Yet Another Unbraced Length question

slickdeals · May 11, 2011

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?

RFreund · May 11, 2011

If the joist and joist connection is stiff enough (see appendix 6) to prevent movement than I would say 5'

Attached is some notes (not mine) that I have found helpful.

EIT

slickdeals · May 11, 2011

RF:
IT will prevent movement between the flange and the joist, but nothing is stopping the lateral movement of whole system. There is nothing providing lateral bracing. I am inclined to think 30', most definitely not 5'.

If one compression flange wants to move, then it will take the other compression flange with it.

Lion06 · May 11, 2011

I'm also of the mind that it's 5'. Assuming of course that the strength/stiffness requirements check out, as RF noted. I'd be surprised if it didn't check out, though.

I think the floor construction plays a part in this, too, though. Is this metal roof deck? Slab on composite deck? Is the W24 composite with the slab?

Lion06 · May 11, 2011

Oh, you're assuming this isn't decked at all?

slickdeals · May 11, 2011

This is an academic question, not a real project. But I am sure the concepts would apply in any real building.

It is decked, just bare metal deck. But what prevents the whole system from laterally displacing? The only stiffness is from the weak axis bending capacity of the W24 web.

Make sense?

Lion06 · May 11, 2011

I think if it is decked (even with bare metal deck), and that deck (diaphragm) is positively attached to the joists, that when the joists want to move to brace the W24 that the diaphragm will restrain it.

I'm assuming that there are W24's orthogonal to the ones you're talking directly about (which would be parallel to the joists) that are anchored to the piers in a similar fashion that would essentially act as the lateral system.

BAretired · May 11, 2011

The unbraced length is 5' with deck...30' without.

BA

RFreund · May 11, 2011

I will try to search for a recent post very similar to this and I had the very same question as you and it never really got answered but here are my thoughts:
If the compression flange wants to move laterally due to LTB then the joist will restrain this movement and the force on the joist would be transfered into the deck, the deck then distributes this force to the lateral load resisting system.
However I have never seen a check for this or this load added to a LFRS design.

EIT

RFreund · May 11, 2011

Here is the thread I was thinking of:

Truss / Joist Girder Stability Bracing
thread507-287322

EIT

JAE · May 11, 2011

I agree with BAretired: The unbraced length is 5' with deck...30' without.

2Hats · May 11, 2011

I'm pretty sure what you're describing is a lean-on bracing system (in the absence of deck). Galambos (Guide to stability design criteria for metal strucures) discusses these systems.
The answer depends on the number of W24 being braced agains each other. As the number gets large, your unbraced length should approach 5ft.

ash060 · May 11, 2011

You could use some cross bracing to prevent the section from twisting, using the joists as the system, but you would have to brace both flanges.

I did something very similar to this, but made a truss with the top flanges using angles. It seems to work.

What you bring up is bridging. Joist bridging is used for exactly the situation you mention. before the deck is on the joist chord it has to be braced for construction loads hence bridging. You can use horizontal or diagonal bridging with diagonal being the better option.

There are some good notes by Larry Griffis called "Steel Design After College" on AISC's website.

delagina · May 11, 2011

based on the salmon and johnson book it should be 30' for global lateral buckling

Lion06 · May 11, 2011

delagina, where do you see that?

slickdeals · May 11, 2011

I don't think it is lean-on. There is nothing to "lean" against.
There are a total of 2 W24. IT represents one bay only.

msquared48 · May 11, 2011

I also would say 5 feet, assuming there is no fixity of the beam at the columns, and that the joists bear on top of the steel beam with decking attached to the joists.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

slickdeals · May 11, 2011

See attached.

slickdeals · May 11, 2011

See attached

slickdeals · May 11, 2011

If I had 5 columns carrying compression loads spaced 5' apart. If I had a mid-height brace, connecting the columns, but not really connected to anything else, the column unbraced length is the full height, right?

The columns would buckle globally because the mid-height beam even though present is not bracing it.

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Yet Another Unbraced Length question

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