Shallow Foundations

Combined Footings and Rafts

G. R. Dodagoudar

DEPARTMENT OF CIVIL ENGINEERING

I I T MADRAS, CHENNAI - 600 036.

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,

the distribution of soil pressure is reasonably

uniform.

In addition, the dimensions of the footing are

chosen such that the allowable soil pressure is not

exceeded. When these criteria are satisfied, the

footing should neither settle nor rotate excessively.

Problems

Proportion a strap footing for the following data: Example 1

Solution:

Example 2 Proportion a rectangular combined footing for uniform

pressure under dead load plus reduced live load. Use

the following data.

Solution:

Example 3 Proportion a trapezoidal combined footing for uniform

pressure under dead load plus reduced live load. Use

the following data.

Solution:

Rafts

A ‘raft’ or a ‘mat’ foundation is a combined footing which

covers the entire area beneath a structure and supports all

the walls and columns.

This type of foundation is most appropriate and suitable when

the allowable soil pressure is low, or the loading is high, and

spread footings would cover more than one half the plan

area.

Also, when the soil contains lenses of compressible strata

which are likely to cause considerable differential settlement,

a raft foundation is well-suited, since it would tend to bridge

over the erratic spots, by virtue of its rigidity.

On occasions, the principle of floating foundation may be

applied best in the case of raft foundations, in order to

minimize settlements.

Mats/Rafts

Problems

A raft, 9 m × 27 m, is founded at a depth of 3 m in sand with

a value of N = 25 upo great depth. Determine the total load

which the raft can support.

If the raft is designed as a floating foundation, what will be

the load it can support ?

Example 2

Solution: