Concrete Moment Frame Question

[dcStrucEng]

If I have a multi-story concrete building with shearwalls as the primary LFRS, this will be considered a non-sway (i.e., braced structure). Therefore, I assume all of the columns in the structure (which don't contribute to the LFRS) would be designed with a k-factor of 1.

Now, if I have a multi-story concrete building with moment frames as the primary LFRS, this will be considered a sway (i.e., unbraced structure). For the concrete columns of the moment frame, I'd use the alignment charts in ACI 318 to determine k (using Ieff values.) However, what about the concrete columns that are not part of the moment frame? What value of k would you use? Doesn't every concrete column integral with the slab contribute in someway to the LFRS even if it technically isn't part of the moment frame?

Just wondering if others have dealt with this situation. Thanks.

 

[RFreund]

I'm surprised no one has responded to this.

At first I thought columns not part of the moment frame would have k=1. However you propose a good point about columns integral with the slab.
As you can tell I have not done many errrr, any full concrete building designs.

EIT

 

[JAE]

Doesn't every concrete column integral with the slab contribute in someway to the LFRS even if it technically isn't part of the moment frame?

Yes. Force will follow stiffness and the columns will receive bending and shear forces from lateral loads despite you classifying them as not part of the moment frame.

 

[slomobile]

Depending on your current code...
ACI 318-05: 21.11
ACI 318-08: 21.13
These sections deal with members not designated as part of the SLRS. As for the actual rigidity of the gravity columns, recommend the commentary to these sections as to the assumptions of flexural yielding and appropriate confinement ties to allow this in gravity columns.
Is your project in SDC D or higher is a question to ask also.

 

[FoxSE14]

JAE has got it right. With traditional, monolithic concrete construction with no shear walls, every frame is effectively acting as a moment frame.

 

[StructSEZ]

Hi dc,

JAE is on the right track. Force will follow stiffness on the basis that deformations are compatible.
Now, from this you can control which elements will respond by either increasing or releasing it's stiffness.

In your case, when you want a column to be part of the moment frame you would detail the steel at the slab/column interface such that it can carry the tension due to the flexure (moment) in its entirety.

Where you want the column to act only in the vertical direction, you would only put nominal steel in the slab/column interface. Granted there would still be some moment transfer, but it would only be as much as the nominal steel can develop, in terms of the tension.

One thing you might want to be careful of is that when you have a sway structure and a column exists in there as a pin-pin, it is essentially an instability (free to translate horizontally). Therefore, you must control your sway deflections such that the 'vertical-only' columns don't develop P-delta moments which are too big in the mid-height zone.

Hope this helps a bit.

 

[hokie66]

Late to this thread, but I think the last post by StructSEZ is misleading. You should not try to selectively reduce the stiffness of columns by decreasing the reinforcement in the column to slab connections. The stiffness is what it is, and the connections need to be designed accordingly. If not, local overstress in the horizontal elements can result.

 

[StructSEZ]

Haven't had anything fail yet, mate!

 

[frv]

That's a contractor's answer to a legitimate issue brought up by a valued contributor to this forum. Try again.

 

[DaveAtkins]

I agree with StructSEZ. If you limit the amount of reinforcing between the top of the column and the underside of the beam, it will yield, and the column will only see a small amount of sway moment. Loads always follow the stiffest path.

DaveAtkins

 

[csd72]

The approach depends on the reason for the analysis. If it is for wind then this could have a large number of repetitions so reducing the reinforcement may lead to significant durability and serviceability issues.

If it is purely fo life safety during a design seismic event then allowing for yield of reinforcement would be a reasonable assumption (in my opinion).

It is also important to note that if the analysis has used these columns to resist some of the lateral loads then these loads need to be redistributed to the lateral force resisting system.