BRBF Design
BRBF Design
(OP)
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
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






RE: BRBF Design
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.
RE: BRBF Design
It is important to understand that designing the braces for gravity loads will build additional overstrength into the brace, which is never great for your seismic fuse. Additionally, if you have a tall building and are designing the braces for gravity loads, it is likely that the brace force due to gravity loads will be significant compared to the brace force due to seismic loads at the upper stories. At the lower stories, the gravity loads will be smaller relative to seismic loads in the braces. This leaves you with high overstrength in the upper stories and low overstrength in the lower stories, which is likely to cause a concentration of ductility demand at the lower levels versus evenly distributing it up the building like you want.
Also note that AISC 341 F4.3 requires that braces shall not be relied on to resist gravity forces.
RE: BRBF Design
...Braces shall be designed, tested and detailed to accommodate expected deformations. Expected deformations are those corresponding to a story drift of at least 2% of the story height or two times the design story drift, whichever is larger, in addition to brace deformations resulting from deformation of the frame due to gravity loading.
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: BRBF Design
RE: BRBF Design
- I was wrong.
- Deker is right.
- 341 is pretty clear that BRBF are not to be designed for gravity loads.
Your explanation was excellent Deker. Thanks for that.
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.
RE: BRBF Design
RE: BRBF Design
I completely agree with you regarding the chevron braces below the beam. Beam needs to be designed to resist all gravity loads.
I am concerned about the component of the gravity forces that will be taken away by the brace from the column based on the brace stiffness. Even these need not be designed for that gravity load component? Does that mean the braces in BRBF need to be sized only for lateral forces?
How do you treat these braces for wind load? What happens when wind load starts controlling?
Also, 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
RE: BRBF Design
Star Seismic is now part CoreBrace, the other major BRBF player.
RE: BRBF Design
So not sure what they really mean in F4.2. that I quoted above.
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: BRBF Design
I'm fairly certain that it's as Deker mentioned: anything where the gravity load support system would great a lateral demand of its own. Sloped columns, cantilevered floor plan, differential shrinkage of vertical elements in tall things.
@BAGW: it would be great if you could tell us:
1) What brace configuration you're using. Sounds like V-bracing or multi story X maybe?
2) What is the ratio of gravity load to yield capacity for the braces? 5%? 90%?
Correct. There will be some gravity loads in the braces at the start of a seismic event. And, I'm sure, there will be some degree of gravity load in the braces during the entire seismic load history barring some serious good fortune. And the presence of those loads will impact the the load history of the braces. I think that the main point is to realize that, so long as the gravity framing doesn't need the braces to be viable, the system can find itself a plastic state where the lateral loads can be resisted as planned.
I keep asking about the load involved here as I think that that is an important consideration. If the gravity load on the braces is, say, 5% of AsFy, then I'd not be concerned and I'd follow the AISC recommendations on not designing the braces for gravity. On the other hand, if the gravity load is 90% of AsFy, then I think that one would have to seriously consider the impact that would have on the hysteric behavior of the braces. More than likely, you'd do something to get back to a more reasonable load ratio.
Your lateral system can't be allowed to yield under wind load as that implies collapse. As such, I would think that you would have to consider the gravity loads in the braces. As I mentioned previously, if you have a symmetric situation, it may be that gravity loads lower one brace's capacity while increasing another's.
See F4.6b. The gusset should not yield in flexure and, depending on the path you choose, should be designed to accommodate the angular change associated with a rotation of 0.025 rad.
I don't think that there is an explicit requirement for it. That said, it's hard to imagine that you wouldn't want a beam, or something akin to it, attached to the column to stabilize it at the floor level.
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.
RE: BRBF Design
RE: BRBF Design
JAE...For what it's worth, my opinion is that the wording "resulting from deformation of the frame due to gravity loading" is chosen deliberately to indicate that lateral gravity loads--which cause frame deformations--need to be considered in addition to seismic loads. This makes sense since the braces are a required part of the load path for stability under lateral gravity loads. Not so with vertical gravity loads.
Out of curiosity, are BRBFs commonly used for low-seismic areas where wind governs? What advantages do they offer if using R=3?
RE: BRBF Design
In wind areas with low seismic not sure BRBF's are worth it.
I don't know the cost magnitudes with these things but seems like they might be more cost than benefit - even with a kicked up seismic with R=3 when compared to typical braces.
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: BRBF Design
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.
RE: BRBF Design
Wilshire Grand
RE: BRBF Design
Its a plane Hangar. Its one story building with roof and bottom chord level. I havent yet modeled the Hangar to determine what percentage of gravity load will be resisted by the brace.
There are no beams framing into column perpendicular to the brace column intersection. We only have perimeter framing. Thats the reason I wanted to know as per the code do we need to have any lateral stability frame perpendicular to the beam column intersection.
The bracing system is multistory X brace.
Summarizing above discussion.
1) BRBF needs to be designed for resisting gravity loads. But also make sure the building performs under gravity load even after the brace fails.
2) There is no code requirement for the lateral stability at brace column intersection perpendicular to the brace
RE: BRBF Design
I don't think that you can have a multi-story BRBF X-brace in a one story building. There's no way you'd want a girt bracing the intersection of the four braces.
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.
RE: BRBF Design
RE: BRBF Design
I guess I'll restate my position to be that I personally question the efficacy of using a girt at the intersection of a two story BRBF X-brace. I haven't actually tried it myself which limits the validity of my opinio, but I'd have to think that the stiffness demand would be enormous.
While the draft provisions speak to the out of plane strut(girt) stiffness, they don't explicitly mention a couple of other items:
1) Column out of plane stiffness. For a multi tiered frame like this, the flexibility of the hypothetical nodal bracing must include the substantial contribution of the columns in concert with the girt.
2) Torsional strength and stiffness of the girt. The girt must be prevented from torsionally buckling about it's longitudinal axis. This is exacerbated by asymmetric brace yielding, fabrication and erection tolerances, and the fact that BRBF usually make for lousy torsional bracing.
These things are all surmountable with enough care. And perhaps these are things that a competent structural engineer should spot without needing to be spoon fed by the code. Suffice it to say, I'd be lot more comfortable having conventional lateral and torsional beam bracing at the brace intersection.
I suppose that the compression braces may well be braced by the tension braces. That stuff's contentious enough in non-seismic applications. I won't bother trying to debate it here for a hysterics system with high ductility demands.
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.
RE: BRBF Design
RE: BRBF Design
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.
RE: BRBF Design
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.
RE: BRBF Design
RE: BRBF Design
It is. That's fundamentally what bifurcation column buckling is generically: the compression load at which flexural stiffness disappears.
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.
RE: BRBF Design
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.
RE: BRBF Design
RE: BRBF Design
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.
RE: BRBF Design
RE: BRBF Design
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.
RE: BRBF Design
See the plan and elevation. I hope this will make it more clearer what I ma talking about.
https://res.cloudinary.com/engineering-com/raw/upload/v1485023692/tips/AA3CA687-27B3-49E0-A9C4-C36947172884_ioioao.SNAG
RE: BRBF Design
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.
RE: BRBF Design
RE: BRBF Design
RE: BRBF Design
@ KootK: I was out of town and dint have access to computer. I am uploading pdf file.
RE: BRBF Design
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.
RE: BRBF Design
RE: BRBF Design