Steel Beam Lateral Bracing

[DTS419]

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. ;-)

 

[frv]

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.

 

[Lion06]

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

 

[DTS419]

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

 

[Lion06]

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.

 

[Lion06]

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.

 

[DTS419]

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?

 

[Lion06]

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.

 

[hokie66]

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.

 

[JrStructuralEng]

"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

 

[Lion06]

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.

 

[DTS419]

StructuralEIT, you can get treatment for that you know...

 

[JrStructuralEng]

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

 

[Lion06]

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).

 

[DTS419]

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. ;-)

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

 

[DonPhillips]

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