Geotechnical Engineering

DHARM

626 GEOTECHNICAL ENGINEERING

The location of the maximum and minimum soil pressures may be determined readily
by observing the directions on the moments. Likewise, the proper signs in Eq. 15.12 may be
determined by inspection for any other point on the base of the footing.

If the minimum soil pressure computed appears to be negative, there exists a zone like
CZZ in which the footing loses contact with the soil and hence, there will be no pressure in the
zone. Equation 15.12 will not be applicable to this case. For the determinations of soil pres-
sures for this situation, the reader is referred to Peck, Hanson and Thornburn (1974), who give
an excellent trial and error procedure.

Useful width concept

For the determination of the bearing capacity of an eccentrically loaded footing, the concept of
‘useful width’ has been introduced. By this concept, the portion of the footing which is sym-
metrical about the load is considered useful and the other portion is simply assumed superflu-
ous for the convenience of computation (Teng, 1976). This is illustrated in Fig. 15.18.

L

L =(L–2e )¢ L

b =(b–2e )¢ b

eL

—–eL 2 L

eb —–e

b 2 b

b

P

Fig. 15.18 Useful width concept for eccentrically loaded footings
If the eccentricities are eb and eL, as shown, the useful widths b′ and L′ are:
b′ = b – 2eb L′ = L – 2eL ...(Eq. 15.13)
The equivalent area A′ is considered to be subjected to a central load for the determina-
tion of bearing capacity:

A′ = b′L′ = (b – 2eb)(L – 2eL) ...(Eq. 15.14)
The procedure may be used even if the eccentricity is with respect to one of the axes
only.

This concept simply means that the bearing capacity of a footing decreases linearly with
the eccentricity of load. This is almost true in the case of cohesive soils; however, the relation-
ship is parabolic rather than linear in the case of granular soils (Meyerhof, 1953).

Therefore, it is considered better to use a reduction factor Re for getting the reduced
bearing capacity due to eccentricity of loading:

q′ult = qult. Re ...(Eq. 15.15)

where q′ult = bearing capacity of an eccentrically load footing size b × L,

qult = beairng capacity of a centrally loaded footing of size b × L, and

Re = reduction factor for eccentricity.