Unsupported compression length of beam 3

[tdawgye]

Hi there,

For beams under gravity loading, what would you consider the unsupported compression flange to be? If its supports a metal roof deck with no joists framing into it, would the unsupported compression length be the total span from column to column? Or do you consider the unsupported length to be 0, as the deck is welded to the beam?

Thank you.

 

[BAretired]

It would be normal to consider the top flange of the beam continuously braced by steel deck.

BA

 

[Lomarandil]

*with the assumption that the deck is connected and geometrically able to act as a diaphragm between that beam and some other structural member.

Also, as BA stipulates, the deck braces the top flange of the beam. This may not solve all of your problems if your beam has negative bending (continuous over supports) or axial compression.

----
just call me Lo.

 

[tdawgye]

Thanks guys, i appreciate it

 

[r13]

Just a reminder, torsion needs to be considered if there is no positive bracing at the bottom flange.

 

[ldeem]

I did a very deep dive into stability bracing for a project recently. I was peer reviewing a roof made of a large number of 180' long trusses with the top chord supported by metal deck. In the past I have generally considered top flanges with metal deck to be braced at 4 or 5 ' centers. That works for most situations but if you have a partially long span you need to think about other considerations. The attached paper is a useful reference as well as AISC Appendix 6.

 

[JAE]

Two things:
1. Depends on the orientation of the deck flutes - if perpendicular then we assume full bracing at fastener spacing. If parallel - then we don't consider deck only unless there is a concrete deck over it.
2. For simple spans, bottom flange doesn't need to be braced as long as top flange is translationally braced.

 

[r13]

I stopped reading the paper, provided above, when seeing the sentences,

". Therefore, a lateral brace restricts twist best when it is located at the top flange. Lateral bracing attached at the bottom flange of a simply supported beam is almost totally ineffective. "

I fully agree on the first, but think the second delivers incomplete message, as it does not address whether the top flange is braced or not. I can only agree that it is difficult to brace bottom flange effectively, but will not go that far to discount the effectiveness of the bottom flange brace completely.

 

[JAE]

For years Dr. Yura at UTexas Austin has taught that you can EITHER brace the compression flange translationally OR you can brace the section against twist AND there's no need to do both.

 

[JoshPlumSE]

Just to be clear, this is a simply supported beam, right?

If the bottom flange goes into compression, then the bottom flange is likely unbraced for it's whole length. This is an issue with continuous beams and / or uplift wind scenarios.

 

[r13]

I will try to get Dr. Yura's paper and understand his whole point. But for now, let me throw something into the mud making the discussion more interesting, also see how other thinks.

Here is a excerpt from AISC Exchange (Feb. 1996), Dr. Yura was quoted in this guy's response to the questions about stability bracing. Please pay attention to the bolded text.

What constitutes lateral bracing? For beams, a brace "must prevent the relative displacement of the top and bottom flanges, i.e., twist on the section" (Stability Bracing Specification Provisions and Commentary, to be published in the next LRFD Specification). A structural member can be considered a brace if it has sufficient strength and stiffness to restrain the compression flange from displacing or prevent the top and bottom flange from twisting (i.e. relative displacement).

Now we have a relatively deep edge beam supporting a flexible roof deck, will you concerned with the "twist", or not concerned at all because the compression flange is continuously braced?

 

[JAE]

If the flutes are parallel in non-concrete applications of course concerned as I stated above.

If the flutes are perpendicular to the beam no concern at all.

Yura, in the statement you quoted, said exactly what I said: [blue]"...member can be considered a brace if it has sufficient strength and stiffness to restrain the compression flange from displacing [red]or[/red] prevent the top and bottom flange from twisting.[/blue]

 

[r13]

So, no torsion concerns as the top flange is braced against translation?

I don't think this is a fair question that can be easily answered in this forum. So please ignore it.

I suggest that if there is potential for the beam subject to torsion, and any doubt on the stiffness of the deck/slab to provide adequate restrain against rotation, then either upsize the beam (larger I, S r),or provide torsional brace, which might include bottom flange bracing. In general, I am against throwing out the idea of bottom flange bracing without further research/evaluation/understanding on this matter.

 

[Tomfh]

If the whole top flange is in compression then I would consider it continually braced.

If the beam is continuous and the bottom flange goes into compression then it’s a different matter entirely, and we could be looking at another 600 post thread...

 

[JAE]

There are perhaps millions of linear feet of simple span beams "out there" with no bottom chord bracing in buildings.

You don't need it unless, as Tomfh has stated, its a continuous beam, or, it is a beam with a superimposed torsional load or lateral load on it from anther source.

For bridge girders, yes you need full twist resistance in general for a multitude of reasons (lateral forces, deck removals, unbalanced deck loading, etc.
But for simple span beams "under gravity loading" as the original post above asks, you don't need bottom chord bracing.

 

[human909]

There are perhaps millions of linear feet of simple span beams "out there" with no bottom chord bracing in buildings.

That is a very poor argument to argue that bottom flange bracing is not needed. There would also be "millions of linear" feet of simple span beams out there WITH bottom flange bracing.

You don't need it unless, as Tomfh has stated, its a continuous beam, or, it is a beam with a superimposed torsional load or lateral load on it from anther source.

So those "millions of linear" of simple span beams with fly bracing are unnecessary?

If the whole top flange is in compression then I would consider it continually braced.

What happened to uplift as others have mentioned.


For roofs with minimal gravitational load then uplift would be expected to govern the design. In areas with snow or larger roof loads this may not be the case but in the absence of these then uplift would normally be the critical case (for flattish roofs). In which case you would often look at bracing the bottom flange. (Or just use a section that is less susceptible to LTB)

 

[JAE]

Oh for cripes sake. Read the original post.

It is a GRAVITY beam. Not talking about uplift and I qualified all my statements to that effect.

Yes if there’s uplift then bottom chord bracing may be required.

Perhaps there are millions of feet of beams WITH bottom chord bracing but that’s irrelevant. My statement was in response to the suggestion that we shouldn’t omit bottom bracing without “more research” etc. That isn’t valid for GRAVITY beams.

 

[Tomfh]

Human909, OP said bracing of compression flange on a gravity loaded beam.

For uplift the answer is obviously different.

 

[human909]

I was aware of that. But he also said roof. Which would imply it is very likely to be subject to wind loads.

 

[r13]

Down thrust gravity loads cause rotation/torsion in cases such as - edge beams, off center loading, unequal beams/joists spacing, one side loading on beam....all gravity load cases, are these cases not possible to have torsion as the beam is braced by metal deck?