huangyhg

concrete encased column
hey folks,
the architect wants to have some new steel columns partially encased in concrete for visual effect. i was thinking that the main thing i would need to do is put in some cracking reinforcement (mesh or the like), but as i'm thinking more and more about it, that might not be enough. i think that i'm going to have to consider the bending stresses that the original steel column would impose on the concrete (based on deformed shape).
there's no shear keys or anything like that to intentionally transfer load from the steel column to the concrete, but i'm sure even without, there would be some substantial force transferred.
thanks for the opinions
drw
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can you isolate the column with some sort of grease or debonding agent to help prevent force transfer?
you didn't provide anywhere near enough detail to get a real response, but i'll try, with assumptions:
assumption #1: you're just encasing a portion of one floor of a steel column, not extended the encasement through joints to create more connection stiffness than you intend.
assumption #2: these columns are not part of the lfrs.
assumption #3: your steel gravity column would work alone to resist the axial load without help from the concrete column.
if these are the case, then i've done this lots of times without worrying about the concrete failing. i usually put a #5 in each corner and then #3 u-shaped ties at about 24" and no studs connecting the steel and concrete. no dowels into the footing. if it's a gravity column, then you're only transferring axial load to the concrete and i don't see how this could hurt anything.
if you're talking about a lfrs column, it's a completely different story and you should be looking at the aisc composite column design guide. you should read that anyway because it's fun!
i hope you re-post with more details.
dbd
ucfse,
debonding agent not a bad idea - can insure that the concrete slips relative to the steel. hmmm...
dbdavis,
sorry for the lack of detail. the archs want it to run effectively from the ground (it is exterior exposure), to the underside of a canopy (oh about 3 metres or so). the column extends up through and picks up more facade - which is mainly lateral loads. it will resist lateral forces (and hence there will be lateral deflections) and is using frame action with effectively pinned foundation. there will naturally be more stiffness in the column near where the beams frame into (inside the canopy) - and i am going to investigate this further.
i will take a closer look at the calculations tonight when i get the opportunity.
i think i'm going to investigate putting some kind of a debonding agent, and making sure that minimum crack reinforcement can withstand the service that this thing will see. i'm tempted just to make the column concrete instead and reinforce it accordingly, however i've got a bunch of drawings & calcs done based on this design already... originally it was supposed to be clad with some architectural panels, but it turns out they are too costly.... ha. who would have thought that anything architectural would be expensive
i hate the look of concrete encased steel columns/posts - especially when one side of the concrete spalls off...lol.
thanks for the thoughts/discussion.
drw
i've used canopy columns like this before and often found that the extra stiffness from the concrete can be helpful. i like the idea of designing it as a composite column per the aisc dg.
good luck.
dbd

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