61
Honour Lectures /
Conférences honorifiques
10 CONCLUSIONS
(a) Current seismic design practice leads most often to very
conservative foundation solutions. Not only are such
foundations un-economical but are sometimes difficult to
implement. Most significantly : they are agents of transmitting
large accelerations up to the superstructure. The ensuing large
inertial forces send back in “return” large overturning moments
(and shear forces) onto the foundation
a vicious circle.
(b) On the contrary, seriously under-designed foundations limit
the transmitted accelerations to levels proportional to their
(small) ultimate moment capacity. This leads to much safer
superstructures. In earthquake engineering terminology the
plastic “hinging” moves from the columns to the foundation-
soil system, preventing dangerous structural damage.
(c) For tall-slender systems that respond seismically mainly in
rocking, underdesigning the footings “invites” strong uplifting
and mobilization of bearing capacity failure mechanisms. It
turns out that the statically determined ultimate moment
resistance is retained without degradation during cyclic loading,
at least for the few numbers of cycles of most events
hence
the geotechnical reliability in such a design. Moreover, the
cyclic response of such foundations reveals that the amount of
damping (due to soil inelasticity and uplifting
−
retouching
impacts) is appreciable, if not large, while the system has a fair
re-centering capability. These are some of the secrets of their
excellent performance.
(d) The key variable in controlling the magnitude of uplifting
versus the extent of bearing
−
capacity yielding is the static factor
of safety F
S
against vertical bearing
−
capacity failure. The
designer may for example, choose to intervene in the subsoil to
increase F
S
and hence enhance uplifting over soil inelasticity.
Such intervention need only be of small vertical extent, thanks
to the shallow dynamic “pressure bulb” of a rocking foundation.
(e) In classical geotechnical engineering, avoiding bearing
capacity failure at any cost is an unquestionably prudent goal.
Seismic “loading” is different
it is not even loading, but an
imposed displacement. Sliding mechanisms develop under the
footing momentarily and hence alternatingly, and may only lead
to (increased) settlement. It would be the task of the engineer to
“accommodate” such settlements with proper design.
The results and conclusions of this paper are in harmony with
the numerous experimental and theoretical findings of Professor
Bruce Kutter and his coworkers at U.C. Davis, and of
Professors Alain Pecker and Roberto Paolucci and their
coworkers in Paris and Milano.
11 ACKNOLEDGMENTS
Τ
he financial support for the work outlined in this paper has
been provided through the research project “DARE”, funded by
the European Research Council (ERC), “IDEAS” Programme in
Support of Frontier Research. Contract/number ERC–2–9–
AdG228254–DARE .
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