Building Design and Construction Handbook - Merritt - Ventech!

6.66 SECTION SIX
TABLE 6.14 Allowable Bearing Pressures
Material type
Allowable bearing
pressure a
Maximum allowable
bearing pressure b
Massive crystalline bedrock 4,000 psf (200 kPa) 12,000 psf (600 kPa)
Sedimentary and foliated rock 2,000 psf (100 kPa) 6,000 psf (300 kPa)
Gravel and sandy gravel (GW, GP) 2,000 psf (100 kPa) 6,000 psf (300 kPa)
Nonplastic soil: sands, silts, and NP silt 1,500 psf (75 kPa) 4,500 psf (220 kPa)
(GM, SW, SP, SM) c
Plastic soil: silts and clays (ML, MH, SC,
CL, CH) c
1,000 psf (50 kPa) 3,000 psf (150 kPa) d
a Minimum footing width and embedment depth equals 1 ft (0.3 m).
b An increase of 20% of the allowable bearing pressure is allowed for each additional foot (0.3 m) of
width or depth up to the maximum allowable bearing pressures listed in Column 3. An exception is plastic
soil, see note d.
c Group symbols from Table 6.8.
d No increase in the allowable bearing pressure is allowed for an increase in width of the footing.
For dense or stiff soils, allowable bearing values in this table are generally conservative. For very loose
or very soft soils, the allowable bearing values may be too high.
Source: Data from ‘‘Uniform Building Code’’ (1997)
There are many charts, graphs, and figures that present bearing capacity factors
developed by different engineers and researchers based on varying assumptions. For
example, Fig. 6.28 presents bearing capacity factors N c, N �, and N q, which automatically
incorporate allowance for punching and local shear failure. Another example
is Fig. 6.29, which presents bearing capacity factors that have not been
adjusted for punching or local shear failure. Figure 6.29 also presents the bearing
capacity equations for square, rectangular, and circular footings. The equations for
granular soil (i.e., cohesionless soil, c � 0) and for a total stress analysis for cohesive
soil (i.e., � � 0 and c � s u) are also shown in Fig. 6.29.
Other Footing Loads. In addition to the vertical load acting on the footing, it
may also be subjected to a lateral load. A common procedure is to treat lateral
loads separately and resist the lateral loads by using the soil pressure acting on the
sides of the footing (passive pressure) and the frictional resistance along the bottom
of the footing.
It is always desirable to design and construct shallow footings so that the vertical
load is applied at the center of gravity of the footing. For combined footings that
carry more than one vertical load, the combined footing should be designed and
constructed so that the vertical loads are symmetric. There may be design situations
where the footing is subjected to a moment, such as where there is a fixed-end
connection between the building frame and the footing. This moment can be represented
by a load Q that is offset a certain distance (known as the eccentricity)
from the center of gravity of the footing. For other projects, there may be property
line constraints and the load must be offset a certain distance (eccentricity) from
the center of gravity of the footing. Because an eccentrically loaded footing will
create a higher bearing pressure under one side as compared to the opposite side,
one approach is to evaluate the actual pressure distribution beneath the footing. The
usual procedure is to assume a rigid footing (hence linear pressure distribution) and
use the section modulus ( 1 ⁄6B 2 ) in order to calculate the largest and lowest bearing
pressure. For a footing having a width B, the largest (q�) and lowest (q�) bearing
pressures are as follows: