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Re: Factor of Safety against Uplift

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	It's interesting that you should ask about the use of dual soil
pressures (factored & working).

	I raised that question waaay back about 1970, which in our office of
the time was in the year 15 B.C. (before computers, that is).  We were
using ultimate strength analysis for concrete building design; having
allowable bearing pressures for foundation loading required separate
analyses for structural design and foundation design.  We (my boss and
I) had a nice lunch with two Ph.D.s (who, as I recall, even paid for the
lunch) who were the top geotechnical people in Calgary at the time and
we presented our case for the desirability of having "factored" soil
pressures and the amount of design time we might be able to save.

	The eventual reply that the geotechnical people made, after due
consideration, was that allowable soil pressures were almost always
determined by magnitude settlements, magnitude of differential
settlements, or by rates of settlements.  The actual structural failure
strength of soil, (they explained,) was almost always at least four
times higher than allowable pressures based on settlement
considerations, hence, knowledge of the actual failure conditions of the
soil would almost never be of any use to us whatever since we were using
load factors of 1.4 and 1.7 AND the knowledge would be fairly costly to

	It's been a long time since 1970; computing techniques have changed
quite a bit; and tolerance of differential settlements may have changed,
particularly for steel structures, partially as a result of improved
calculation techniques.  Never-the-less, my engineering intuition tells
me that it is unlikely that the use of factored soil pressures will
become commonplace. (Unless, possibly, the "factor" is some multiple of
the present allowable pressure rather than the actual soil failure
stress.)  But, hey, I've been wrong before.

	I'll be interested in any other people's comments on this subject.


				H. Daryl Richardson

Paul Martin wrote:
> John,
> I am sure you realize that allowable stress design had been used for many
> years, so your question about how can one possibly design reinforcing in the
> toe of a retaining wall footing can be answered by looking in a text book.
> Do you view the recommended soil pressure for design as the limit for design
> based on factored or unfactored loads?
> When factored loads are used to determine soil pressure, it seems that there
> should be a limiting value ... a value higher than that for serviceability.
> For this to work, that value is going to have to be more than 1.5 times
> greater. I have not seen such a value, but I haven't requested a limiting
> value from a geotech engineer either.
> I am jumping in here because there is confusion among new grads, and some
> not so new on how to merge "factored" loading and "allowable" soil bearing
> pressure in the design process.
> Does anyone know if two recommended soil pressures are coming in to use?
> Then there is the "net allowable soil pressure" discussion ...
> Paul Martin
> Wichita, KS

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