55
Ishihara Lecture
Soil-Foundation-Structure Systems Beyond Conventional Seismic Failure
Thresholds
Conférence Ishihara
Les systèmes sol-fondation-structure qui dépassent les limites de la rupture parasismique
conventionnelle
Gazetas G.
Professor, National Technical University of Athens, Greece
ABSTRACT: A new paradigm has now emerged in performance
–
based seismic design of soil
foundation
structure systems. Instead
of imposing strict safety limits on forces and moments transmitted from the foundation onto the soil (aiming at avoiding pseudo-static
failure), the new dynamic approach
“
invites
”
the creation of two simultaneous
“
failure
”
mechanisms: substantial foundation uplifting
and ultimate-bearing-capacity slippage, while ensuring that peak and residual deformations are acceptable. The paper shows that
allowing the foundation to work at such extreme conditions not only may not lead to system collapse, but it would help protect (save)
the structure from seismic damage. A potential price to pay: residual settlement and rotation, which could be abated with a number of
foundation and soil improvements. Numerical studies and experiments demonstrate that the consequences of such daring foundation
design would likely be quite beneficial to bridge piers and building frames. It is shown that system collapse could be avoided even
under seismic shaking far beyond the design ground motion.
RÉSUMÉ : Un nouveau paradigme a émergé dans la conception sismique de la performance des systèmes sol
–
fondation
–
structure.
Au lieu d'imposer des coefficients de sûreté sur les forces et les moments transmis par la fondation sur le sol (pour éviter la rupture
pseudo-statique), la nouvelle approche dynamique permet la création de deux modes de rupture simultanés : le soulèvement important
de la fondation et le dépassement de la capacité portante ultime, tout en assurant que les déformations maximales et résiduelles sont
acceptables. L’
article montre que, quand on permet à la fondation de travailler dans ces conditions extrêmes, l'effondrement du
système peut être évité et de plus la structure peut être protégée du dommage sismique. Un prix potentiel à payer : le déplacement et la
rotation résiduels, qui peuvent être contrôlés avec différentes méthodes d'amélioration de la fondation et des sols. Des études
numériques et expérimentales montrent que les conséquences d'une telle conception audacieuse de la fondation seraient certainement
très bénéfiques pour les ponts et les bâtiments. On montre que l'effondrement du système pourrait être évité, même pendant des
secousses sismiques qui dépassent le mouvement de calcul.
KEYWORDS: seismic analysis, performance-based design, foundation rocking, bearing capacity failure, nonlinear vibrations
1 CURRENT STATE OF PRACTICE:
THE CONVENT
IONAL “WISDOM”
Seismic design of structures recognises that highly inelastic
material response is unavoidable under the strongest possible
shaking of the particular location and for the specific soil where
the structure is founded. “Ductility” levels of
the order of 3 or
more are usually allowed to develop under seismic loading,
implying that the strength of a number of critical bearing
elements is fully mobilized.
In the prevailing structural
terminology “plastic hinging” is allowed to develop as long a
s
the overall stability is maintained.
By contrast, a crucial goal of current practice in seismic
“foundation” design, particularly as entrenched in the respective
codes is to
avoid
the mobilisation of “strength” in the
foundation. In the words of EC8 (Part 2, § 5.8) :
“…foundations shall not be used as sources of hysteretic
energy dissipation, and therefore shall be designed to
remain elastic under the design seismic action.”
In
structural
terminology :
no
“plastic hinging” is allowed in
the foundation. In simple
geotechnical
terms, the designer must
ensure that the below-ground (and hence un-inspectable)
support system will not even reach a number of “thresholds”
that would conventionally imply failure. Specifically, the
following states are prohibited :
plastic structural “hinging” in piles, pile
-caps, foundation
beams, rafts, and so on
mobilisation of the so-called
bearing-capacity failure
mechanisms under cyclically
uplifting shallow foundations
sliding at the soil
–
footing interface or excessive uplifting of
a shallow foundation
passive failure along the normal compressing sides of an
embedded foundation
a combination of two or more of the above “failure” modes.
In this conventional approach to foundation design,
“overstrength” factors plus (explicit a
nd implicit) factors of
safety larger than 1 (e.g. in the form of “material” factors) are
introduced against each of the above “failure” modes, in a way
qualitatively similar to the factors of safety of the traditional
static design. Thus, the engineer is certain that foundation
performance will be satisfactory and there will be no need to
inspect and repair after strong earthquake shaking
a task
practically considered next to impossible.
Some of the above thresholds stem not just from an
understandable engineering conservatism, but also from a
purely (pseudo) static thinking. It will be shown that such an
approach may lead not only to unnecessarily expensive
foundation solutions but also, in many situations, to less safe
structures.
Ishihara Lecture
Soil-Foundation-Structure Systems beyond Conventional Seismic Failure
Thresholds
Conférence Ishihara
Les systèmes sol-fondation-structure qui dépassent les limites de la rupture parasismique
conventionnelle
Gazetas G.
Professor, National Technical University of Athens, Greece
