Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 267

loads can have both low- and high-frequency cyclic components in which the
rates of load application and duration are measured in seconds. Storm and ice
loadings can have several thousand cycles of applied forces, while earthquakes
can induce several tens of cycles of forces.
For most fixed offshore platforms supported on piles, experience has proven
the adequacy of determining pile penetration based on static capacity evalua-
tions, static ultimate design loads and commonly accepted factors of safety
that, in part, account for the cyclic loading effects.

Cyclic Loading Effects
A study by Briaud et al. (1984) on the axial capacity and performance of
piles showed that, as compared with long-term, static loadings, cyclic load-
ing may have the following important influence on pile axial capacity and
stiffness: it may decrease capacity and stiffness due to the repeated loading.
On the other hand, it may increase capacity and stiffness due to the high rates
of loading.
The cyclic loading effect on pile capacity is caused by resistance from the
pile and soil and the load itself.
Cyclic loading can provide a positive effect by stiffening and strengthening
or, on the contrary, may may be a reason for softening and weakening of the
soil around the pile; also, it may cause accumulation of pile displacements.
For earthquakes, the free-field ground motions, which are natural phenom-
ena, are not affected by the piles and structure but, due to their vibration, cyclic
straining develops in the soil; these effects may influence pile capacity and stiff-
ness. The soil will damp the earthquake vibration and absorb the loading
energy.

Analytical Models
A primary objective of these analyses is to ensure that the pile and its penetra-
tion under static and cyclic loading are adequate to meet the structure ' s service-

ability and capacity based on ultimate limit state requirements.
Different analytical models have been developed and applied to determine
the cyclic axial response of piles, as presented in API RP2A. In general,
these models can be categorized into two types, discrete element models and
continuum models, noting that the two models use the finite difference method
or finite element method. The two types of analytical models can be described
as follows.

Discrete Element Models
The soil around the pile is modeled by a series of uncoupled springs or elements
attached between the piles, and the far-field soil in most cases is assumed to be