56
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 SOME COMPELLING REASONS TO GO BEYOND
CONVENTIONAL THRESHOLDS
A growing body of evidence suggests that soil–foundation
plastic yielding under seismic excitation is unavoidable, and at
times even desirable; hence, it must be considered in analysis
and perhaps allowed in design.
[See for an early recognition
:
Pecker 1998, Faccioli & Paolucci 1999, Martin & Lam 2000,
FEMA-356 2000, Kutter et al 2001, Gazetas & Apostolou
2003.]
The urgent need to explicitly consider the possibility of
the foundation system to go beyond “failure” thresholds, and the
potential usefulness of doing so, have emerged from :
(a)
The large (often huge) effective ground acceleration, A,
and velocity, V, levels recorded in several earthquakes in the
last 25 years. A f
ew examples
:
•
1994 M
s
≈
6.8 Northridge
:
A = 0.98 g, V = 140 cm/s
;
•
1995 M
J
≈
7.2 Kobe
:
A = 0.85 g, V = 120 cm/s
;
•
1986 M
s
≈
5.6 San Salvador
:
A = 0.75 g, V = 84 cm/s
;
•
2003 M
s
= 6.4 Lefkada
:
A
≈
0.55 g, V = 50 cm/s
;
•
2007 M
J
≈
6.9 Niigata
:
A =1.20 g, V = 100cm/s .
With the correspondingly large accelerations in the (above–
ground) structure from such ground motions (spectral
S
a
values
well in excess of 1 g), preventing “plastic hinging” in the
foundation system is a formidable task. And in fact, it may not
even be desirable
:
enormous ductility demands might be
imposed to the structure if soil–foundation “yielding” would not
take place to effectively limit the transmitted accelerations.
Several present-day critically–important structures on relatively
loose soil could not have survived severe ground shaking if
“plastic hinging” of some sort had not taken place in the
“foundation”
usually unintentionally.
(b)
In seismically retrofitting a building or a bridge, allowing
for soil and foundation yielding is often the most rational
alternative. Because increasing the structural capacity of some
elements, or introducing some new stiff elements, would then
imply that the forces transmitted onto their foundation will be
increased, to the point that it might not be technically or
economically feasible to undertake them “elastically”. The new
American retrofit design guidelines (FEMA 356) explicitly
permit some forms of inelastic deformations in the foundation.
A simple hypothetical
example
referring to an existing three–
bay multi–story building frame which is to be retrofitted with a
single–bay concrete “shear” wall had been introduced by
Martin & Lam 2000. Such a wall, being much stiffer than the
columns of the frame, would carry most of the inertia-driven
shear force and would thus transmit a disproportionately large
horizontal force and overturning moment onto the foundation
compared with its respective small vertical force. If uplifting,
sliding, and mobilisation of bearing capacity failure
mechanisms in the foundation had been all spuriously ignored,
or had been conversely correctly taken into account, would have
led to dramatically different results. With “beyond–threshold”
action in the foundation the shear wall would “shed” off some
of the load onto the columns of the frame, which must then be
properly reinforced ; the opposite would be true when such
action (beyond the thresholds) is disallowed.
The Engineer therefore should be able to compute the
consequences of “plastic hinging” in the foundation before
deciding whether such “hinging” must be accepted, modified, or
avoided (through foundation changes).
(c)
Many slender historical monuments (e.g. ancient
columns, towers, sculptures) may have survived strong seismic
shaking during their life (often of thousands of years). While
under static conditions such “structures” would have easily
toppled, it appears that sliding at, and especially uplifting from,
their base during oscillatory seismic motion was a key to their
survival (Makris & Roussos 2000, Papantonopoulos 2000).
These nonlinear interface phenomena cannot therefore be
ignored, even if their geometrically–nonlinear nature presents
computational difficulties.
In fact, it is worthy of note that the lack of recognition of the
fundamental difference between pseudo-static and seismic
overturning threshold accelerations has led humanity to a gross
under-estimation of the largest ground accelerations that must
have taken place in historic destructive earthquakes. Because,
by observing in numerous earthquakes that very slender blocks
(of width
b
and height
h, with h >> b
) or monuments in
precarious equilibrium that had not overturned, engineers had
invariably attributed the fact to very small peak accelerations,
less than
(b/h)g
, as would be necessary if accelerations were
applied pseudostatically in one direction. Today we know that
sometimes even five times as large peak ground acceleration of
a high-frequency motion may not be enough to overturn a
slender block (Koh et al 1986, Makris & Roussos 2000, Gazetas
2001). Simply stated: even severe uplifting (conventional
“failure”) may not lead to overturning (true “collapse”) under
dynamic seismic base excitation.
(d)
Compatibility with structural design is another reason for
the soil
−
structure interaction analyst to compute the lateral load
needed for collapse of the foundation system, as well as (in
more detail) the complete load–displacement or moment–
rotation response to progressively increasing loading up to
collapse. Indeed, in State of the Art (SOA) structural
engineering use is made of the so-called “pushover” analysis,
which in order to be complete requires the development of such
information from the foundation analyst.
In addition to the above “theoretical” arguments, there is a
growing need for estimating the “collapse motion” : insurance
coverage of major construction facilities is sometimes based on
estimated losses under the worst possible (as opposed to
probable) earthquake scenario.
(e)
Several persuasive arguments could be advanced on the
need
not to
disallow structural plastic “hinging” of piles:
•
Yielding and cracking of piles (at various critical depths) is
unavoidable with strong seismic shaking in soft soils, as the
Kobe 1995 earthquake has amply revealed.
•
Refuting the contrary universal belief, post-earthquake
inspection of piles is often feasible (with internally placed
inclinometers, borehole cameras, integrity shock testing,
under-excavation with visual inspection ), although certainly
not a trivial operation. Again, Kobe offered numerous
examples to this effect.
•
The lateral confinement provided by the soil plays a very
significant role in pile response, by retarding the development
of high levels of localised plastic rotation, thereby providing
an increase in ductility capacity. Sufficient displacement
ductility may be achieved in a pile shaft with transverse
reinforcement ratio as low as 0.003 (Butek et al 2004).
•
The presence of soil confinement leads to increased plastic
hinge lengths, thus preventing high localised curvatures
(Tassios 1998). Therefore, the piles retain much of their axial
load carrying capacity after yielding.
Thus, a broadly distributed plastic deformation on the pile
may reduce the concentrated plastification on the structural
column
so detrimental to safety.
Furthermore, when subjected to strong cyclic overturning
moment, end-bearing piles in tension will easily reach their full
frictional uplifting capacity. It has been shown analytically and
experimentally that this does not imply failure. The same
argument applies to deeply embedded (caisson) foundations.
(f)
The current trend in
structural
earthquake engineering
calls for a philosophical change
:
from strength-based design
(involving force considerations) to performance-based design
(involving displacement considerations) [Pauley 2002,
Priestley et al 2000, 2003, Calvi 2007].
Geotechnical
earthquake engineering has also been slowly moving towards
performance–based seismic design
:
gravity retaining structures
