Vertical Spacing of Column Ties

[artnkman]

I have a semi-circle column, 14" in diameter supporting a balcony slab. The equivalent circular compression member for this column shape is adequate to carry the design load. ACI 318 (7.10.5.2) states that the vertical spacing of the ties should not exceed the least dimension of the compression member, i.e., 7" for this matter. If the builder spaced the ties at 8", should I be concerned?

 

[jimaitken]

I would determine the equivalent rectangular area of the column and then determine the least dimension assuming the maximum to be 14". This would give you a least dimension of around 5.5" (assuming column is exactly 1/2 circle). Given that the contractor used 8" spacing I would say the the code requirement is not being met.

Does this column tie into a load bearing wall that you can use to contribute to the least dimension or increase the equivalent area of the member?

 

[civilperson]

My arithmetic gets 77 sq inches as area of column; thus a equivalent square column would be 8.8 inches on a side. Would accept the spacing as suitable and be more specific in call outs in future plans.

 

[JAE]

I can get a square column of 6 1/4" each face within the 14" dia. semi-circle. That's a little better than jimaitken's 5 1/2" but doesn't help you in that regard.

The question is, does section 7.10.5.2 stand alone, or do tie spacings get affected by the maneuver used in 10.8.3?

10.8.3 doesn't say anything specifically about ties (nor does the commentary), but it does use the phrase: [green]"required percentage of reinforcement...shall be based on that circular section".[/green]

So with that, it appears that the logical interpretation of the text forces you into 7" max. spacing.

Perhaps you could contact ACI with this question.

 

[JAE]

One other thought - if, with your smaller 7" dia. circle, you have excess strength - i.e. the unity is a good deal less than 1.0, then perhaps you could use a strength reduction factor on the column to account for the longer tie spacings - 7"/8" = 0.875 factor on the axial load capacity.

I don't know what the relationship between tie spacing and axial capacity is (whether its linear or other) so the 0.875 factor idea may not be truly accurate.

Also, this brings up another question - does 10.11.5 apply here (is your kL/r > 100 and thus dumps you into 10.10.1 for design?

Lastly, I believe that PCA's column design program (used to be called IRRCOL) can do irregular shapes without resorting to 10.8.3.

 

[JAE]

civilperson - ACI doesn't allow you to simply take an equivalent square column with the same area as a substitute for the irregular shape. Read in ACI - 10.8.3.

 

[artnkman]

10.8.3 would give me an equivalent 7" diameter column for the half section that I have. I agree that the builder didn't comply with the code of 7" max tie spacing but the commentary R7.10.5 also mentions that limited tests were done on full-size, axially-loaded, tied columns containing full-length bars (w/o splices) showed no appreciable difference between ultimate strengths of columns with full tie requirements and no ties at all. For a 12-ft high column, I may have missed 3 ties over the full height of the column... should I be concerned structurally?

 

[UcfSE]

How much capacity are you using? If you aren't anywhere close to using or needing full capacity, it might not be a problem to be missing a few ties. That will also be subject to how much ductility your column needs. Technically, it does not meet code so it is up to your judgment as to whether to accept it or not. I also agree with JAE's post on 24 May 06 13:41, meaning your equivalent section should be used to determine tie spacing.

Another thing you might look at is whether the column works without the contribution of the strength of the steel. Lateral ties serve to prevent the longitudinal bars from buckling as well as provide confinement for the concrete core. If your column works as plain concrete perhaps that is a way to justify too large of a tie spacing. I would check the strain and stress in the longitudinal bars as well just to make sure they wouldn’t buckle with the strain in the concrete. Even if you don't need the steel strength you don't want the bars to buckle and damage the column.

 

[JAE]

artnkman - what is this column supporting? It seems quite small for a building column.

 

[artnkman]

JAE -- There is one pair of this half-circle column at the ground floor that supports 2 upper small balcony floors (2nd & 3rd); one column at each end of the balcony.

 

[artnkman]

JAE -- I forgot to mention that the upper floor balcony columns are complete 14" dia. circular columns. For some reason, the architect wanted the half circle columns at the ground floor.

 

[JAE]

I guess I just ask because I traditionally have never tried to design columns less than 10" square. Just seems like we'd be at the limit of actual tests and the potential for rebar to be placed wrong would have much greater effect on the strength for such small sections...i.e. the [Φ] factor wouldn't necessarily cover you at those small sizes.

 

[artnkman]

JAE -- I'd appreciate your view of the commentary R7.10.5 regarding the limited tests done on tied columns as well as columns without ties.

 

[JAE]

artnkman - Here is the abstract of reference 7.15 that is referred to in the commentary:

[red]Title: Influence of Ties on the Behavior of Reinforced Concrete Columns
Author(s): James F. Pfister
Publication: Journal Proceedings
Volume: 61
Issue: 5
Pages: 521-538
Date: May 1, 1964


Abstract:

To aid development of the 1963 ACI Building Code, 11 rectangular tied columns were tested under concentric load to explore the influence of arrangement and spacing of lateral ties on the strength and behavior of tied columns. In three of the columns, full ties were provided as required by the 1956 Code, and in another three columns only exterior ties were used. Two columns had ties only at the ends and at midheight of the columns, and three columns were provided with ties only at their ends. It was found that the primary function of the ties was to restrain the concrete laterally so that it could develop its full strength in a gradual type of compression failure.

Exterior ties surrounding the longitudinal reinforcement were found to be as effective as combined interior and exterior ties conforming to the 1956 Code. It is concluded that the new tie requirements of the 1963 Code should be entirely adequate.[/red]

So this seems to be referring to the need for interior ties vs. no interior ties, not ties in general. Also, the first sentence of the commentary states, "All longitudinal bars in compression should be enclosed within lateral ties".

I don't see that you can justify going to 8" instead of 7" based on the commentary. But I do think that if the columns are over-designed a bit, that the longitudinal bars are not stressed too high under your design loads, then the few ties "missing" shouldn't be that critical.

 

[JAE]

Here's another one:

[blue]Title: Effect of Tie Spacing on Inelastic Buckling of Reinforcing Bars
Author(s): S. T. Mau
Publication: Structural Journal
Volume: 87
Issue: 6
Pages: 671-677
Keywords: buckling; columns (supports); finite element method; loads (forces); reinforcing steels; strain hardening; stress-strain relationships; ties (reinforcement); Structural Research
Date: November 1, 1990

Abstract:

The inelastic buckling and load-carrying capacity of reinforcing steels in concrete columns are studied by an accurate finite element method. A critical tie spacing exists for reinforcing steels under monotonic loading. The peak loads for cases with a spacing below and above the critical spacing differ greatly. Below the critical spacing, the tangent modulus theory used in previous works for tie spacing and tie stiffness determination may be applied; above the critical spacing, the theory may not be expected to predict the load-carrying capacity accurately. The critical spacing varies with the shape of the stress-strain curve of the reinforcing steel. A parametric study is carried out to establish the range of critical spacing for typical high-strength steels.[/blue]

These articles can be ordered on the ACI website.

 

[artnkman]

Thanks JAE and to the rest of you, guys. Cheers.