DHARM
622 GEOTECHNICAL ENGINEERING
(viii) Service loads for all other columns are computed.
(ix) The area of the footing for each of the other columns is obtained by dividing the
corresponding service load by the reduced allowable bearing pressure of step (vii).
The advantage in this procedure is that the allowable bearing pressure of the soil is
never exceeded under any circumstances and the reduced or service loads, which are effective
during most of the time are expected to result in equal settlements.
The procedure, as standardised by the ISI, is set out in ‘‘IS: 1080-1985 Code of Practice
for Design and Construction of simple spread foundations (Second Revision)”.
15.4.6Footings Subjected to Moments—Eccentric Loading
Footings supporting axially loaded columns and which are symmetrically placed with respect
to the columns will be subjected to uniform soils pressures. However, footings may often have
to resist not only axial loads but also moment about one or both axes. The moment may exist at
the bottom of an axially loaded column, whence it is transmitted to the footing; alternatively,
it may be produced by an axial vertical load located eccentrically from the centroid to the base
of the footing, positioned unsymmetrically with respect to the column. If the moment in the
first case is equal to the product of the axial load and eccentricity in the second, the soil pres-
sure distribution will be just identical. Thus, the substitution of an equivalent eccentric load
for a real moment is considered a convenient method which simplifies computations in some
cases.
Foundations for retaining walls may have to resist moments due to the active earth
pressure and those for bridge piers may have to resist moments produced primarily by wind
and traction on the superstructure. These foundations also have to be treated in a somewhat
similar manner as footings subjected to moments.
Once the soil reactions are determined, the design data such as critical moments and
shears may be obtained as a prerequisite for the structural design. Fundamental to all these
computations are the laws of statics. The distribution of vertical soil pressure at the base must
satisfy the requirements of statics that (i) the total upward soil reaction must be equal to the
sum of the downward loads on the base, and (ii) the moment of the resultant vertical load
about any point must equal to the moment of the total soil reaction about the same point. In
addition, an adequate horizontal soil reaction must be available, by virtue of frictional resist-
ance at the base, to oppose the resultant horizontal load.
Ordinary footings are commonly assumed to act as rigid structures. This assumption
leads to the conclusion that the vertical settlement of the soil beneath the base must have a
planar distribution since a rigid foundation remains plane when it settles. Another assump-
tion is that the ratio of pressure to settlement is constant, which also leads to the conclusion
regarding the planar distribution of soil pressure. Although, neither of these assumptions is
strictly valid, each is considered to be sufficiently accurate for ordinary purposes of design.
Two distinct cases arise:
(1) Resultant force within the middle third of the base; and,
(2) Resultant force outside the middle third of the base.