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Re: Differential Settlement

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Depending on your approach differential settlement can be no problem or can
cause you grief even when there is a rational basis.

For slabs on grade I don't worry much if the deflection is consistent with
that of a suspended slab or beam.  For example if they estimate a
differential settlement of 1" in 20 feet I would compare this to the
allowable deflection in a beam with a span equal to 2* 20'.  (2 * 20ft *
12"/ft)/ 1"=480.  Thus since this is greater than 360 (L/360) I would not
worry too much.  I would put in a fair amount of reinf to control crack
size and to deal with local variations.

The logic is that if you are happy with deflections of this magnitude for a
suspended structure why should you have a problem with them on a slab on
grade.

If you try to design for the forces from the imposed displacements you will
end up chasing your tail.  You are probably better off designing the
foundation to carry the loads on the structure over the depressed area.

An other rational approach includes getting subgrade modulas values from
the geotech and using these to analyze the mat.  To do this the correct way
you would provide the geotech with the resulting bearing pressures and he
would revise the subgrade modulas values leading to another iteration. 
This is what they do on many tall buildings built on mats.

When dealing with differential settlements two of the most important things
are working with the geotechnical engineer to provide you with likely
values and working with stake holders so that their expectations are
reasonable.

Mark Gilligan

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