Civil Engineering Reference
In-Depth Information
8.16
Use the following values:
b
9.7 kN/m
3
N
21
(entering Fig. 8.11 with
32° and intersecting
N
curve,
N
from vertical axis is 21)
B
1.5 m
for strip footings
2.5 m
for spread footings
r
u
0.3
(entering Fig. 5.15 with factor of safety against liquefaction
1.2,
r
u
for sand varies from 0.12 to 0.46. Using an average value,
r
u
0.3)
Insert the above values into Eq. (8.10).
For the strip footings:
q
ult
1
⁄
2
(1
r
u
)
b
BN
1
⁄
2
(1
0.3) (9.7 kN/m
3
) (1.5 m) (21)
107 kPa
and using a factor of safety of 5.0 gives
q
all
q
ult
FS
107 kPa
___
_______
5.0
21 kPa
For the spread footings:
q
ult
0.4 (1
r
u
)
b
BN
0.4 (1
0.3) (9.7 kN/m
3
) (2.5 m) (21)
140 kPa
and using a factor of safety of 5.0 gives
q
all
q
ult
FS
140 kPa
___
_______
5.0
28 kPa
Thus, provided the strip and spread footings are at least 1.5 and 2.5 m wide, respec-
tively, the allowable bearing capacity is equal to 21 kPa for the strip footings and 28 kPa
for the spread footings. These allowable bearing pressures would be used to determine the
size of the footings based on the anticipated dead, live, and seismic loads.
From Fig. 8.12, the fully weakened shear strength
25 lb/in.
2
3600 lb/ft
2
and
8.17
Q
1000 (20,000 kips)
___
________________
BL
5300 lb/ft
2
q
75 (50)
Using Eq. (8.11
b
) gives
1
0.3
B
1
0.3
B
__
__
q
ult
cN
c
L
5.5
s
u
L
50
___
5.5
s
u
1
0.3
75
6.6
s
u
(6.6) (3600)
23,800 lb/ft
2
FS
q
ult
q
23,800
___
______
5300
4.5
