Rigid and fully baced steel structures 8

[honat]

Anyone out there can tell me why would anyone design a steel frame structure as a fully rigid moment framed structure and at the same time design the same structure as a fully brace structure?
That is the structure is a fully rigid moment frame and a fully braced structure.
Is this supported by any code?
If so which one?

 

[weab]

Sounds like a waste. Could be designed by an engineer with little confidence. Could be that someone designed a moment frame and thought that they could add a little steel in the form of braces to get extra ductility. I'd be surprised that any code supports this as advantageous.

Essentially, you have a braced frame structure with oversized beams and connections.

 

[JAE]

The Top 10 Reasons for both a brace and a moment frame:

10
"An engineer with little confidence"
9
An engineer with a dual personality!
8
Twin engineers working together who can't agree.
7
An engineer who chews Doublemint gum.
6
Simply a glitch in the Revit model.
5
The X brace for wind and the moment frame for seismic!
4
An architect posing as an engineer. "such symmetry!"
3
The moment frames takes the load and the X brace keeps the drift
below L/500.
2
Trying to keep the welders busy.
1
We just know the plumbers are going to cut the brace anyway...

 

[wannabeSE]

I have not really looked into this. But, I suggest you read the new requirements for special concentric braced frames and the corresponding commentary in AISC 341-10, Seismic Provisions for Structural Steel Buildings (

http://www.aisc.org/content.aspx?id=2884#seismic

). The beam to column connection requirements have changed.

 

[wannabeSE]

The 2010 Seismic Provision's beam to column connection requirements are at F2.6b. Where a brace or gusset plate attaches to both members, the beam to column connection may be a simple connection capable of 0.025 rad rotation or a moment resisting connection.

The commentary has two figures (C-F2.7 and C-F2.8) depicting simple connections that allow this rotation. In C-F2.7, the gusset plate appears to be attached to the column with a double angle connection bolted to the column (in lieu of welding the gusset directly to the column). In C-F2.8, there is stub beam with the gusset plate welded to the column and the stub beam, the stub beam web is welded to the column, and a hinge connection is placed between the stub beam and beam outside the gusset plate.

If a moment connection is used, it is a fully restrained moment connection that is capable of resisting the expected moment strength of the column or beam. The commentary states the moment connection must meet the requirements of an ordinary moment frame connection.

 

[jdgengineer]

Sometimes the code gives you a higher R value for using braces and moment frames together. Take for example the case of BRBFs. I believe the code gives you a higher R value if you use moment connections together with your BRBFs. Is his what they are doing?

 

[UcfSE]

You'll need to fill out form EZ2C at the Department of Redundancy Department.

 

[ToadJones]

Are you sure that is how it was designed?

I have worked on a few power plant projects where the connections appeared to be as you are describing.
The fabricator wanted to replace the clip angles on the beams and the clip angles on the gussets (above and below the beams) with on single end plate.
Some guys in our office re-worked all the connections as such to satisfy the fabricator.
I was only tangentially involved, but in the end, the connections "looked like" moment end plates with braces.