Civil Engineering Reference
In-Depth Information
Table 12.1
Maximum Allowable Soil Pressure
Maximum allowable soil pressure
U.S. Customary Units
(kips/ft
2
)
SI Units (kN/m
2
)
Class of material
20% of ultimate
20% of ultimate
Rock
crushing strength
crushing strength
Compact coarse sand, compact fine sand,
hard clay, or sand clay
8
385
Medium stiff clay or sandy clay
6
290
Compact inorganic sand and silt mixtures
4
190
Loose sand
3
145
Soft sand clay or clay
2
95
Loose inorganic sand-silt mixtures
1
50
Loose organic sand-silt mixtures, muck,
0
0
or bay mud
strength method using this ultimate soil pressure. This simple procedure is used for the
footing examples here.
The 1971 ACI Commentary (15.2) provided an alternate method for determining the
footing area required that will give exactly the same answers as the procedure just de-
scribed. By this latter method the allowable soil pressure is increased to an ultimate value
by multiplying it by a ratio equal to that used for increasing the magnitude of the service
loads. For instance, the ratio for
D
and
L
loads would be
1.2
D
1.6
L
Ratio
D
L
Or for
D
L
W
, and so on
1.2
D
1.6
W
1.0
L
0.5(
L
r
or
S
or
R
)
Ratio
D
L
W
(
L
r
or
S
or
R
)
The resulting ultimate soil pressure can be divided into the ultimate column load to deter-
mine the area required.
12.5
DESIGN OF WALL FOOTINGS
The theory used for designing beams is applicable to the design of footings with only a few
modifications. The upward soil pressure under the wall footing of Figure 12.3 tends to bend
the footing into the deformed shape shown. The footings will be designed as shallow beams
for the moments and shears involved. In beams where loads are usually only a few hundred
pounds per foot and spans are fairly large, sizes are almost always proportioned for moment.
