Spread Footings - Shallow Foundation
In case of spread footings, the base of the member transfering the load to the soil is made wider so as to distribute the load over wide and larger area. The various spread footings described under this classification are
(1) Wall footings
(2) Reinforced concrete footings
(3) Inverted arch footings
(4) Column footings.
(1) Wall Footings:
Wall Footings consists of several courses of bricks, the lowest course will be twice the breadth of the wall above. The increase in base width of the wall is achieved by providing 5 cm (½ brick length) offsets on either side of the wall. The depth of each course is usually 10 cm (thickness of one brick). In some cases, however,the bottom courses are made 20 cm. (depth of two bricks) deep. The footing of various wall thicknesses have been shown in the pic below. In case of footings for stone walls, the size of offsets is slightly more than that of the brick wall footing. A bed of lean concrete of uniform thickness is first spread over the entire length of the wall. The depth of lean concrete bed is normally not less than 15 cm. and its projection on either side of the wall base varies between 10 to 15 cm. In any case, the depth of the concrete bed should never be less than its projection beyond the wall base. The concrete bed provides a plain surface upon which the brick masonry work for wall footing can be started directly. It not only rectifies the inequalities of excavation but also bridges over soft patches in the soil below.
(2) Reinforced Concrete Footings:
In buildings where the walls are subjected to relatively heavy loading and the bearing capacity of the soil is low on which the wall footing is to rest, then thewall footing described above will work out to be massive in size. In such situations, it is desirable to provide reinforced concrete footing below the wall. This appreciably reduces the volume of masonry work in footing and depth of bed concrete and as such proves to be economical. A 7 to 8 cm. thick bed of lean concrete is usually provided below the reinforced concrete footing to perform thc function of concrete bed block.
(3) Inverted Arch Footings:
This type of foundation used to be provided for multistoried buildings in olden times. However, with the advent of reinforced cement concrete construction practice, inverted arch construction is rarely done these days. One of the drawbacks in this type of construction is that the end piers have to be specially strengthened by buttresses to avoid the arch thrust tending to rupture the pier junction. However, the advantage of inverted arch construction is that in soft soils the depth of foundation is greatly reduced. Inverted arch footing is shown below.
(4) Column Footings (Independent footings):
An independent footing is one which is provided under a column or other similar member for distributing the concentrated loads in the form of uniformly distributed load on the soil beneath. The footing may be square, rectangular or circular in plan. Depending upon the load to be carried and the bearing capacity of the soil, independent footing may be of brick masonry, stone masonry, R.C.C., steel grillage etc.
(i) Footing for brick pillars:
Square footing is the simplest and most economical to be provided under pillars, columns or stanchions. The area of the base is calculated by dividing the total load to which the pillar is subjected to, by the safe bearing capacity of the soil. The desired base width is obtained by means of offsets which run symmetrical all-round the pillar.
(ii) R.C.C. Column footings:
On account of their low bending strength the footings constructed with brick, stone or plain concrete require considerable depth to be safe to carry heavy loads. The depth of plain concrete footing can be appreciably reduced by providing reinforcements at its base to take up tensile stresses. R.C.C. column footings may be circular, rectangular or square in plan. The footing is reinforced both-ways by means of mild steel ribbed bars placed at right angles to one another at equal distances apart.
(iii) Stone pillar footings:
This type of footing is similar in construction to the footing for brick pillar. In this case, the regular offsets on all the four sides of pillar are slightly bigger in width and depth.