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
------------------------------------------------------------------------
|