Classifying a Moment Frame per ASCE 7-16 Chapter 12

[dmitrikylev]

Hello, first post. I appreciate any advice.

I am designing a foundation and structural supports for a prefabricated shelter. The support itself consists of a mat foundation, concrete columns, and steel beams that are anchored to the top of the concrete columns. The prefab shelter will be fastened to the top of the steal beams.

How would the structure i am designing be classified per ASCE 7-16 chapter 12? I know it would be some kind of moment frame, but I don't see an option for a moment frame consisting of concrete columns and steel beams. Would this be a "composite steel and concrete ordinary moment frame"? I am not sure because this will not be composite members. The concrete columns are just ordinary reinforced concrete, and the steel will just be hot rolled wide flange steel, anchored through the bottom flange and to the top of the column with conventional anchor rods.

Thank you for helping.

 

[JAE]

Wouldn't you be "Cantilevered Column Systems" - Type G.5 (ordinary reinforced concrete moment frames)?

 

[Ganesh Persaud]

JAE, I think his senario is if we look at the framing plan, he is basically resting the concrete columns and typically bolted through the lower flange with about 4 bolts. You’ll have a very rigid frame on concrete columns. I’ve actually seen this in use by local contractors here where I live in the Caribbean. However this is not a good practice and I would avoid this if I were you.

To the question, you wouldn’t have a moment frame with the configuration you mentioned, you would have a high horizontal shear which the anchor bolts would have to provide resistance to seismic and wind loads. The framing plan being rigid, the torsional behavior would be a something to consider before you construct something like this.

Like JAE mentioned, you could have rolled sections with an end plate connection anchored to the side of the concrete column. There are provisions in ACI to check tear out and break out given the geometry of the concrete column.

To help you understand this, a moment frame with rolled sections are typically connected to rolled section column flanges through the strong axis.

I don’t think asce 7 allows the use of rolled sections resting on concrete columns and anchored.

It’s safer and quicker to have steel columns and beams. Looking at the moment frame also, connection design is where you bring the life into the moment frame.

Hope this clears up anything.

 

[r13]

Your shelter is supported by the cantilever columns, which are tied on the top by the steel beams. There is no framing action between the column and the steel beams.

 

[JAE]

I don’t think asce 7 allows the use of rolled sections resting on concrete columns and anchored.

Well, ASCE 7 doesn't prohibit it either.

What frame type G.5 is describing is exactly what dmitrikylev is doing ... concrete columns that are cantilevered out of the ground (on footings designed to take lateral shear and moment from the columns, and pinned beams or whatever, on top. No moment connection is required at the top to satisfy G.5. The phase "ordinary concrete moment frames" in ASCE 7 actually reads: "Cantilevered Column Systems Detailed to Conform to the Requirements for: Ordinary reinforced concrete moment frames."

So it is not a moment frame - but you have to detail the column as though it was.

 

[dmitrikylev]

Thanks for the responses.

I can visualize now how it would act like cantilevered columns. The structure will be symmetrical in plan view, so I do not think global torsion would be an issue but that is a good point that I will be sure to check.

So from what I understand, for the calculations as it is currently designed, the concrete column to steel beam connection would be treated as pinned?

 

[JAE]

Yes you can assume it pinned and detail it that way - and use G.5.

 

[r13]

Note that older code had specified a minimum eccentricity to accommodate incidental torsional effect. Even without that requirement, it is always prudent to include in your design consideration and check it.