Civil Engineering Reference
In-Depth Information
Table 17.3
Foundation modeling methods
Foundation type
Modeling method I
Modeling method II
Spread footing
Rigid
Foundation spring required if footing
flexibility contributes more than
20
%
to pier displacement
Pile footing with
pile cap
Rigid
Foundation spring required if footing
flexibility contributes more than
20
%
to pier displacement
Pile bent/drilled shaft
Estimate depth
to fixity
Estimate depth to fixity or soil
springs based on
p
-
y
curves
1
G
& ν
K
= α
β
K
0
3
2
Circular
footing
stiffness
K
0
R
2
B
2
L
Embedment
factor
D
D
β
R
R
2
B
2
L
L
B
Shape
factor
γ
Figure 17.13
Half-spaced method for spread footings. (Data from NHI Course No. 13063
“Seismic Bridge Design Applications,” April 25, Publication No. FHWA-
SA-97-017 [Part One] and -018 [Part Two], 1996.)
(NHI 1996). One is elastic foundation method, and another is elastic half-
space method (Figure 17.13).
In elastic foundation method,
k
s
for vertical stiffness (or subgrade reac-
tion coefficient) and
k
r
for rotation stiffness can be determined by
P
3
3
k
s
=
(
in kip/ft or kN/m
)
(17.23)
(
Area Deflection
)(
)
L B
3
in
kip ft
rad
or
kN m
rad
−
−
k
=
k
(17.24)
r
s
16
where:
P
is the vertical load on the mat foundation
L
and
B
are the half sizes of the mat in their respective longitudinal and
transverse directions
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