huangyhg

residential concrete column
i am currently working with an architect on his new residence. he is using 16" dia sonotube formed concrete columns to support a porch. they are only carrying 5.5k of load and are 6'-8" above grade. the size is purely for aesthetics. do i need to follow aci requirements for reinforcing of these columns or is there an exemption for such lightly loaded elements? aci requires (5)-#6 with #4 ties at 3" o.c. max. seems a bit much.
thanks.

have not designed in ages, haven't a clue what klu/r means anymore but i bet you could get away with ties consisting of no.3 size w/spacing of 1@ between 3" to 6 " e.e bal @ 12 in o.c enclosing the 5 no.6 verts. the forum gurus will confirm so do not take my word for it.
the area of the 5 no.6 verts/ag is about 0.01 so i don't know if you can drop it any further. aci 318 sec 10.9.1
out of interest, what aci sections did you use to come up with size and spacing of lateral reinforcement/ties?

where did you get that requirement for ties? that seems pretty excessive. is it because you are in a seismicly active area?
i would go with the 1% minimum myself, but that's me. it seems like #3 ties at 12" should be ok. you do have the option of arranging the reinforcing in a rectangular cage if you want. that's usually cheaper than cicular ties and vertical bars in a circular pattern.
i believe there is an aci provision that allows you to go down to .005 for longitundinal steel for columns that are larger than needed for asthetic reasons. do not use less than 4 longitudinal bars. i would think that #3 at 12" oc are adequate ties for #6 longitundinal bars.
see aci 318-02 section 10.8.4. this allows you to effectively go down to .005*ag for the column if it is larger than required with a minimum of four bars. this is done quite often in my area for lightly loaded columns. based on aci 318-02 section 7.10.5 it looks like #3 @ 12" is what you need, based on no shear.
we usually like to add a second, additional tie at the top, located 3" below the very top tie, just to add some extra confinement where any applied moment or twisting might occur.
we also do what jae recommends.
why not try to design it as plain concrete and just place one bar in the middle for integrity?
axial stress is less than 30 psi on the gross area. the height is only 6'-8" (if i understand correctly) what is all the reinforcing going to do other than insure that is won't break up when it falls over? even a single bar might provide that.
connections at the top and bottom of the column would seem to be far more critical than a 4 bar cage in the thing. maybe 4 dowels out of the foundation would provide some fixity at the base to promote safety but above that i'd expect very little bending stress and likely well within the capacity of plain concrete. also, the code allows reinforcement 33% greater than that needed by analysis.
i haven't look at aci for plain concrete in a while but i would expect that there are some provisions there for such a case.
houseguy, check the last paragraph of aci 318 sec 22.2.2. it states "the use of structural plain concrete columns shall not be permitted". chapter 22 permits use of structural plain concrete for pedestals ref: 22.2.2 (c) and 22.8
thank you all for your helpful comments. i do not typically design with concrete, with the exception of foundations, so you're insights are much appreciated. the project is currently on hold (go figure), so i will deal with it on another day!

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