Yet Another Unbraced Length question

[slickdeals]

I am sure this has been beaten to death already......but thought I should ask anyways.

Assume you have a 2' high concrete pedestal spaced on a grid of 30' x 20'. A W24 steel beam spans between the pedestals in the 30' direction. There are end stiffeners welded to the W24 when it bears over the column to prevent rotation @ support. In the other direction, steel joists spaced at 5' o.c. span into the top flange of the W24.

For gravity loading only, is the unbraced length of W24 :
1. 5' ?
2. 30' ?
3. Something in between?

 

[ble31980]

I'll be the first to admit that I'm no where close to being an expert on beam stability. That said, here's my opinion.

I agree with those that say the unbraced lengths are 5' with the deck and 30' without. This is assuming that the end stiffeners are rigid enough to prevent end rotation. The way that I visualize the problem is by comparing it to a textbook wood-framed diaphragm and shear wall example. While it's true you might have lateral displacement of the beam, similar to the displacement of a diaphragm, the deck will keep the top of the beams from rotating. If you don't have the deck, I don't see how you can prevent the top flanges from rotating without the deck.

 

[JennyNakamura]

Lion/csd,

I am re-reading my last post and now I don't even understand what I am talking about! So I will just stick to CMU warehouses with CFS roof purlins for now...

Fascinating subject, wish I could spend more time studying up on it. You could probably spend 40 hrs/week for a couple of months doing nothing but researching this topic, but who has time for that when we're all working so much free overtime (just a joke on a different thread)!

Thanks for opening my eyes, guys.

 

[csd72]

Never stop learning

 

[slickdeals]

All:
Based on my OP and the responses, I am going to try to make a summary of the various conditions that can alter the behavior (with deck & without deck). To be continued........

 

[slickdeals]

See attached sketch. I am keeping this as a work in progress. There are various scenarios with possible (??) explanations.

I think it might be a good idea to suggest what the unbraced length might be and have a narrative of why it is so. I think having a metal deck makes things easier. I have purposely omitted the deck in order to help understand behavior better.

I appreciate you guys taking the time to review and comment.

 

[RFreund]

Slick- first off, nice work!

Thoughts based on rereading Guide to Stability Design Criteria (GSDC) for Metal Structures By Ronald D. Ziemian Chapter 12.9:

Case A:
Scenario 1: Agree
Scenario 2: I believe this would be a "lean-on" system. In this case the beams are linked together and lateral buckling cannot occur at the links unless all the members buckle. In this case the beams in the system cannot buckle until the sum of the maximum moment in each beam exceeds the sum of the individual buckling capacities of each beam. Also the buckling of an individual beam can occur only between the cross members in a lean on system. That is what the GSDC says however I need to do a little more research here because in the Yura notes that I have found the bracing ability of the lean on member depends on how heavily it is loaded. Basically one beam would need to have sufficient strength and stiffness in the weak-axis to brace the other beam. However I should mention that references to "Bracing for Stability" 2009, Yura and Helwig, short course notes, North American Steel Construction Conference, Phoenix, AZ and "Global Lateral Buckling of I-Shaped.

Case B:
This seems as though it would be classified as Torsional Bracing. The GSDC reads 'If two adjacent beams are interconnected by a properly designed cross frame or diaphragm at midspan, that point can be considered a braced point when evaluating the beam-buckling strength. Because the beams can move laterally at midspan, the effectiveness of such a bracing system is sometimes questioned. As long as the two flanges move laterally by the same amount, there will be no twist. If twist is prevented, the beam can be treated as braced.'
I believe that overall stability would need to be considered see my above post 11 May 11 16:10

Case C:
I need to give this more thought. But I would like to put kickers at the end for some reason and say 5'

Case D:
Torsional Brace so 5' same as case B. Consider overall global buckling of the system.

I still may need to do a little research and might be able to improve these answers.




EIT

 

[BAretired]

Slickdeals,

I'll take a shot at it.

Case A

Scenario 1. I agree that unbraced length for lateral torsion buckling is 5'.

Scenario 2. The beams are unstable. Unbraced length is meaningless.

Cases B, C and D

Beam rotation is restrained to some degree in all of these cases. If the bracing satisfies code requirements respecting strength and stiffness, the unbraced length for lateral torsional buckling is 5' but the unbraced length for lateral buckling is 20'.

The section resisting lateral buckling is the weak axis section modulus of the two W24 members. If the W24 members are highly stressed by gravity load, the flange tips will not be very effective in resisting lateral buckling.

Personally, I would not use Cases B, C or D. In the absence of steel deck, I would add bracing in the horizontal plane to form a truss. In that case, the stiffeners and kickers at the intermediate beams could be eliminated.

BA