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Re: 1 or 2 piles under a cap

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These are isolated footings supporting a precast exterior deck. It's a very simple condition, actually - much like a residential deck, but with precast plank and foolishly large piers. The piers may drop to 16" sonotube when the client gets the estimate on the stone-clad, pyramidal piers. I suppose I could just encase the top of the helical pier in the concrete pier and forgo the separate pile cap, but I'm concerned about the potential for moment in the helical pier shaft if the installation isn't perfectly centered under the post above. 3-4" off with a 40+kip load would likely exceed the capacity of the shaft. Most of the contact I've had with these types of piles is either as shoring of existing buildings with light loads (maybe 10k), or using multiples under caps for capacity reasons.


David Maynard wrote:
John Pack, PE, of IMR, Inc. out of Denver, CO, has done extensive studies
with AB Chance Helical Piers.  If you do an installation torque of 10,000
ft. lbs, you could get an Ultimate Capacity of the pier to be 100 kips.
With a factor of safety of 2.0, that's a 50 kips design capacity.  If they
have the materials to build a 50 kip (design capacity, 100 kip ultimate
capacity) helical pier (heavier shaft, several helix flights, etc., then you
can use a local helical pier for this particular load and isolate it from
the rest of the foundation.

Otherwise, with the spread that you have, you would have to design the
foundation as a concrete beam between these two piers.  I might not have a
clear understanding of your problem.  If you had a diagram to share, I might
have more information.

Dave Maynard
Gillette, WY

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint2(--nospam--at)] Sent: Wednesday, June 11, 2008 9:28 AM
To: seaint(--nospam--at)
Subject: 1 or 2 piles under a cap

Does anyone have a rule of thumb for using less than 3 piles (helicals, in my case) under a pile cap? I've got a condition where I have some lightly loaded pile caps - about 45k - but their supporting pyramidal piers which have a 40" square base. The tops of the piers will support a beam line and concrete plank, so there's no lateral loads. The piers are 16-20' apart and are in a (roughly) straight line, so I can't efficiently combine multiple piers under a single footing.

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