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Butbycontrasttothepiledfoundation,thecapabilityofthecaissontotransmitthe
vertical load would be hardly affected.
(4) For S
13m, the rupture path “hits” the base corner of the caisson and “defracts”
to the right, emerging at the ground surface at a distance of S
=
18m, i.e. 5m to
the right of the free-field outcrop. The caisson essentially follows the movement
ofthe“hangingwall”,therebyexperiencinganappreciablerotationofabout3
◦
for
h
=
=
2m.
In conclusion, it appears that the response of deep embedded foundations (“caissons”)
would in most cases be quite satisfactory, especially if structural provisions are taken
to accommodate their unavoidable rotation at large fault offsets. Once again, one of the
limitationsinourmodelling,namelytheassumptionofaperfectly-bondedinterface,may
have exaggerated the lateral displacement/rotation of thecaisson.
3. Nonlinear response of shallow foundations to strong seismic excitation
3.1. INTRODUCTION
The conventional approach to foundation design introduces factors of safety against
sliding and exceedance of ultimate capacity, in a way similar to the traditional sta-
tic design. This approach involves two consecutive steps of structural and foundation
analysis:
(a) Dynamic analysis of the structure is performed in which the soil is modelled as an
elastic medium, represented by suitable translational and rotational springs (and,
sometimes,withtheassociateddashpots).Thedynamicforcesandmomentstrans-
mittedontothefoundationarederivedfromtheresultsofsuchanalysesalongwith
considerations for inelastic structural response (e.g. by reducing the moments in
columns through thebehaviour [“ductility”] factor q).
(b) The foundations are then designed in such a way that these transmitted horizontal
forces and overturning moments, increased by “overstrength” factors, would not
induce sliding or bearing capacity failure.
The use of “overstrength” factors isnecessitated by the so-called “capacity design” prin-
ciple, under which plastic hinging is allowed only in the superstructural elements—not
in the below-ground (and thus un-inspectable) foundation and soil. Therefore, structural
yielding of the footing and mobilization of bearing capacity mechanisms is not allowed.
Only a “limited” amount of sliding deformation and uplifting at the foundation-soil
interface is allowed. However, there is a growing awareness in the profession of the
need to consider soil-foundation inelasticity, in analysis and perhaps even in design (see
Paolucci, 1997; Pecker, 1998; Martin and Lam, 2000; Allotey and Naggar, 2003). This
need has emerged from:
•
The large (often huge) acceleration (and velocity) levels recorded in several earth-
quakes which are associated with even larger elastic spectral accelerations (of the
