Use spirals as ties in piers? 1

[Kramer]

I have designed a large residence in California which is supported by 16 inch diameter drilled piers (caissons). I have specified #3 ties with a spacing of 6" o/c for the full length of the piers. The contractor is proposing to use a continuous spiral instead of individual ties. The code (UBC 1997) specifies that the maximum clear spacing between spirals is 3". I believe this is because when you use spirals, you are counting on the additional vertical load capacity provided by the confinement of the spirals. In my case, the vertical loads are very low (compared to the capacity) and I only need to provide shear reinforcing. Does the 3" minimum still apply?

 

[JAE]

Is the pier totally in the ground? Most drilled piers I've designed have used spirals at 12" pitch. The 3" pitch typically is found in specs for above-grade columns, not buried drilled piers.

But in California, perhaps the UBC has a tighter restriction in high seismic zones.

 

[youngstructural]

I would guess that the intent is to ensure ductility, which would again be more a concern in a column rather than your drilled piers. At the same time, I would prefer to stick with the code requirements all the same. Your design works, and it's the contractor who wants to make a change. I like to follow Rick's advise in these situations "I stick my neck out for nobody" [Casablanca, 1942]. It's quite common for a contractor to ask to make a change, and in my opinion these changes, particularly when they require calculations to make work, are a source of a great deal of waste of time and money. If you can built it as drawn, and the change is being requested for the contractor's convenience (or whim!), then I prefer to stick precisely to the code requirements.

Don't mis-understand me, contractors and their criticism of designs are essential, and desirable! I love hearing feed-back and always pay close attention to concerns: Helps me become a better engineer. But I do not breach code clause for contractor convenience.

Good luck,

YS

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

 

[shin25]

I do work with lot of driven concrete cylinder piles and square piles. As ties we always prescribe spirals, the standard spacing of spirals is 6" at the center of the length but is smaller at the top and bottom. This is because to handle the driving stresses. You may want to check out PCI design hand book for similar recommendation.

 

[miecz]

For what it's worth, AASHTO Standard Specs (4.6.6.2 and 8.18.2.2) call for a 3 inch pitch for drilled piers.

 

[Kramer]

Thanks for all your comments. I found that in the commentary portion of ACI 2005, section 7.10.5.4, it states that "continuouly wound bars or wires can be used as ties .....". I read this to mean that even though it looks like a "spiral", it isn't, and the 3" maximum spacing doesn't apply.

 

[civilperson]

If more than the 3" pitch for spirals is used, then the spirals fall into the class of normal ties. Still OK for column design.

 

[civilperson]

If the maximum spacing is greater than 3", then the spirals become classified as ties. 6" pitch would be equivalent to 6" spacing if the ends are bent around a vertical bar or addition turn is used at ends.