Geotechnical Engineering

DHARM

402 GEOTECHNICAL ENGINEERING

maximum value, especially during the early stages of settlement. However, the structural
members such as the columns and footings must be designed for maximum loads.

The structural engineer determines the column loads based on the assumption that all
columns undergo equal settlement. This assumption is reasonable for a large and perfectly
flexible structure, such as one with timber framing and brick bearing walls, in which consider-
able unequal settlements can occur without causing significant changes in load distribution.
However, in the case of small structures of concrete or steel framing, the settlement of any
individual footing causes considerable readjustment in the load on this and the adjacent footings.

This is analogous to the settlement of supports of a continuous beam. For example, if
the middle support of a three-span continuous beam settles, the reaction or the load on it gets
reduced and that on the other supports gets increased correspondingly. Depending on the
magnitude of the settlement, the middle support may not carry any load at all, having trans-
ferred the entire load to the other supports. Although it is possible to predict the changes in
the column loads consequent to known differential settlements, the procedures are cumber-
some. Thus, it is common in settlement analysis to assume column loads for equal settlement.
The assumption of flexible construction is always on the safe side since it leads to greater
differential settlements than actually occur. The effect of rigidity is, therefore, a desirable one,
both for the building as a whole and with reference to local irregularities. In rigid construction,
the start of settlement at a footing immediately transfers much of the footing load to the
adjacent footings, thus greatly relieving all undersirable effects.

For a clearer understanding let us consider the two types of buildings founded on a
compressible foundation, as shown in Fig. 11.6.

Flexible building

1

4

Rigid building

1

4

3

2

(a) (b)

3

2

Fig. 11.6 Pressure distributions and settlement patterns for flexible and rigid structures

underlain by buried compressible strata (After Taylor, 1948)