Dear Brian,
Factored strength works fine when one is dealing with undrained loading
where the design parameter is the undrained shear strength. Here,
factoring the undrained shear strength will not alter the critical
deformation mechanism. However, if one is dealing with drained loading
(e.g. foundation on sandy soils or long-term response of a foundation on
a clayey soil), the design parameter is the critical state angle of
friction. Here, factoring the critical state angle of friction (or its
tangent?) will invariably alter the critical deformation mechanism. I
would be interested in getting some feedback on this issue from the
geotechnical community.
Regards,
Jitendra Sharma
Associate Professor of Geotechnical Engineering
University of Saskatchewan, Saskatoon, Canada
Brian Simpson wrote:
>
>
>I would suggest it is this sort of question which leads to the
>conclusion that factors of safety are more usefully applied to soil
>strength than to bearing resistance. Although it is often possible to
>work out a "best option" to for application of factors to derived
>resistances (bearing capacity, passive forces, etc) in specific
>individual cases, it is much harder to make generalisations which are
>sensible. Factors applied to strength are fairly sensible regardless of
>whether you believe their purpose is to prevent ultimate collapse or
>control serviceability. (Malcolm is right, of course, that
>consolidation settlement also must be considered.)
>
>Incidentally, most of the modern codes use the term "design" value to
>mean the factored value, not the expected or working value. I think
>your usage was different. This is all a matter of definition and
>preference, but it is certainly desirable that we all settle on the same
>definitions to avoid confusion.
>
>Regards - Brian Simpson
>________________________________________________________
>Brian Simpson
>T: +44 20 7755 3206
>www.Arup.com/Geotechnics
>
>
>
>-----Original Message-----
>From: Geotechnical Engineering Email List
>[mailto:[log in to unmask]]
>Sent: Wednesday, May 11, 2005 4:52 PM
>To: [log in to unmask]
>Subject: Use of net bearing pressure for ultimate limit state design of
>spread foundations
>
>Dear colleagues,
>
>To assist in curriculum development, I'm curious to find out whether
>there is a general preference within the geotechnical community to
>define factors of safety for spread foundations in terms of net
>pressures:
>
> FOS = (q_ult - q_0) / (q_design - q_0)
>
>or in terms of gross pressures:
>
> FOS = q_ult / q_design
>
>I'm aware that practice varies between organisations and companies.
>However, I wonder whether there is a general preference for one or the
>other.
>
>Philosophically, I think that it makes more sense to define an overall
>factor of safety as the ratio of the load / pressure / action at the
>ultimate limit state to that at the design state, rather than as the
>ratio of the increase in load / pressure / action at the ultimate limit
>state to the increase at the design state. It seems to me that using the
>net pressures leads to odd results for special cases, such as buoyant
>structures. Leaving aside the question of partial factors (as in, for
>example, Eurocode 7), I also think that the gross pressure approach is
>more consistent with what would usually be done for design of
>non-geotechnical structures in civil engineering.
>
>Yours sincerely,
>
>John D. McKinley
>------------------------------------------------------------------------
>Dr. John D. McKinley +44 (0) 28 9097 4690
>Lecturer in Environmental Engineering
>School of Civil Engineering, Queen's University Belfast
>www.prb-net.qub.ac.uk/eerg/People/Academic_staff/jmckinley/jmckinley.htm
>------------------------------------------------------------------------
>
>
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