316
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
The allowable bending stress would then be 31.83 ksi. The section modulus of
the section (14,117/16.5) yields a bending moment at failure of 27,233 in.-kip.
By entering Figure 11.19 with the bending moment, a latéral load of about 105
kips is found. The computed pile-head deflection at that load was about 6.7
in. A portion of the latéral load could be taken by dynamic loading and some
load factor, according to the design criteria, to find the loading under service
conditions.
Conclusions
In addition to the comments made in the présentation about the importance
of obtaining the best possible prédiction of p-y curves, the following points can
be made:
1. The nondimensional solution can yield solutions that agréé fairly well
with the computer solution if the soil profile shows favorable values of shear
strength with depth (sands and normally Consolidated clays).
2. The nondimensional solution can (a) serve as a check to computer solutions and (b) reveal clearly the nature of the lateral-load problem and the
importance of various parameters.
3. The plot of bending moment with respect to depth shown in Figure 11.18
clearly demonstrates the importance of analyzing the piles and superstructure
as a unit to ensure the best estimate of rotational restraint at the top of the
pile.
4. The engineer can achieve the most efficient design of the piles by taking
into account the length of the extension of the jacket, the details of grouting
between pile and jacket, the wall thickness of the pile with respect to depth,
and careful estimation of loadings on a platform as a function of time.
11.5
RESPONSE OF PILES TO DYNAMIC LOADING
Earthquakes
Several steps are taken in order to design the foundation for a pile-supported
structure in a seismic région. The location of the fault with respect to the
structure must be found and the expected magnitude of the event must be
selected. Then, the characteristics of the soils and rocks at the site must be
considered in order to perform microzonation. With such data at hand, the timedependent, free-field motion of the supporting soils and rocks at the building site
can be computed or estimated. The engineer can then décidé if there is a chance
of liquéfaction of any loose granular soil below the water table. If liquéfaction
appears to be likely for the selected earthquake, steps must be taken to improve
the supporting soil or to design a structure that remains stable, even though
some of the supporting soil liquéfiés.
If the free-field motion of the soils at the site are known as a function of
depth, a fully rational solution can be undertaken which will require extensive
and complicated computations. The validity of the results of such computations
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
The allowable bending stress would then be 31.83 ksi. The section modulus of
the section (14,117/16.5) yields a bending moment at failure of 27,233 in.-kip.
By entering Figure 11.19 with the bending moment, a latéral load of about 105
kips is found. The computed pile-head deflection at that load was about 6.7
in. A portion of the latéral load could be taken by dynamic loading and some
load factor, according to the design criteria, to find the loading under service
conditions.
Conclusions
In addition to the comments made in the présentation about the importance
of obtaining the best possible prédiction of p-y curves, the following points can
be made:
1. The nondimensional solution can yield solutions that agréé fairly well
with the computer solution if the soil profile shows favorable values of shear
strength with depth (sands and normally Consolidated clays).
2. The nondimensional solution can (a) serve as a check to computer solutions and (b) reveal clearly the nature of the lateral-load problem and the
importance of various parameters.
3. The plot of bending moment with respect to depth shown in Figure 11.18
clearly demonstrates the importance of analyzing the piles and superstructure
as a unit to ensure the best estimate of rotational restraint at the top of the
pile.
4. The engineer can achieve the most efficient design of the piles by taking
into account the length of the extension of the jacket, the details of grouting
between pile and jacket, the wall thickness of the pile with respect to depth,
and careful estimation of loadings on a platform as a function of time.
11.5
RESPONSE OF PILES TO DYNAMIC LOADING
Earthquakes
Several steps are taken in order to design the foundation for a pile-supported
structure in a seismic région. The location of the fault with respect to the
structure must be found and the expected magnitude of the event must be
selected. Then, the characteristics of the soils and rocks at the site must be
considered in order to perform microzonation. With such data at hand, the timedependent, free-field motion of the supporting soils and rocks at the building site
can be computed or estimated. The engineer can then décidé if there is a chance
of liquéfaction of any loose granular soil below the water table. If liquéfaction
appears to be likely for the selected earthquake, steps must be taken to improve
the supporting soil or to design a structure that remains stable, even though
some of the supporting soil liquéfiés.
If the free-field motion of the soils at the site are known as a function of
depth, a fully rational solution can be undertaken which will require extensive
and complicated computations. The validity of the results of such computations