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API REPORT 8 Document Information:
Title
Centrifuge Studies of Cyclic Lateral Load-Displacement Behavior of Single Piles
American Petroleum Institute
Publication Date:
Jun 15, 1978
Scope:
1. Introduction
In the period following the completion of the previous report of
December 1977 and the end of the contract on OSAPR Project 8 with
California Institute of Technology, additional pile loading tests were
conducted.
Two different test series were followed through: additional
load-unload cyclic tests again in simulation of the Mustang Island
(MI) tests and pile vibration tests. The reasons for the first series
were: (a) the model pile dimensions originally chosen for the Mustang
Island simulation did not correctly represent either the El of the
prototype pile, nor its width; and (b) it was desired to perform the
tests in a soil more closely resembling the fine-grained MI sand, at
higher relative densities than had been achieved in the earlier tests.
It is worth pointing out here that it is not easy to produce a model
pile with the correctly scaled EI, since, although the dimensions can
be correctly calculated, they are based on an assumed E for the
material which may be slightly different in the metal actually
machined, and the strain gauges, leads and moisture-protection coating
employed increase the EI in the final product. A further deficiency of
the first test pile was that, although the model pile had been
instrumented with 5 strain gauges, they had been installed in
locations and at intervals that proved inconvenient in the actual
centrifuge tests. Only three gauges could be positioned below ground
surface. On the model pile whose use is described here, 6 strain
gauges were bonded, closer together near the top, at sites such that
all six gauges were at or below the soil surface. As will be seen
later, this enabled much better curves of moment in the pile as a
function of depth to be plotted.
The dynamic test series was designed to explore resonant behavior in a
model pile-soil system at model frequencies representative of those
produced in a prototype earthquake. The centrifuge scaling laws for
frequency (see first report, Table 2.1) require that the model
frequencies be 100 times those of the prototype at 100 g acceleration.
Since earthquake frequencies of interest to structures lie generally
in the range 0.2 to 20 Hertz, model frequencies of 20 to 200 Hertz are
required. Earthquake strong motion durations of 10 to 50 seconds at
full scale correspond to model durations of 0.1 to 0.5 seconds.
Prototype earthquakes have peak strong motion lateral acceleration in
the range 0.3 to 1.0 g; model simulation therefore requires 30 to 100
g equivalent accelerations. Ideally, therefore, a centrifuge
earthquake to apply to a structural model would consist of a 0.1 to
0.5 second burst of essentially random vibration containing
frequencies of tens to hundreds Hertz, at peak lateral g's of 30 to
100 g. Equipment to do this has not yet been developed, and a
different approach was adopted for the preliminary work.
One method of treating the effects of earthquakes on a prototype
structure is modal analysis which requires a knowledge of the modes of
vibration of a structure in the frequency range of interest. Since an
electrical signal generator was available with the capability of
sweeping through a selected range of frequencies at a constant
amplitude, as well as emitting a burst of signal at constant frequency
for a preselected duration, it was employed to find, first, the
resonant frequencies of a pile-mass system, and second, to vibrate the
system at one of these frequencies for a short time.
The results of both of these studies are reported here. In addition,
some time was devoted previously to analytical studies of pile-soil
behavior under load; an appendix is attached describing the technique
of the analysis.
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