Steel Column Encased in Concrete

[JNEnginr]

Yet another Friday morning question...

If you take a steel column (W12x87), 22ft tall and encase it in 24" of concrete, up the entire length of the column, does this change the unbraced length?

I'm leaning towards "No", but just wanted to get some feedback.

Thanks!

 

[Lomarandil]

(This is just gut feel, I haven't designed one myself to know the specifics of the code)

It wouldn't change the unbraced length for axial, but I would certainly consider an increase in the radius of gyration.

If it's a beam-column, I'd also presume that LTB or FLB effects are prevented by the concrete.

 

[SAIL3]

short ans...no

 

[TehMightyEngineer]

Short answer; maybe. Though it would take so much work to prove it that I wouldn't consider it worth it to consider the concrete (especially for a relatively long column).

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[allgoodnamestaken]

I would treat it as a composite column that is unbraced. New section and material properties will improve its capacity but bracing is different than "bigger section".

 

[KootK]

If you take a steel column (W12x87), 22ft tall and encase it in 24" of concrete, up the entire length of the column, does this change the unbraced length?

It's a matter of perspective. I'm going to assume that you're working on strengthening an existing column. I'm also going to assume that we're dealing with axial loads for the sake of simplicity. If we need to get fancier, let us know.

There are two fundamentally different approaches that one can take:

1) The steel and concrete act compositely and that combined section buckles in a half sine wave over the entire height of the physical column. The new composite column has a higher radius of gyration but an unchanged kL as Lomariandil mentioned.

2) The steel and concrete do not act compositely. As the designer, you decide on where you'd like to see the steel column braced and establish your bracing strength and stiffness requirements (AISC appendices in the US). Next you design the concrete column, acting on its own, to satisfy those bracing strength and stiffness requirements. In this sense, the concrete column can most definitely alter the effective length of the steel column. See the sketch below for an illustration of this concept.

Within these two general approaches, I know of a few ways to approach the computations to expedite the design. Let us know if that's something that you're interested in pursing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JNEnginr]

Hey! Sorry, I didn't get any notifications until yesterday for some reason. Thank you for all your responses.

My intent is not to reinforce the column...not yet anyway. I have an existing building and I'm checking the max capacity of the 2nd floor structure. Axial only. Currently the columns are the controlling element, but I'm calc'ing them as completely unbraced. In the field, they are currently encased in concrete, so i have no way of determining if there were any welded studs or any other formal connection.

Would you agree in my choice to just ignore the concrete and treat the column as unbraced?

Thanks,

 

[KootK]

Would you agree in my choice to just ignore the concrete and treat the column as unbraced?

I would agree. In many cases with older buildings the "concrete" is really just fire protection. Limited thickness; unreinforced or nominally reinforced; low density. In summary, tough to do much with structurally.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JNEnginr]

Great, thanks as always.

 

[Enginerdad]

It wouldn't change the unbraced length for axial, but I would certainly consider an increase in the radius of gyration.

If it's a beam-column, I'd also presume that LTB or FLB effects are prevented by the concrete.

Lomarandill has it exactly right; the KL stays the same, but your r increases substantially, which for axial loads has the same effect as reducing L. This is of course assuming that the column and concrete are made sufficiently composite.