#4 Tie 2

[Althalus]

I have a 24" diameter drilled pier. It is 6" to 1' above grade and 3' below grade. I'm using #6 verticals and #4 circular ties to confine it.

Question:

Do the #4 ties require the same lap calculation as a straight bar? Is it considered confined because it is circular? Or is it unconfined because it *IS* the confining element? OR???

What is the lap requirement for it?

 

[JAE]

See attached- from CRSI

 

[Althalus]

OK, that seems simple enough. But I need to reconcile what I've seen many times. The development for my case is 19". Yet I've seen multiple details approved by engineers more experienced than I with lap lengths 12" to 15" for the same situation. What am I missing?

 

[slickdeals]

I had this issue on a previous project and I requested the Deep Foundation Institute for answers. Sadly, no replies.

http://www.dfi.org/qaforum.asp?class=ACIP&sid=10447

 

[hokie66]

Never used lapped circular ties. Circular columns normally have 135 degree hooks on alternate bars, and bored piles use spirals.

 

[Althalus]

WOW! I just discovered the 2011 ACI:

7.10.5.4 — Where longitudinal bars are located
around the perimeter of a circle, a complete circular tie
shall be permitted. The ends of the circular tie shall
overlap by not less than 6 in. and terminate with
standard hooks that engage a longitudinal column bar.
Overlaps at ends of adjacent circular ties shall be
staggered around the perimeter enclosing the longitudinal
bars.

In other words the simple lap splice is no longer allowed. (Kudos to Hokie). I read an article earlier that it was only for seismic areas. But this ACI excerpt indicates no such exception. The reasoning was that too many circular piles and piers were spalling on the outer layer. This reduced the required cover to the point where the laps no longer provided any tension capacity.

So, hooks it is.

@Slick,

This may be the reason why your contractor thought 6".

 

[cliff234]

What a great resource this website is. It's like a virtual water cooler for structural engineers. I was unaware of this change. I have used lapped circular ties for 30 years and have never had a problem or heard of a problem. This change makes me want to use four bar patterns (where possible) in round columns that are less than 24" in diameter with standard square ties - or use spiral ties. The round tie with hooks seems complicated.

I have never used spiral ties. How are we supposed to terminate the end of a spiral tie? With a hook?

 

[TXStructural]

Spirals are "tied off" at the ends with an extra 1-1/2 wraps. The change to ACI for circular hoops was because of research and field observations which found a loss of confinement once cover spalls. Short laps create a strong splitting force in the plane of the laps, which is why we see fewer and fewer places where unconfined laps are recommended.

 

[TXStructural]

As to the original post:
"I have a 24" diameter drilled pier. It is 6" to 1' above grade and 3' below grade. I'm using #6 verticals and #4 circular ties to confine it."
This seems like a very shallow foundation, unless it is into rock. Also, the longitudinal bars will not really develop along the length of the pier. Will the bars extend into an element at the top of the pier?

 

[hokie66]

I suspect the main reason for spalling of columns with lapped circular ties may have been inadequate cover. Too many loose ends flapping around in the form. If that method has been prohibited, good riddance.

 

[Althalus]

@TXStructural,

It is a shallow foundation. It doesn't need to be any deeper. Very little load. It would be shallower if the soil allowed it.

#6 has a development length of 29". We're at least that height.

There is no true column or beam action. To me it is a reinforced pedestal. But we have a standard detail that needs to cover instances where the T.O.C. will be a foot or two above grade.