ACI 318 - Spacing of longitudinal bars in tied column

You only have to have 4 vertical bars in a square or rectangular column.
Bars in corners of columns must have ties around them.
If any bar is more than 6" from an adjacent longitudinal bar, it must be in a corner of a tie.
Across a face with longitudinal bars spaced closer than 6 inches, every other bar (alternating bars) must be in the corner of a tie.

The purpose is to restrain bars against buckling in compression (as well as helping confine laps in tension.)

So if I understand right, only 4 vertical bars are all that's needed in a square or rectangular column. BUT if there are any additional vertical bars to the corner vertical bars: (1) if the bar is more than 6" from an adjacent longitudinal bar, it must be located in the corner of a tie, (2) OR if the bars are spaced closer than 6", every other bar needs to be in the corner of a tie.

Am I understanding you right? So it sounds like the easier design/situation would be to have bigger vertical bars so you only have to deal with 4 corner bars and one set of lateral ties. Thanks.

Upon re-reading ACI, where do you gather that you can space the bars further apart than 6" if you locate it in the corner of a tie? Based on how the code reads, it doesn't appear like it ever allows them to be spaced along the column face at greater than 6".

So if I understand right, only 4 vertical bars are all that's needed in a square or rectangular column. BUT if there are any additional vertical bars to the corner vertical bars: (1) if the bar is more than 6" from an adjacent longitudinal bar, it must be located in the corner of a tie, (2) OR if the bars are spaced closer than 6", every other bar needs to be in the corner of a tie.

Am I understanding you right?

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Yes.

Upon re-reading ACI, where do you gather that you can space the bars further apart than 6" if you locate it in the corner of a tie?

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See Figures 13 & 14, on pages 35 & 36 of ACI Detailing Manual 2004 (SP66).

A good structural engineer is often a blessing for others.

LearnerN said:

Am I understanding you right? So it sounds like the easier design/situation would be to have bigger vertical bars so you only have to deal with 4 corner bars and one set of lateral ties.

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It is. And, often, I'll do that. However, the simpler tie arrangement needs to be balanced against:

1) Easy of handling the rebar. I usually stay below 30M for heavy commercial work and below 20M for residential.

2) Sometimes joint detailing favors the use of smaller bars which can be developed/anchored over shorter distances.

An interesting cheat that I've seen for eight bar columns is to have the middle bars shifted off center such that they are only 6" from the corner bars.



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.

KootK,
I think that would indeed be cheating, and incorrect. The intent is that the 6" is measured in both directions.

LearnerN,
Read TXStructural's post again. He clearly answered your question.

hokie66 said:

KootK, I think that would indeed be cheating, and incorrect.

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You'll get no argument from me.

I've always wondered about something related. See the sketch below. My understanding of why alternate bars don't require direct restraint is that they derived indirect restraint from the flexural stiffness of the tie legs spanning between neighboring bars. See case B below. Now check out case C. There, it would seem that the presence of the intermediate tie between vertical bars A & B does nothing to help restrain them.

I suspect that the answer is probably just "we tested it and it works". However, I rarely get any intellectual satisfaction from those kinds of answers. I think that all testing should culminate in a physical understanding of some sort.

Anybody got any insights to share? Is it simply that one unrestrained bar every six inches doesn't generate enough instability to generate cover spalling?



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.

I suspect that it is the concrete itself rather than the ties in bending which restrains those bars. How the rules were derived, I don't know.