COMPOSITE COLUMN

[deereman]

I have a condition where the bottom floor of a 2 story building will be open. The end columns I have designed as moment frames. The columns are round HSS14x.625 columns. For protection, the arch would like to wrap the columns in concrete. I was thinking of having studs welded to the column so that it would act as a composite column and the concrete would move with the steel. Currently, I've designed it to meet all loads as a regular steel column with 1" of drift. If I was to do this should I show any reinforcement, wire or ties around the column for crack control?
Another option is to use a double sono tube around the column so that there is a hoop of concrete around the steel column that could move independently of the steel.
Any thoughts on either method?

 

[BridgeSmith]

I would think that if the concrete has to accommodate 1" of drift, you'll definitely need reinforcement to control cracking, but even with that, I would expect cracking to still be visible, unless you use some kind of polymer modified concrete that gives it some elasticity.

Isolating the concrete from the steel column seem like the more aesthetically palatable option. That said, a concrete sleeve seems like it would be difficult to cast. Can you talk the arch into faux concrete columns, or is shotcrete an option?

 

[Nor Cal SE]

I agree with HotRod, difficult constructability for cast-in-place sleeve, plus lots of cracking potential even if reinforced well. Some type of faux column and/or precast option that doesn't directly connect to the steel column sides should be a better option.

I would present your concerns to the architect, and then defer the column cover item directly to him if possible. Let him have the responsibility for the aesthetic elements.

 

[WARose]

For a real composite column (and this sounds more like a protection situation), yes, you'd have ties and some vertical re-bar as well (along with the studs, to hold the concrete together.)

 

[BridgeSmith]

If the concrete is being added for fire protection, as WARose suggested, then it would need to be tight to the column. For that case, it would certainly need to be reinforced, and the closer the spacing of the wires, the better. If that the case, I reiterate my suggestion to look into modified concrete, or possibly fiber reinforcing of some kind.

 

[Agent666]

What are you protecting the columns from (impact/fire/others unforseen events)? Is concrete filling them an option to improve their robustness? Paint them to look like concrete if it's an issue.

(usually architects are trying to make the columns smaller, not larger. What's the world coming to....)