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RE: 1 or 2 piles under a cap

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"stone clad pyramidal piers" - I have never used something like this.

Helical pier installation, in my experience, is actually quite easy, and
even more so in a strictly vertical installation, and it is very accurate.
If the pier is 3-4" off center, then it's time to get another pier
installer.  I've used helical piers in 8" wide grade beams, cast within the
grade beam using the "new construction load transfer bracket" (a chunk of
tube steel with a couple #5 rebar sticking up, kinda like rabbit ears).

This bracket was tested using a 3,000 psi concrete (cylandars testes at 7,
14, and 28 broke at 3250, 4090, and 4780 respecitvely) to a total load of
106K.  The bracket was HSS2x2x1/4 by 6" with a top plate of 2" by 4" by 1/2"
and two (2) #5 rebar by 12" long welded to the top plate and HSS.  This was
cast in a 10" sonotube and sent to compression testing.  The unreinforced
concrete failed under compression testing, and it sheared from all
imbedments.  In short, the concrete failed, the bracket did not.

If you build a design capacity 50K pier (100K ulitimate), and it is strictly
vertical load, then you could do a 12" by 18" to 24" sonotube pier cap at
the location of the load to react it specifically.  Use other, smaller,
piers to cover holding the weight of the precast concrete deck and any
lateral loads you may have.

I've used piers for 25K and 50K capacity for foundation remediation and new
foundation construction, as well as isolated conditions that you are
experiencing.  The installation is quite accurate, and again, if you have a
contractor that misses the target by 3" to 4", they shouldn't be installing
piers to begin with.  If they don't meet location requirements, have them
take the piers out and try again (at the contractor's dime of course).
After a couple of zinged piers, he's going to get the idea that he is either
going to have to do it right, or go broke doing it "they way they always
have."  *laugh*

Does this help at all?

Dave Maynard
Gillette, WY


-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint2(--nospam--at)truesdellengineering.com] 
Sent: Wednesday, June 11, 2008 7:31 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: 1 or 2 piles under a cap

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.

Jordan



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)truesdellengineering.com] 
> Sent: Wednesday, June 11, 2008 9:28 AM
> To: seaint(--nospam--at)seaint.org
> 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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