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Termination of Circular Ties

Posted Date: 18 Dec 2014

Circular ties are used in a number of applications, with a common one being concrete piles.  Depending on the seismic design category assigned to a structure, the reinforcement in the concrete pile is subject to certain detailing requirements.  Here is a question that we received on a very specific detailing requirement as it relates to the termination of circular ties and our answer.

Q: ACI 318-11 Section 7.10.5.4 is clear on its requirement for a minimum 6-inch lap and termination in standard hooks in circular ties as illustrated in Figure 1.  On the other hand, 2012 IBC Section 1810.3.2.1.1 requires only that the ends of hoops, spirals and ties used in concrete deep foundation elements in structures assigned to SDC C or higher be terminated with seismic hooks, as defined in ACI 318.

2012 IBC:

1810.3.2.1.1 Seismic hooks. For structures assigned to Seismic Design Category C, D, E or F, the ends of hoops, spirals and ties used in concrete deep foundation elements shall be terminated with seismic hooks, as defined in ACI 318, and shall be turned into the confined concrete core.

ACI 318-11:

2.2 — Definitions

Seismic hook — A hook on a stirrup, or crosstie having a bend not less than 135 degrees, except that circular hoops shall have a bend not less than 90 degrees. Hooks shall have a 6db (but not less than 3 in.) extension that engages the longitudinal reinforcement and projects into the interior of the stirrup or hoop. See 7.1.4 and Chapter 21.

We have two possible interpretations of the above requirement:

A) My interpretation is that for circular ties, the minimum 6-inch lap must remain, but the termination in a “standard hook” should become a “seismic hook.”

B) Another engineer’s interpretation is that the minimum 6-inch lap would NOT be required and that only the “seismic hook” is required – terminating on the same longitudinal bar (since there would be no lap).

Can you advise whether interpretation A or B above is correct?

A:  Interpretation A is correct.

Two things are important here. First, please look at the definition of a “seismic hook” in ACI 318-11 Section 2.2. Please note “…, except that circular hoops shall have a bend not less than 90 degrees.” No 135-degree bends are involved here. Second, the detailing rules in ACI 318-11 Section 7.10.5.4 would still apply, because they have not been superseded by the seismic requirements of ACI 318-11 Chapter 21. In general, all the provisions in ACI 318 Chapters 1 through 18 remain applicable when you are detailing by ACI 318 Chapter 21, except where a provision has specifically been superseded by Chapter 21. Section 21.7.5, for instance, has superseded Section 12.5, if seismic detailing needs to be special. Note that 2012 IBC Section 1810.3.2.1.1 addresses structures assigned to SDC C, D, E, or F only, which must be detailed by ACI 318 Chapter 21.

In summary, the detailing required by 2012 IBC Section 1810.3.2.1.1 would look exactly like Figure 1 if you are using circular hoops, except that there is now a 3-in. minimum on the 6db extension, because the hook must now be a seismic hook, rather than a standard hook. The 3-in. minimum obviously makes a difference only for #3 bars.