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Re: Modified footing design

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Phillip,

	You probably can't rely on finding a sample problem that fits this
situation; that is where your education and engineering ingenuity come
into play.

	Your solution is generally O.K. but I would modify it slightly as
follows:

1.)	I would notch the new "footing" into the caisson, or at least
roughen the surface to increase bond.  You might also use an adhesive. 
This bond plus bearing from the top or the notch must transfer all of
the added load from the caisson to the footing and hence to the soil. 
Having a few dowels drilled into the caisson will probably not provide
the necessary shear transfer.

2.)	The soil below the new footing and adjacent to the caisson may not
support loading as well as you think due to differences in consolidation
and some other effects.  You would be wise to get a good geotechnical
evaluation on the soil performance of the new combined foundation before
you have complete confidence in it.  It may be necessary to have some
new settlement of the caisson in order to "activate" the collar footing
you are adding.  This would be related to "compatibility of
displacements" effects, not unlike shored vs. unshored composite
construction; it may not follow Hook's Law to the letter of the law but
the effects are not dissimilar.

	Sorry I can't give you any more time; but I have a "fire" to put out.

				Regards,

				H. Daryl Richardson

Phillip Rogers wrote:
> 
> I am a young engineer interested in learning more
> about foundation engineering.  I  understand spread
> footing design.  My problem is finding the method of
> determining the adequacy of a modified footing.
> Specifically:
> 
> There is an existing 4'X 13.5' caisson supporting a
> tower structure.
> The problem lies in the need to modify this footing to
> make up for the tower's extension and additional load.
> The proposed method is to place an 8' square, 2' thick
> collar footing around the caisson.  This addition is
> held on to the caisson by steel bars drilled into
> place.  This addition is 1.5' below grade and 3.5' to
> the base.  The addition is reinforced with #11 bars
> top and bottom (3 on the outer part of all sides of
> the caisson and 6 U-Bars in the middle sections on all
> sides).
> 
> How does one go about checking the reinforcment and
> dimensional adequacy for such a modification?  What
> kind of calculations must be done that differ from
> that of a spread footing design?
> 
> I understand the need for such a design.  But where
> can I find the supporting calculation methodology for
> such a case?
> 
> thanks
> 
> 
> 
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