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?

 

[BAretired]

The end stiffeners and the deck.

BA

 

[Lion06]

I think the case that you show in the sketch is slightly different than the previous discussion (at least based on some of the assumptions I made).

Some of this will come down to detailing and whether the connection between the stacked beams can transfer moment to brace the W24. Let's assume for a second that it can't.

If the connection between the stacked beams can't transfer moment AND there is no beam parallel to the high beams that can take a diaphragm force into the piers, then I agree that the unbraced length of the W24's would be 30'.

Although, just to talk about it. The last stacked beam at each end (that stacks directly on top of the end plate of the W24.......... I think that end plate and connection detail would be stiff enough to give the diaphragm a "lateral system" such that it would still be bracing the S24's.

 

[RFreund]

I see now, this is a 1 bay system. Then I would say that your Lb is still 5' (I think) but you may need to check overall lateral torsional buckling.

I don't have the book with me but I believe on page 231 of
Guide to Stability Design Criteria for Metal Structures By Ronald D. Ziemian

Try this link:

http://books.google.com/books?id=DD...&resnum=5&ved=0CEgQ6AEwBA#v=onepage&q&f=false

EIT

 

[slickdeals]

@BA:
Interesting....I had not thought of that. I do apologize if this is rudimentary question, but I am having trouble visualizing it all.

The deck then spans as a deep beam between the 4 stiffeners? The deck is not attached to the beam, but to the joists. Do you mind explaining further?

 

[BAretired]

slickdeals,

In your sketch, you asked "what is stopping the lateral displacement?" The answer is the end stiffeners and the steel deck.

In your last post, you are correct. Five columns, braced only with a mid-height brace are not really braced because all columns can buckle in the same direction.

BA

 

[delagina]

fig 9.2.2

 

[RFreund]

Actually for my case to apply you would need cross frames or moment resistance from the top beams that Lion is alluding to.

EIT

 

[BAretired]

slickdeals,

The end stiffeners prevent the ends of the beams from rotating.

The steel deck acts as a diaphragm or deep girder. It prevents the joists from translating relative to each other. If the joist ends are held in a straight line, the top flange of the beam cannot buckle over its full 30' length but it can buckle over the 5' spacing of the joists.

BA

 

[Lion06]

BA-
I think that's totally awesome that we posted so close to each other (yours must have come up when I was typing mine, because I didn't see it before-hand) on such a theoretical topic and reached the exact same conclusion.

There, that makes up for the last time my gut was so off with that wood capacity!

 

[azcats]

I'll quote the commentary of appendix 6, Sec 6.3:

"Beam bracing must prevent the twist of the section, not lateral displacement. Both lateral bracing (for example, joists attached to the compression flange of a simply supported beam) and torsional bracing (for example, a cross frame or diaphragm between adjacent girders) can effectively control twist."

IMO, your situation (with or without deck) is braced against lateral-torsional buckling at 5' provided there is some sort of reasonable connection between the joists and that the joists meet the bracing requirements of Appendix 6.

Also, I'm not sure I like the sketch on how you envision the buckling. The flanges don't stay parallel in LTB.

http://www.adina.com/newsgH49.shtml

You can see how a member of reasonable stiffness would restrain the buckling in the animation linked above.

But, I could be wrong...

 

[Lion06]

Az- I don't think I agree for joists with no deck. The deck takes the brace forces to the lateral system. Without deck, the joist is just pushing in the other beam. I don't thing that constitutes bracing. I would analogies that to bottom chord bracing for OWSJ. The bottom chord bracing isn't effective unless it actually connects back to something capable of taking out the brace force..

 

[azcats]

IMO, if that beam top flange can't rotate, it can't buckle. The joist (again, if it meets appendix 6 requirements) restrains the twisting.

Are there really push/pull forces to resolve in LTB bracing or do the braces themselves just need to be strong enough not to buckle while doing bracing work?

Fun (for us - my wife would just be looking at me weird) discussion here.

 

[JAE]

The braces need to be BOTH strong enough and stiff enough.

Appendix 6 outlines this for a nodal brace (joists with deck) and relative braces (joists without deck).

 

[RFreund]

Damn.

After reading Lion and BA I should have stuck with my original post regarding the load path.

However, if you have no deck then I believe not only do your braces need to be strong and stiff enough to prevent twist (Lb then = 5') but also you must consider overall lateral torsional buckling which is what I posted about earlier. See the the link above or search for twin girder stability / buckling.

Or at least I think...

Or you could check to see one of the W24's is sufficiently strong and stiff enough to resist the force that is applied from the brace (joist in this case) in weak axis bending and deflection. However you would need to check bending strength in both directions.

Or at least I think...

EIT

 

[csd72]

In practice I would do as others have said and use 5' with deck and 30' without, though as the load applied by the joists is technically destabilising then you could potentially get more than 30'.

If the joists were connected to the web of the beam with a full height fin plate and two well spaced bolts then I would think that you could treat them as a torsional restraint and reduce down the 30' but in this case you would rely on the thin web to transfer torsion which would be inadequate.

 

[SAIL3]

I have always believed that if you prevent the compression flange of a beam from lateral displacement then it eliminates LTB.
For LTB to occur, the compression fla must first displace laterally as the rotation occurs....it can not rotate "in-place" without lateral displacement of the compression fla.
So in essence, I disagree with the reg'd stiffeness of bracing members in the AISC 13th edition..

 

[csd72]

there has to be some sort of stiffness, otherwise people could weld a half inrod to the top and call it a bracing strut.

 

[Lion06]

The only reason the brace prevents movement is because of the stiffness. The brace isn't a hard point that is unyielding. It is itself a spring with a stiffness of it's axial stiffness. If you tried bracing the top flange of the beam with a spring shock absorber from a car, I think we can agree that it can be positively attached to the beam's top flange, but not prevent lateral movement that would occur when the beam wants to buckle.

 

[csd72]

lion06, thats what i was trying to say. should have said half inch rod.

 

[SAIL3]

Sorry, I should have said rotational stiffeness....axial was covered by designing the brace for the old 2to4% of the axial load in the fla of the bm....and I am not aware of any problems that arose from that method...going through the extra time and effort of checking rotational stiffeness to me is another example of the the recent shotgun approach of the AISC code of checking every nit-picking possibility wheather it applies or is actually meaningful or not