Dear John et al,

It is an interesting question.  I think the main point is that the
engineer should think what the issues are, and not just blindly follow a
single approach or set of rules.  If you think about why you are doing
something, it will become apparent whether one approach or the other is
right in a particular situation, or perhaps that it really makes no
difference.  As far as factors of safety on bearing capacity are
concerned, considering that the high factors usually used are really
about controlling settlements, provided that a proper assessment of
settlement is attempted, it really makes no difference.  Otherwise, one
should follow the procedure for which the conventional factors of safety
have been developed through experience.  I think this may be Malcolm's
point, too.

Paul McCombie
Director of Studies for Civil Engineering
Department of Architecture and Civil Engineering
University of Bath
United Kingdom

Parminder Singh wrote:

>Dear Dr McKinely,
>
>Off late (after the tsunami), there are Qs on the inter-dependence of
>geotechnical and structural designs. The performance criteria of the
>overall system including superstructure and foundation such as in
>earthquakes often requires a complicated understanding of the two
>responses. In general it is difficult to determine whether a stiffer
>of softer foundation would result in greater displacements and there
>are some cases where the relative stiffness of different foundation
>components exist within the structure. Applying different safety
>approaches could lead to uneconomical designs and possibly fatal
>ones also, hence, I agree there should be some overall consistency
>in the approaches.
>
>Best regards,
>
>Parminder Singh
>
>-----Original Message-----
>From: Geotechnical Engineering Email List
>[mailto:[log in to unmask]]On Behalf Of John McKinley
>Sent: Wednesday, May 11, 2005 11:52 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
>------------------------------------------------------------------------
>
>