DHARMSHALLOW FOUNDATIONS 621Footings on clay will experience settlement which consists of three components (Skempton
and Bjerrum, 1957):
S = Si + Sc + Ss ...(Eq. 15.1)where, S = Total settlement,
Si = Immediate elastic settlement,
Sc = Consolidation settlement due to primary compression, and
Ss = Settlement due to secondary compression of the clay.
These and other details of settlement analysis have already been dealt with exhaus-
tively (Chapter 11).15.4.5 Proportioning Sizes of Footings and Choice of Column Loads
A structure is usually supported on a number of columns. These columns usually carry differ-
ent loads depending upon their location with respect to the structure. Differential settlements
are minimised by proportioning the footings for the various columns so as to equalise the
average bearing pressure under all columns.
But each column load consists of dead load plus live load. The full live load does not act
all the time; further live loads such as those due to heavy wind do not produce significant
settlement since they act only for short durations; this is especially true in the case of cohesive
soils. Hence, dead load plus full live load is not a realistic criterion for producing equal
settlement.
What is known as the ‘service load’ is a better criterion. This is the actual load expected
to act on the foundation during the normal service of the structure, i.e., for most of the time. In
ordinary buildings, this is taken as the dead load plus one half the live load; a larger fraction of
the live load should be used for warehouses and other industrial structures.
The following procedure is given by Teng (1976) based on the recommendations of Peck,
Hanson and Thornburn (1974):
(i) Dead load, inclusive of self-weight of column and estimated value for footing, is
noted for each column footing.
(ii) The live load for each column is calculated (appropriate values are chosen from the
relevant I.S. Codes of Practice).
(iii) The ratio of live load to dead load is calculated for each column footing; the maxi-
mum value of this ratio is noted.
(iv) The allowable bearing pressure of the soil is determined by the procedures given in
Chapter 14.
(v) For the footing with the largest live load to dead load ratio, the area of footing re-
quired is calculated by dividing the total load (dead load plus maximum live load) by
the allowable bearing pressure of the soil.
(vi) The service load for the column with the maximum live load to dead load ratio is
computed by adding the appropriate fraction of the live load to the dead load.
(vii) The allowable bearing pressure to be used for all the other column footings is ob-
tained by dividing the service load for the column with maximum live load to dead
load ratio by the area of the footing for this column (This pressure will be obviously
somewhat less than the computed allowable bearing pressure of step (iv).