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
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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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