Sn = distance between nth pile and axis of
caps or basement walls will control lateral spring
stiffness; therefore, estimates of lateral spring
rotation.
stiffness can be computed using elastic solutions as
described in Paragraph 9-2c(2)(a).
In instances
Whereas the effects of group action and the influence
where piles may contribute significantly to lateral
of pile batter are not directly accounted for in the
stiffness (i.e., very soft soils, battered piles) solutions
form of the above equations, it can be reasonably
using beam-column pile models are recommended.
assumed that the latter effects are accounted for in
Axial pile group stiffness spring values, ksv, may be
the range of uncertainties expressed for axial pile
assumed to be in an upper- and lower-bound range,
stiffness.
respectively, given by:
(c)
Capacity parameters.
Vertical load
0.5 A E N 2 A E
capacity of piles (for both axial compression and
N
k sv = Σ
to Σ
(9-12)
L
L
axial tensile loading) should be determined and
n=1
n=1
documented in the geotechnical investigation report.
The investigation should be based on accepted
where:
foundation engineering practice using best estimate
of soil properties. Consideration should be given to
A = cross-sectional area of a pile
the capability of pile cap and splice connections to
take tensile loads when evaluating axial tensile load
capacity. The moment capacity of a pile group
should be determined assuming a rigid pile cap,
L = length of piles
leading to an initial triangular distribution of axial
pile loading from applied seismic moments.
Full
N = number of piles in group.
axial capacity of piles may, however, be mobilized
when computing ultimate moment capacity, leading
The rocking spring stiffness values about each
to a rectangular distribution of resisting moment in a
horizontal pile cap axis may be computed by
manner analogous to that described for a footing in
assuming each axial pile spring acts as a discrete
Figure 9-6. The lateral capacity of a pile group is
Winkler spring.
The rotational spring constant
largely dependent on that of the cap, as it is
(moment per unit rotation) is then given by:
restrained by passive resistance of the adjacent soil
material.
The capacity may be assumed to be
N
k sr = Σ kvn S n
2
(9-13)
reached when the displacement reaches 1.0 percent
n=1
of the depth of the cap in a manner similar to that for
where:
a shallow bearing foundation.
kvn = axial stiffness of the nth pile
9 - 12
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