Vertical Spacing of Column Ties
Vertical Spacing of Column Ties
(OP)
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?





RE: Vertical Spacing of Column Ties
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?
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
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: "required percentage of reinforcement...shall be based on that circular section".
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.
RE: Vertical Spacing of Column Ties
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.
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
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.
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
RE: Vertical Spacing of Column Ties
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.
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.
RE: Vertical Spacing of Column Ties
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.
These articles can be ordered on the ACI website.
RE: Vertical Spacing of Column Ties