Lateral Torsional Buckling

[GalileoG]

Say you have floor joists spanning over cross beams (beam sitting on top of the flange of another beam,) does this constitute a brace point if the bottom flange is welded to the top flange of the beam it is sitting on? If not, then how can we make it a brace point? (Stiffener?)

 

[dgkhan]

Can you explain little more, Joist, cross beam or another beam

 

[civeng80]

Probably better to fly brace it to bottom flange.

 

[youngstructural]

You need a stiffener as well as the ability to develop a moment. The key here is that you restrain against twist... There is a very good paper on this from HERA (Heavy Engineering Research Association) here in New Zealand.

Using this as a brace point is all about stiffness and detailing. I would encourage you to consider a bolt plate welded to the top of your beam and fastened through the joists, or directly welding (NOT bolting) the joist to the top of the beam. Again, stiffeners are required.

Hope that helps,
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[apsix]

If the joist provides lateral restraint of the critical flange (typically the compression flange) then it is a brace point (as I understand the definition of brace point).

Clansman's situation appears to fit the criteria provided the joists are suitably restrained longitudinally. Welding of the joists to the beam will improve the restraint by also providing some torsional restraint.

 

[youngstructural]

apsix is spot on if and only if you're asking about the BOTTOM beam, OR if the top beam is a simple span element. If your critical flange is away from your bracing element, my post applies.

Again, apsix was quite right in saying that it is the compression flange that is key, however my comments were made with the understanding that you are considering a continuous beam accross a supporting member. In that case the top beam sees this as a brace point only if a stiffener is provided and the connection is torsionally rigid.

I think dgkhan has the most appropriate post: Clarify what you meant!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[apsix]

I agree. I've assumed we are designing a simply supported floor beam which is supporting joists on the top flange.

 

[cap4000]

I am assuming you are referring to your bottom "simple span" beam top flange which requires the brace. Beams involved with double curvature bending are much more complicated. If you can refer to AISC Engineering Journal Article by James Fisher 1st Qtr 2006 titled "Bracing of Beams with Joists"... Essentially the brace must be designed for both strength and stiffness. Typically a joist welded to the top flange of a beam is a rigid brace point and the metal deck welded to the joists provides the stiffness for the brace. Composite slab design is also cosidered rigid for both displacement and twisting.

 

[csd72]

As noted above, this provides restraint to the top flange but none to the bottom unless you put in fly bracing or full height stiffeners.

If it is a continuous beam or is subject to uplift then it needs restraint to the bottom flange and the configuration is not sufficient.

 

[xxpegasus11]

Bracing beam's other end should obviously be looked at as well - i think this is implied in all posts made. For example, similar detail at other end, sitting on a parallel "braced" beam wouldnt be bracing either of the two beams - or to be more correct, it would provide twist restraint but not lateral.

 

[youngstructural]

csd72: Are you advocating the use of flybraces to restrain the compression flange if your beam is subjected to uplift? I would think a full depth stiffener would be sufficient if attaching to a torsionally rigid element; At least from what I have learnt.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[csd72]

Either way works.

 

[GalileoG]

Thanks for the responses.

I was referring more specifically to the beam that sits on a supporting orthogonal beam.

 

[apsix]

"I was referring more specifically to the beam that sits on a supporting orthogonal beam. "
That would be classed as 'partially restrained' by AS4100.

 

[csd72]

Same rules apply but upside down.

Need either fly braces or welded stiffeners to restrain the top flange.

There is a good gouide put out by the Australian institute of steel construction (or whatever they are called now) that gives good examples of what is what.

I think its called 'design of unrestrained beams' I suggest your office gets a copy if it doesnt have one already.