52
STRUCTURE-ENVIRONMENTAL FORCE INTERACTIONS
Soil Foundation Restraints
The properties of the soils on the sea floor are needed to predict the dynamics
of fixed offshore structures. In some cases, it may be appropriate to assume
that the soil foundation for such structures behaves elastically and that the soil
properties are unaffected by the motion of the contacting structure. Consider,
for example, the single cantilevered pile that pénétrâtes the flexible foundation of
the sea floor to a depth £u, as shown in Figure 2.23a. The pile’s bending stiffness
is El and its height above the sea floor is I. Under the horizontal tip load Fx, the
horizontal tip displacement is 6. Using a static analysis, Kocsis (1976) computed
the équivalent length t, > £ for a uniform pile partly submerged in sandy or in
clayey soil of constant, elastic properties. This équivalent length, depicted in
Figure 2.23b, is that for a hypothetical pile with full fixity at the base, which
gives the same horizontal deflection ô under the same horizontal load Fx as for
the pile with the flexible soil foundation. For a sandy soil, that length is
(2.74)
in which
/ 102.9EZX1/5
I
I
\ No y
(2-75)
Figure 2.23 Static model for pile-soil stiffness.
u mergtx sandy soil, the horizontal subgrade reaction constant Nq in the
as equa ion .as a range of 4 tons/ft3 to 34 tons/ft3 for relatively loose to
, q -, ! san^‘
1X11’ (Terzaghi, 1955). With the value of £e from équations
p,,..!:)111 'jL 'u
* r
* '
can ca^cu^ated from classical beam theory for
antilerered beam of length t. as: k, = iEI/Ç. Kocsis (1976) also presented
STRUCTURE-ENVIRONMENTAL FORCE INTERACTIONS
Soil Foundation Restraints
The properties of the soils on the sea floor are needed to predict the dynamics
of fixed offshore structures. In some cases, it may be appropriate to assume
that the soil foundation for such structures behaves elastically and that the soil
properties are unaffected by the motion of the contacting structure. Consider,
for example, the single cantilevered pile that pénétrâtes the flexible foundation of
the sea floor to a depth £u, as shown in Figure 2.23a. The pile’s bending stiffness
is El and its height above the sea floor is I. Under the horizontal tip load Fx, the
horizontal tip displacement is 6. Using a static analysis, Kocsis (1976) computed
the équivalent length t, > £ for a uniform pile partly submerged in sandy or in
clayey soil of constant, elastic properties. This équivalent length, depicted in
Figure 2.23b, is that for a hypothetical pile with full fixity at the base, which
gives the same horizontal deflection ô under the same horizontal load Fx as for
the pile with the flexible soil foundation. For a sandy soil, that length is
(2.74)
in which
/ 102.9EZX1/5
I
I
\ No y
(2-75)
Figure 2.23 Static model for pile-soil stiffness.
u mergtx sandy soil, the horizontal subgrade reaction constant Nq in the
as equa ion .as a range of 4 tons/ft3 to 34 tons/ft3 for relatively loose to
, q -, ! san^‘
1X11’ (Terzaghi, 1955). With the value of £e from équations
p,,..!:)111 'jL 'u
* r
* '
can ca^cu^ated from classical beam theory for
antilerered beam of length t. as: k, = iEI/Ç. Kocsis (1976) also presented