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, *k*sv, 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

to Σ

(9-12)

axial tensile loading) should be determined and

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

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,

leading to an initial triangular distribution of axial

pile loading from applied seismic moments.

Full

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

2

(9-13)

reached when the displacement reaches 1.0 percent

of the depth of the cap in a manner similar to that for

where:

a shallow bearing foundation.

9 - 12

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