3)/(48EI).
Now your stiffness is 1/0.019862 = 50.35 K/in.
Compare that to your required stiffness.
RE: Steel Beam Lateral Bracing
The rigid support is what I'm talking about. Don't you always need this to resist the lateral force in the braces? Without it, how is there sufficient lateral stiffness to resist lateral buckling?
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
In Australia, the screws are through the crowns of the steel roofing, with neoprene washers. Intended to shed water, not brace the roof. Some of our decks are concealed fastener systems, where the deck is intended to slide. Similar to decks used in some of the US PEMB systems, which also make poor diaphragms. We provide bracing between the rafters and sometimes, but not always, the purlins are part of that bracing system.
csd72,
I know, except I don't agree with the better quality control bit.
RE: Steel Beam Lateral Bracing
Where do you find the time for this stuff! I don't know about you but our office is SWAMPED. Just bugging you :)
RE: Steel Beam Lateral Bracing
Here is a scan of a portion of the discussion from S&J.
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
RE: Steel Beam Lateral Bracing
"All work and no play makes Jack a dull boy."
RE: Steel Beam Lateral Bracing
Don Phillips
http://worthingtonengineering.com