Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 631

It may be inferred from this figure that the two individual footings is a problem of

statics if the allowable soil pressure is known and if the dimension b of the exterior footing is

either fixed or assumed. Also, the centroid of the two areas must lie on the line of the action of

the resultant load. This requirement may not be obvious because the two areas are usually

found rather independently from ractions determined from the principles of statics.

15.5.2Basis for Design of Strap Footings

Strap footings are designed based on the following assumptions:

(i) The strap footing is considered to be infinitely stiff. It serves to transfer the column

loads onto the soil with equal and uniform soil pressure under both the footings.

(ii) The strap is a pure flexure member and does not directly take soil reaction. The soil

below the strap will be loosened up in order that the strap does not rest on the soil

and exert pressure.

With these assumptions, the procedure of design is simple. With reference to Fig. 15.25,

it may be given as follows:

e l

Ri q Re

q

Pe Pi

q = Allowable bearing pressure
Ri = Pe(1 + e/l)
Re = Pi – Pe. e/l
Fig. 15.25 Design of strap footing
Assume a trial value of e and compute the reactions Ri and Re from statics. The tenta-
tive areas of the footing are equal to the reactions Ri and Re divided by the allowable bearing
pressure q. The value of e is computed with tentative sizes. These steps are repeated until the
trial value of e is identical with the final value.
The shearing force and bending moment in the strap are determined, the strap being
designed to withstand the maximum values of these.
Each of these footings is assumed to be subjected to uniform soil pressure and designed
as simple spread footings. Under the assumptions given above, the resultant of the column
loads Pe and Pi would coincide with the centre of gravity of the areas of the two footings.

15 .6 Combined Footings

The use of combined footings is appropriate either when two columns are spaced so closely

that individual footings are not practicable or when a wall column is so close to the property

line that it is impossible to center an individual footing under the column.

A combined footing is so proportioned that the centroid of the area in contact with the

soil lies on the line of action of the resultant of the loads applied to the footing; consequently,