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RE: Pile Interconnection (CBC & IBC)

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Paul, Mark and Roy:


Thanks for the comprehensive comments.

The "lateral resisting system" that the designer has used is basically "cantilevered concrete columns".

Essentially these columns are the piles that are projecting 15 feet above the ground and support the a good size swimming pool and deck above.

Since the property is sloped I would be inclined to introduce grade beams at the ground level.

(And yes there's a 4 inches slab on grade at the ground level.)


Casey (Khashayar) Hemmatyar, PE


From: Paul Feather [mailto: pfeather(--nospam--at)]
Sent: Friday, April 14, 2006 6:25 PM
To: seaint(--nospam--at)
Subject: Re: Pile Interconnection (CBC & IBC)


The purpose of the pile cap or caisson interconnection is to limit differential movement at the ground level.  A typical pile cap system usually consists of a hinge point, one column transferring to multiple piles.  The system you refer to is more like a cantilevered column system under 1807.5.  If the beams at the second level are to be considered as providing the necessary restraint, than the structure and slab above need to be capable of resisting the additional applied moments from a lateral displacement of the pile at the soil, as well as any companion forces from relative settlement during displacement.  Also, the piles need to be designed as free standing columns for the clear height plus point of fixity, vertical and lateral.  The construction you describe is more like a wharf than a typical pile system.


At a minimum, since it is single family, do they at least have a slab at grade level that captures all the pile bases?  If there isn't a slab, I would strongly recommend the potential for individual displacement be accounted for.  I do not believe the pile cap and caisson requirement of 1807.2 applies, but I also think a minimum amount of restraint is good practice.


The same is true for pad footings separate from the slab envelope.  I have seen severe slab distress where an outlying column was not tied back to the rest of the structure in more than one instance.  Soil migrates, and without tie beams or a restraining membrane like a slab, the movement induced moments can be considerable.  It may take 20 years, but it is the difference between good design and poor design.



Paul Feather PE, SE