BRBF Design 2

[BAGW]

Hi All,

I have couple of questions regarding BRBF (AISC Code)

•Should the BRBF brace be designed to resist gravity loads along with lateral loads? When the brace connects to column, small portion of the column load may be taken up by the brace. Should the brace be designed for this?

•When BRBF attaches to the column, are there any special detailing requirements? Should we have a beam spanning laterally at the brace column intersection like in brace beam intersection?

Thanks

 

[Deker]

Agree that the column stiffness needs to be included in the bracing checks, just pointing out that it can be done. I think a box or flat wide flange girt framing into the weak axis of the columns and moment connected could be a viable system. Perhaps BAGW can provide a sketch of his frame for us to comment.

 

[KootK]

Yeah, having noodled on it a bit more:

1) The stability demands on the girt may not be all that bad. As you noted, one of the important features of BRBF is keeping over-strength capacity in check.

2) Really, my concerns are not at all specific to BRBF or even seismic. They'd apply to any tiered system with struts between diaphragm levels.

This got me thinking about another interesting point that I'd never considered previously. The stability of the intersection joint requires:

1) Some flexural stiffness in the girt.

2) Some flexural stiffness in the tension column.

3) Some flexural stiffness in the compression column.

What's the flexural stiffness of a column that's loaded in compression to nearly it's buckling capacity? Nearly zero. Kinda scary. That said, you'd often have a couple of things going for you:

1) For high seismic, design requirements probably prevent your column compression load getting too close to Fcr. And I bet you'd have to get pretty close for it to compromise stability.

2) If your columns are oriented with the strong direction out of the plane of the brace, then your buckling load at the weak axis limit likely would not compromise your flexural stiffness out of the plane of the brace.

Sometimes structural engineering feels so complicated as to be almost intractable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Perhaps BAGW can provide a sketch of his frame for us to comment.

Indeed. Being an airplane hanger, I'm guessing portal frames in one direction and BRBF in the other.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

Is the flexural stiffness of the column dependent on the axial load prior to buckling? Do you know of any references that cover this? Or are you referring to p-delta effects?

 

[KootK]

Is the flexural stiffness of the column dependent on the axial load prior to buckling?

It is. That's fundamentally what bifurcation column buckling is generically: the compression load at which flexural stiffness disappears.

Do you know of any references that cover this?

Off hand, I don't know of anything brief and succinct such as you're probably hoping for. It's almost always covered in some fashion in stability texts although they rarely do a great job of saying "compression = reduced stiffness" as loudly as I feel they ought to. This is one of my favorite references that deals with the subject in the context of how it's handled in software. It's free. Should be somewhere around the discussion of the geometric stiffness matrix. It's been a while.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Or are you referring to p-delta effects?

And yes, it's absolutely about p-delta. Little delta, not big.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

Thanks. I'll check it out. Is it fair to say that your concerns regarding stiffness of the compression column are really about not taking p-delta effects from the bracing force into account when designing the column?

 

[KootK]

Is it fair to say that your concerns regarding stiffness of the compression column are really about not taking p-delta effects from the bracing force into account when designing the column?

No, I don't think that it is fair. That said, I'm struggling to be certain of your intent because you're not using the big/little convention for P-delta that I'm used to.

1) P-Big Delta, resulting from joint translation. This is what I think you're referring to. It hasn't entered into my thinking. At least not yet. I'd think it not a big deal because:

a) Lateral shear translation should be relatively slight (more with seismic of course).

b) It doesn't mean much in terms of moment amplification unless you're column connections are fixed at the base and/or strut connections.

c) This would be amplifying moments about the plane of the brace anyhow. It's stiffness out of plane that matters.

2) P-Little Delta. This results from compression load being significant enough that any displacement of the column between joints would fail to generate the necessary restoring moment because of the P-delta effect. Imagine a simple span beam with a pin at the left, a roller at the right, and Pcr applied axially at the roller. That beam has no flexural stiffness. That's what I'm getting at.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

Sorry, was referring to p-little delta. I think we're on the same page.

 

[KootK]

No sweat. In that case, I'd say that there really is no situation in which P-baby-delta effects from the bracing forces are not accounted for in our designs. It's baked into our column design methodology, even if it's just pure axial load and pin-pin.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAGW]

@ kootk and Deker

See the plan and elevation. I hope this will make it more clearer what I ma talking about.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1485023692/tips/AA3CA687-27B3-49E0-A9C4-C36947172884_ioioao.SNAG[/url]

 

[KootK]

My computer is scared of your .snag file type. Can you post a PDF or more conventional image file?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

BAGW...Saw this video and thought it might be helpful (Link). It illustrates how to apply the upcoming multi-tier braced frame provisions in AISC 341. The part of the presentation specific to BRBF starts around the 44 minute mark.

 

[T_Bat]

If we may, I'd like to go back to the gravity load question to make sure I'm tracking. Deker, the chevron example makes sense. We want to be sure the beam can handle the gravity load post brace buckling. However, if there were a case where the P-Delta (that's "big" delta) demands were large, say braced frames supported by transfer level with varying stiffness, would you consider this outside the requirement of AISC 341-10 F4.2 since it is more stability than pure vertical resistance?

 

[BAGW]

@ DEKER. Thanks for the video file. I will go through that.

@ KootK: I was out of town and dint have access to computer. I am uploading pdf file.

 

[KootK]

Arghh!! I still can't download it BAGW. Might be a technical deficiency on my part.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Remove any spaces from the file name... engineering.com doesn't like spaces in file names.

 

[BAGW]

try now