Helical ties

[woodman1967]

I am working on a project on the construction side of things. And have a question about helical ties. The concrete colummn is 12" in diameter and the engineer has specified No. 10 (Canada) helical ties. I have never used them and am unfamiliar with them. I would like to use No. 10 circular ties. Before I go back to the design engineer is this a legitimate question? What would we expect for a result?

Thanks for all your help

 

[youngstructural]

Firstly, one of my own pet beliefs: All questions are legitimate questions. Failures typically occur from a series of mistakes/ommissions/etc, and I have seen as well as read about numerous where the first error was in fact a question which was not asked.

I think you have a legitimate question, however the engineer may have good reason for the specification of helical ties. I'm not entirely certain I understand correctly, however, as I always think of helical ties as a wall remediation tool, where the continuous ties in a column would be called "spiral", with the single loops called "hoops".

Please correct me if I've got the wrong end of the stick, but in the mean time I'd pick up the phone if I were you.

Good luck with it,
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[BAretired]

A 12" pile is pretty small to get any kind of tie in place. How much concrete cover will you have over the ties? Maybe you should suggest 16" diameter piles instead. Then circular ties would appear to be reasonable.

BA

 

[hokie66]

I prefer spiral ties in circular columns, but in some areas, the bending equipment for spirals is not readily available. The ACI Code actually allows for higher axial capacity of spirally reinforced columns, with conditions on the spacing. I don't know if the Canadian code has a similar provision. You have to ask, as you don't know the engineer's reasons for specifying spirals.

 

[csd72]

Most engineers will treat spirals as equivalent to circular ties. There is a small reduction in shear capacity due to te inclination of the spiral but this is usually ignored.

There was an article on this in the istructe magazine a few years ago, I will see if I can locate it.

 

[BAretired]

In reading the question again, I note that you are talking about a column, not a pile. Please disregard my earlier comment.

hokie,

The Canadian code permits about 6% more axial load resistance for a spirally reinforced column than for a tied column.

BA

 

[Lion06]

Spirals are better - they provide greater confinement and ACI recognizes this with a higher phi factor for spirally reinforced columns. It's likely not a shear issue. The spirals provide much greater ductility and better ultimate strength behavior than do ties.

That being said, it can't hurt to ask the question. Spirals typically have a VERY close spacing.

 

[kelowna]

Interesting

I have had the oposite issue. Designer specifying circular ties and the rebar suplier wanting to change them to
helical ties. It was cheaper and faster to produce, although he probably had the equipment to do so.

As already said, it is a pertinent question and there are design implications.

 

[hokie66]

kelowna,

In your case, there is no reason not to accept the helical ties. As long as the spacing and size are the same, and there are 1 1/2 extra turns at the end, the helical ties are equal or better than the individual ties.

 

[kelowna]

This was sometime ago Hokie66, and yes, I did accept them. I had a chat with the designer, who was happy with the change. I found it a much neater detail