317
response of piles to dynamic loading
Will be difficult to ascertam because of the lack of experimental data on the
detailed performance of a variety of structures in a variety of seismic events
A procedure sometimes used for the design of the pile foundation is to select
a horizontal accélération, which is some function of the accélération of gravity
The accélération can be used to obtain a pseudo-horizontal load, which is some
function of the mass of the superstructure. A judgment about the safety of the
foundation can be made on the basis of the pseudo-horizontal load.
The American Petroleum Institute (1993) requires that structures be designed to be safe in seismic zones. The évaluation of the area where the structure is to be placed with respect to seismic activity is required. Designs may
be made using procedures from dynamic analysis such as response-spectrum
analysis or time-history analysis.
Zeevaert (1983) studied data from records of seismographs during two significant earthquakes. The instruments were located at sites in the Valley of Mexico
where deep beds of soft soils are présent. In addition to the valuable records on
accélérations, Zeevaert had extensive information on structures that were not
damaged during the earthquakes or damaged to various degrees of severity. He
studied the records of accélération in detail and developed pseudo-acceleration
spectra as a function of various amounts of damping. The characteristics of
the soil were studied experimentally and analytically. Zeevaert developed a prédiction of ground motion as a function of depth and extended the work into
predicting the bending moment in a pile at a spécifie location during the earthquake selected for design. A photograph was included to show the rupture of
a reinforced-concrete pile due to high bending moments. With regard to piles
broken by an earthquake, Professor Ishahara at a breakfast in Tokyo years ago
told the writer that he had not seen broken piles except due to liquéfaction
(Ishahara, 1977).
Time-Dependent Loading Above the Mudline
A completely different approach to the design of the piles is needed for the
case where a forcing function is applied to the superstructure. The loading
may corne from machinery on the platform deck, from waves, and possibly from
ship impact. The problem can be solved in either the time domain or in the
frequency domain. The responses of a single pile or group of piles to dynamic
loading in the frequency domain hâve been analyzed by a number of engineers.
For example, Ensoft, Inc. (1999) has prepared a computer code that yields
the dynamic stiffness of a pile-supported foundation, which is defined as the
ratio between the applied force and the resulting displacement under the stea y
State vibration at a given frequency. Specifically, the stiffness can be relat < t
lateral-load versus deflection, axial-load versus deflection, and moment versus
rotation, and the configurations of the piles. The close spacing of piles is ta
into account by pile-soil-pile interaction.
_
The Ensoft code is based principally on papers by Kausel (1 < )• ■' ' J
et al. (1975), Poulos (1971), and Roesset and Kausel (1975). The principa
analytical techniques employed are the consistent boundary matrix m<
ihe finite element method. This code requires the entry of the geom •
response of piles to dynamic loading
Will be difficult to ascertam because of the lack of experimental data on the
detailed performance of a variety of structures in a variety of seismic events
A procedure sometimes used for the design of the pile foundation is to select
a horizontal accélération, which is some function of the accélération of gravity
The accélération can be used to obtain a pseudo-horizontal load, which is some
function of the mass of the superstructure. A judgment about the safety of the
foundation can be made on the basis of the pseudo-horizontal load.
The American Petroleum Institute (1993) requires that structures be designed to be safe in seismic zones. The évaluation of the area where the structure is to be placed with respect to seismic activity is required. Designs may
be made using procedures from dynamic analysis such as response-spectrum
analysis or time-history analysis.
Zeevaert (1983) studied data from records of seismographs during two significant earthquakes. The instruments were located at sites in the Valley of Mexico
where deep beds of soft soils are présent. In addition to the valuable records on
accélérations, Zeevaert had extensive information on structures that were not
damaged during the earthquakes or damaged to various degrees of severity. He
studied the records of accélération in detail and developed pseudo-acceleration
spectra as a function of various amounts of damping. The characteristics of
the soil were studied experimentally and analytically. Zeevaert developed a prédiction of ground motion as a function of depth and extended the work into
predicting the bending moment in a pile at a spécifie location during the earthquake selected for design. A photograph was included to show the rupture of
a reinforced-concrete pile due to high bending moments. With regard to piles
broken by an earthquake, Professor Ishahara at a breakfast in Tokyo years ago
told the writer that he had not seen broken piles except due to liquéfaction
(Ishahara, 1977).
Time-Dependent Loading Above the Mudline
A completely different approach to the design of the piles is needed for the
case where a forcing function is applied to the superstructure. The loading
may corne from machinery on the platform deck, from waves, and possibly from
ship impact. The problem can be solved in either the time domain or in the
frequency domain. The responses of a single pile or group of piles to dynamic
loading in the frequency domain hâve been analyzed by a number of engineers.
For example, Ensoft, Inc. (1999) has prepared a computer code that yields
the dynamic stiffness of a pile-supported foundation, which is defined as the
ratio between the applied force and the resulting displacement under the stea y
State vibration at a given frequency. Specifically, the stiffness can be relat < t
lateral-load versus deflection, axial-load versus deflection, and moment versus
rotation, and the configurations of the piles. The close spacing of piles is ta
into account by pile-soil-pile interaction.
_
The Ensoft code is based principally on papers by Kausel (1 < )• ■' ' J
et al. (1975), Poulos (1971), and Roesset and Kausel (1975). The principa
analytical techniques employed are the consistent boundary matrix m<
ihe finite element method. This code requires the entry of the geom •