CSA A23.3 Spiral tie spacing requirements for piles

[canwesteng]

Is anyone aware of a way to weasel out of the maximum spiral pitch in CSA A23.3? Specifically regarding clause 7.6.4.3 (7.6.4.4 in 2004), giving the max clear of 3". Contractors often want to use spirals for piles (understandable as they hold their shape well), but usually just suggest swapping tie spacing with spiral pitch. I'm thinking there must be a way to weasel out of this, otherwise why are yards even set up to make spirals at an 8"/10"/12" pitch?

 

[jdgengineer]

In the ACI there is an exception if it is considered a circular tie rather than a spiral.

 

[jayrod12]

Do you need the compression reinforcement in your piles or only the tension?

If only the tension capacity of the vertical reinforcement, then the spiral is only there as a placeholder and I don't think it needs to meet the confinement requirements.

 

[dik]

Use circular ties @ 3' o/c with 3 ties at 12" o/c top and vertical bars 19'8" long (not full length)... a lot cheaper than spirals... or a single 15M or 20M bar...

Dik

 

[hokie66]

Does anyone know the reasoning behind the provision which canwesteng cited? I am not familiar with Canadian codes, but find that a strange requirement. I have seen specifications which provide that the minimum spacing of spirals is 3", but not the maximum.

By any chance is this a spirally reinforced column requirement rather than for bored piles? Bored piles are normally lightly loaded, as their size is dependent on the material they bear on at the bottom. So the reinforcement is typically light, and as Dik suggested, often not full length.

canwesteng, as the normal wire/bar used for spirals is relatively flexible, the spirals fold together, so you determine the pitch when you tie them. All the fabricator needs to know is the diameter and the number of turns you want.

 

[dik]

If you want to use a spiral, then use a small chunk every 3 feet.

Dik

 

[hokie66]

I have no idea what that means, Dik.

 

[dik]

Use 2 or 3 turns of a spiral and place them at 3' o/c

Dik

 

[hokie66]

OK, now I know what you mean, and it is silly. Where did the 3' come from?

 

[jdgengineer]

I could be wrong,but my understanding is the tight spacing for the spiral is to provide better confinement for the concrete core and therefore you get the benefit of a higher phi factor (and more ductility) for compression loads.

However, using a continuous helical as tie reinforcing does not require that you design this as a spiral. ACI code (I'm assuming CSA is similar) allows you to use a larger pitch and classify the reinforcement as a circular tie rather than a spiral. (R7.10.5 ACI 318-11)

 

[hokie66]

jd,

Is that for columns? I tried to make the distinction above for the difference between columns and bored piles. Unless it has been changed, ACI318 does not apply to bored piles, unless in a high seismic area.

 

[jdgengineer]

Yes that is for columns and I'm assuming that is the intent of the CSA reference as well. (Although someone familiar with the code should confirm)
I agree, does not need to apply to drilled piers.

 

[canwesteng]

CSA states the spiral pitch requirement under "Ties for Compression Members", not explicitly for columns. I'm not aware of clause stating that spirals can have the same spacing as ties if they are designed as circular ties, but maybe I can use the ACI clause as justification if there is some explanation in their commentary. The CSA commentary on spirals is awfully light.

As an aside, if you are working in Canada, CSA A23.3 does apply to piles. It has some onerous requirements there as well, but that's off topic.

 

[jayrod12]

As I said, do you need the compression reinforcement? If not then the spirals, or circular ties are strictly to help hold the vertical rebar in place. this in my mind absolves you of having to meet the code requirements for spacing and pitch.

 

[dik]

Hokie... it's a spacing I've used for nearly 50 years... wasn't a textbook at the time that said otherwise... spacing that still allows some support for the vertical bars.

The tight spiral spacing is for confinement with colunms, not friction piles. I've designed friction piles where the top is unsupported by soil and have used regular column tie spacing.

Dik

 

[dik]

jdg... you are correct re CSA A23...

Dik

 

[canwesteng]

The solution then is that I should design piles as plain concrete for compression/shear, to avoid this issue. I do like the suggestion of extra spirals to hold the cage though.

 

[hokie66]

dik,
canwesteng's question was about bored piles, not friction piles. Friction is sometimes used in the design of bored piles, but their best use is for end bearing on rock, and for applications where a lot of moment capacity is required.

As to your 50 years experience, I consider that one year, repeated 50 times. Just because you and I are old doesn't mean what we have done is correct.

 

[canwesteng]

FWIW, these piles are end bearing, but not on rock. Drilled piles relying on friction are actually quite common around my neck of the woods though. For lighter loads screw piles are used, rarely do we see driven piles.

 

[dik]

Hokie:
Bored piles in these environs are friction piles; we have 50' of good clay... as for 50 years, I would suggest they have been a good 50 years, not 50x1 year... I've done just about everything except for bridges and nothing fallen down yet. Must be doing something reasonably OK and have done a lot of stuff before it was ever included in building codes... Doesn't matter much about friction or end bearing, the reinforcing stays the same and concrete is treated as plain concrete for bearing...

Dik