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

 

[KootK]

Tricky. If you've got a system that draws serious load into the braced core after consideration of construction sequencing etc, then I don't see how that could be ignored. It will change what and under how much load your braces will yield with implications for drift and strength. There might be some offsetting effects between the T&C braces if things are symmetric. How much load are we talking about relative to the brace yield strength?

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[Deker]

BRBF braces should not be designed to resist gravity loads. Imagine a chevron braced frame with a point load applied directly over the braces. 50% of the load is resisted by the vertical component of each brace force. During an earthquake, the brace resisting seismic loads in compression will yield earlier than the brace resisting loads in tension. However, after the first load cycle, both braces have yielded and are in equilibrium with each other (assume equal compression and tension strength). This leaves the point load to be resisted entirely by the beam. As the braces unload, the compression and tension unload in unison and the point load continues to be resisted by the beam.

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.

 

[JAE]

Section F4.2. Basis of Design (AISC Seismic Provisions 341-10):
[blue]...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.[/blue]

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[Deker]

JAE...The way I interpret that section is that lateral loads that are delivered to the frame by gravity loading, such as by inclined columns, should be considered in addition to seismic loads. I don't think it means that braces should be designed to support gravity loads.

 

[KootK]

I reviewed AISC 341 when I got back to the office this afternoon and have concluded that:

- 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.

 

[jdgengineer]

Make sure to reach out to the brace manufacturers when you get into design / detailing. It's been a few years since I've worked on one, but I believe they handled all gusset plate detailing and we provided brace forces. Star Seismic & Nippon Steel are the two biggest manufacturer's (I believe). Ian Aiken of SIE, Inc. I believe provides a lot of the support for Nippon Steel.

 

[BAGW]

@ Deker,

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

 

[wannabeSE]

jdengineer,
Star Seismic is now part CoreBrace, the other major BRBF player.

 

[JAE]

Deker - after looking at 341 it does specifically say not to design for gravity loads in section F4.3

So not sure what they really mean in F4.2. that I quoted above.

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[KootK]

So not sure what they really mean in F4.2. that I quoted above.

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%?

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?

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.

How do you treat these braces for wind load? What happens when wind load starts controlling?

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.

Also, When BRBF attaches to the column, are there any special detailing requirements?

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.

Should we have a beam spanning laterally at the brace column intersection like in brace beam intersection?

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.

 

[sandman21]

What type and size of building are we talking about? Transfer girders at BRBF beams? If you are talking a typical office building DL and LL would not need to be considered in the design of the braces. If you have a unique system you may need to consider gravity loading on the BRB. I always provide a beam at the column connection. You are providing a large plates attached to the beam and column you are going to get moment transfer at the connection.

 

[Deker]

BAGW...I chose a chevron braced frame to illustrate my point because I think it represents the worst case for gravity loads in the braces. The same concept would apply to your condition. I agree with KootK on his responses to your other questions and think he has provided nice explanations. I'm not clear on which direction you are asking about for the beam. If same direction as the brace, yes you would usually need a beam to deliver loads to and from the braces. If perpendicular to the brace, yes you would need to brace this point somehow which is usually accomplished by providing a beam framing into the column.

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?

 

[JAE]

Deker - agree

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.

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[KootK]

I don't know if anybody's actually doing it but the snip below comes from NEHRPS BRBF design guide's Novel Applications section.

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.

 

[sandman21]

Yes, many high rises use outriggers. An article on one of the more recent uses.
Wilshire Grand

 

[BAGW]

@ Kootk

[I keep asking about the load involved here as I think that that is an important consideration]

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.

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

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

 

[KootK]

Code required or not, you're going to want to brace the tops of those columns by something more robust than metal deck. What's the framing look like behind the braced frame?

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.

 

[Deker]

Check out section F4.4d of the AISC 341-16 public draft for guidance on multi-tiered frames. A girt could be designed with adequate out of plane strength and stiffness to brace the BRBs. AISC allows this in section F4.4a for V-braced frames.

 

[KootK]

Interesting. In concept, I do of course agree that a sufficiently stiff girt would get the job done. After all, a floor diaphragm is really just a kick-ass girt after a fashion.

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.