in accordance with Table 3-1 in order to assign the
difficulties in determining soil properties and static
appropriate site coefficients, Fa and Fv.
foundation loads for existing buildings, together with
the likely variability of soils supporting foundations,
b.
Load Deformation Characteristics for
an equivalent elasto-plastic representation of load-
Foundations.
deformation behavior is recommended. In addition,
to allow for such variability or uncertainty, an upper-
(1)
General.
Load-deformation
and lower-bound approach to defining stiffness and
characteristics are required where the effects of
capacity is recommended (as shown in Figure 9-1a)
foundations are to be taken into account in linear
to
permit
evaluation
of
structural
response
elastic analyses or in nonlinear static (pushover) or
sensitivity. The selection of uncertainty represented
nonlinear
dynamic
(time
history)
analyses.
by the upper and lower bounds should be determined
Foundation
load-deformation
parameters
jointly by the geotechnical and structural engineers.
characterized by both stiffness and capacity can have
a significant effect on both structural response and
(2) Shallow bearing foundations.
load distribution among structural elements. Load-
deformation parameters, represented by appropriate
(a)
Stiffness parameters.
The shear
soil springs, can provide significant reduction and/or
modulus, G, for a soil is related to the modulus of
redistribution of seismic force levels in some
elasticity, E, and Poisson' ratio, v, by the
s
buildings.
Vertical soil springs may effectively
relationship:
lengthen the fundamental period of slender, stiff
buildings such as aircraft control towers, and could
E
have a beneficial effect for buildings at relatively
G=
(9-1)
2 (1 + v )
stiff soil sites.
Rotational soil springs at interior
column footings will tend to relieve the fixed end
1. Most soils are intrinsically nonlinear
moment at the column base, and cause redistribution
and the shear modulus and the shear wave velocity
of seismic forces in the story. Foundation systems for
decrease with increasing shear strain. Experimental
buildings can in some cases be complex, but for the
values obtained by laboratory testing at low strains
purpose of simplicity, three foundation types are
need to be modified to reflect expected effective
considered in these guidelines:
values at strains corresponding to the design ground
motion.
Shallow bearing foundations;
Pile foundations; and
2. To reflect the upper- and lower-bound
Drilled shafts.
concept illustrated in Figure 9-1 the upper-bound
stiffness of rectangular footings should be based on
While it is recognized that the load-deformation
behavior of foundations is nonlinear, because of the
9-2
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