Geoscience Reference
In-Depth Information
order of 2g). Enormous ductility demands would be imposed to structures by such
accelerations if soil and foundation “yielding” did not effectively take place to limit
thetransmitted accelerations.
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In seismically retrofitting a building or a bridge, allowing for soil and foundation
yielding is the only rational alternative. Because increasing the structural capacity
of some elements would imply that the forces transmitted onto the foundation be
increased, to the point that it would not be technically or economically feasible
to undertake them “elastically”. Thus, new retrofit design guidelines (FEMA 356)
explicitly permit inelastic deformations inthe foundation.
Evenwithnewstructures,ithasbeenrecognizedthatwithimprovedanalysismethodswe
need to better evaluate performance in terms of levels of damage. For the superstructure,
“performance-based” design or equivalently “displacement-based” design have been
used for a number of years, with inelastic “pushover” analyses becoming almost routine
in seismic design practice. It is logical to extend the inelastic analysis to the supporting
foundation and soil.
3.2. NEW DESIGN PHILOSOPHY: “PLASTIC HINGING” IN SHALLOW
FOUNDATIONS
Excluding structural yielding in the isolated footing or the foundation beam, three types
of nonlinearity can take place and modify the overall structure-foundation response:
(a)
Sliding at the soil-foundation interface
:Thiswouldhappenwheneverthetransmit-
ted horizontal force exceeds the frictional resistance. As pointed out by Newmark
(1965), thanks to the oscillatory nature of earthquake shaking, only short periods
of exceedance usually exist in each one direction; hence, sliding is not associated
withfailure,butwithpermanentirreversibledeformations.Thedesignermustonly
ensure that the magnitude of such deformations would not be structurally or oper-
ationally detrimental. Although this philosophy has been applied to the design of
earth dams and gravity retaining walls, its practical significance for foundations
might be somewhat limited in view of the large values of the coefficient of friction
at soil-footing interface and the passive-type resistance often enjoyed by embed-
ded foundations.
(b)
Separation and uplifting of the foundation from the soil
: This would happen when
the seismic overturning moment tends to produce net tensile stresses at the edges
of the foundation. The ensuing rocking oscillations in which uplifting takes place
involve primarily geometric nonlinearities, if the soil is competent enough. There
isnodetrimenttotheverticalloadcarryingcapacityandtheconsequencesinterms
of induced vertical settlements may be minor. Moreover, in many cases, footing
uplifting is beneficial for the response of the superstructure, as it helps reduce the
ductility demands on columns. Housner (1963), Pauley and Priestley (1992), and
many others have reported that the satisfactory response of some slender struc-
tures in strong shaking can only be attributed to foundation rocking. Deliberately
