Civil Engineering Reference
In-Depth Information
FIGURE 8.7
Illustration of a punching shear analysis. The dashed lines represent unliq-
uefiable soil that is underlain by a liquefied soil layer. In the analysis, the footing will punch
vertically downward and into the liquefied soil.
where
R
shear resistance of soil. For strip footings,
R
is the shear resistance per unit
length of footing, lb/ft or kN/m. For spread footings,
R
is the shear resistance
beneath entire footing perimeter, lb or kN.
P
footing load. For strip footings,
P
is the load per unit length of footing, lb/ft or
kN/m. For spread footings,
P
is total load of footing, lb or kN. The footing load
includes dead, live, and seismic loads acting on footing as well as weight of foot-
ing itself. Typically the value of
P
would be provided by the structural engineer.
T
vertical distance from the bottom of footing to top of liquefied soil layer, ft or m.
f
shear strength of unliquefiable soil layer, lb/ft
2
or kPa.
B
width of footing, ft or m.
L
length of footing, ft or m.
L
) represents the entire perimeter of the spread
footing. When this term is multiplied by
T,
it represents the total perimeter area that the
footing must push through in order to reach the liquefied soil layer. For an assumed foot-
ing size and given loading condition, the only unknowns in Eqs. (8.1
a
) and (8.1
b
) are the
vertical distance from the bottom of the footing to the top of the liquefied soil layer
T
and
the shear strength of the unliquefiable soil layer
Note in Eq. (8.1
b
) that the term 2(
B
f
.
The value of
T
would be based on the
liquefaction analysis (Chap. 6) and the proposed depth of the footing. The shear strength of
the unliquefiable soil layer
f
can be calculated as follows:
1.
For an unliquefiable soil layer consisting of cohesive soil (e.g., clays), use a total
stress analysis:
f
s
u
(8.2
a
)
or
f
c
h
tan
(8.2
b
)
