Knee Braced Moment Fame 2

[rajbhat]

I am evaluating 115' tall industrial structure. The structure is inverted V braced (concetricaly braced per AISC seismic provision)in one direction and knee braced in other direction. Sesismic zone is D.

My question is how to categorize Knee Braced frame per AISC seismic provision and detail the connections. Is it a
1. Special or Ordinary Moment Frame or
2. Ordinary Brace Frame (however, OBF are limited to 60' height in Zone D)

 

[JAE]

I believe it would fall under the Eccentrically Braced Frames under AISC (EBF)

 

[rajbhat]

JAE, thanks for the reply.
The 'Link'is relatively long, I assume it will be flexture controlled, do you agree?

 

[JAE]

Without knowing anything about it I couldn't say.

 

[Taro]

If this was a new design, knee-bracing would be forbidden because it can cause plastic hinges to form in the columns and collapse is more likely. For evaluation of an existing structure, I think you would want to ensure that the column remains elastic to make sure this collapse mechanism does not occur.

 

[SacreBleu]

Taro,
1) Where in the AISC manual or IBC does the knee-brace prohibition appear?
2) If the column is specially stiffened at the knee-brace joint to prevent a plastic hinge from forming there first, is that addressed in the Code?

 

[Taro]

Knee braces aren't explicitly prohibited. Rather, they are not addressed at all. They are clearly not concentric braced frames. And they do not qualify as eccentric braced frames either because the EBF links are defined as portions of the beams. Note that AISC requires EBF columns to remain elastic. So for a new design you would not have a defined R-factor value and would have to provide analytical and test data to substantiate the system's performance.

 

[SacreBleu]

Is the main difference between "knee-brace" and EBF is that the EBF diagonal brace is concentric to the beam-column joint at the next lower level, or at the base of column in a one-story situation, and the knee-brace connects to the column at a small dimension (such as 3 feet) just below the top of column?

Taro, your explanation makes sense.

 

[TFL]

ok i am just thinking here.....

but how close the diagonal of the knee brace is to the beam/column intersection would effect the behavior of the system.

When the knee brace is as far away from the joint as possible would it not behave similiar to a concentric braced frame (inverted v type) as the diagonal moves closer to the joint it behaves more as an eccentric braced frame. And when it is finally very close to the joint is it not just behaving as a moment frame.

Now i understand that the seismic requirments may not recognize this behavior in the higher seismic regions but in the wind controled regions were the seismic detailing provisions are not required, why not use an approximate r value based on the how how close it is behaving to a moment frame to check you seismic forces and proceed to design for wind.

even in regions were seismic controls but the detailing requirments are not required it may work.

i'm sure someone has a comment
thanks for listening

 

[SacreBleu]

Is this structure one-story, 115' tall? With knee-bracing, the drift will be huge, totally unacceptable.

 

[whyun]

Knee bracing is similar to K-bracing which is prohibited by code. I agree with Taro that knee bracing is not EBF unless the lower end of the knee brace aims at the lower column/beam joint.