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Steel Beam Lateral Bracing

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.  winky smile

 

RE: Steel Beam Lateral Bracing

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.  

RE: Steel Beam Lateral Bracing

Generally, the top flange is braced at every purlin/joist connected to it and the bottom flange is only braced where you specify fly bracing (these are usually at half or 1/3 span but may be closer).

http://www.draftsperson.net/index.php?title=Fly_Brace

RE: Steel Beam Lateral Bracing

(OP)
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?

RE: Steel Beam Lateral Bracing

DTS-
With regard to your last post....... it depends which way your roof system (presumably metal deck) is spanning.

RE: Steel Beam Lateral Bracing

(OP)
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?

RE: Steel Beam Lateral Bracing

In regard csd72's post, when do you need bottom chord bracing?  I thought that was only for negative bending moment over column.

RE: Steel Beam Lateral Bracing

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.

RE: Steel Beam Lateral Bracing

DTS-
the deck (acting as a diaphragm) takes those buckling loads to the lateral force resisting system.

RE: Steel Beam Lateral Bracing

(OP)
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.

RE: Steel Beam Lateral Bracing

(OP)
PS - In other words, what is the load path of a lateral bracing system that is not a diaphragm?

RE: Steel Beam Lateral Bracing

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.

RE: Steel Beam Lateral Bracing

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.

RE: Steel Beam Lateral Bracing

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.  

RE: Steel Beam Lateral Bracing

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.  

RE: Steel Beam Lateral Bracing

(OP)
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?

RE: Steel Beam Lateral Bracing

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.  

RE: Steel Beam Lateral Bracing

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.

RE: Steel Beam Lateral Bracing

(OP)
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...

RE: Steel Beam Lateral Bracing

DTS419-

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

See AISC 360-05 appendix 6 for bracing stiffness requirements.

RE: Steel Beam Lateral Bracing

(OP)
So how do you calculate your actual bracing stiffness to compare it to the required stiffness?

RE: Steel Beam Lateral Bracing

It would typically be the axial stiffness of the member bracing, but you really have to look at how the brace is doing the bracing.  If it is providing the brace force by axial resistance, then it is an axial stiffness, if it is providing the brace force through flexure, then look at the bending stiffness.  You would apply a unit load to your "beam and check the deflection, then turn that into a k/in stiffness to compare to the requirements.

RE: Steel Beam Lateral Bracing

Just for an example........
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

(OP)
AISC 360-05 Page 16.1-422.  Figure C-A-6.1

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

If you go through the derivation of the lateral buckling stiffness requirements you will see.  It doesn't need to be infinitely rigid.  You can have acompression member with a lateral displacement at midspan that is still stable.   

RE: Steel Beam Lateral Bracing

frv,

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

"If you go through the derivation of the lateral buckling stiffness requirements you will see.  It doesn't need to be infinitely rigid."

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

Our office is swamped as well, believe me.  I don't like to read, however, unless it is technical stuff.  So, when we're at the beach and my wife is reading some trashy romance novel or some other book that her dad gave her, I am reading "Structural stability of steel" by Galambos/Surovek, or reading through my S&J steel text, or at least thinking in some way about engineering.  It doesn't matter where I'm at or what I'm doing.... I can't get away from it.  It's almost annoying sometimes.  
Here is a scan of a portion of the discussion from S&J.

RE: Steel Beam Lateral Bracing

(OP)
StructuralEIT, you can get treatment for that you know...

RE: Steel Beam Lateral Bracing

StructuralEIT, before you get treatment can I hire you? You seem like the ideal engineer who actually loves his job (which is a very tough find). I believe we could offer you double your salary :)

RE: Steel Beam Lateral Bracing

Thanks.  I do love my job.  I have an excellent perspective on it, too.  I used to work in a storm door manufacturing plant for 3 years before I actually started school and then for 6 years while I was in school, so I certainly appreciate the variety of the job and the level of thinking involved more than most (though certainly not all).

RE: Steel Beam Lateral Bracing

(OP)
Before my engineering career I was a carpenter, plumber, and enlisted man in the US Army, so I share your appreciation for the job in which we work "from the shoulders up."  However, my chair spins at 4 o'clock sharp and then it's time for one of the several hobbies that I need this job to fund.  winky smile

"All work and no play makes Jack a dull boy."

RE: Steel Beam Lateral Bracing

S-EIT, I am similar.  On my bedstand is the Mechanical Engineering Reference Manual by Lundberg.  I think my wife is reading "Men are from Mars...."    

Don Phillips
http://worthingtonengineering.com

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