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
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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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