66. Footings under walls are used for two purposes: 1. To spread the weight over a greater area. 2. To add to the stability of the wall. Under buildings of only two or three stories, the latter function is generally the more important.
The width of the footings should be at least 12 inches wider than the thickness of the wall above, and also such that the pressure per square foot under the footing will not exceed the safe bearing power of the soil or the material on which it rests. (See Section 16.)
For nearly all classes of buildings built on solid ground cement concrete makes probably the best material for the bottom footing course, especially for the money expended. Concrete possesses the advantage over large blocks of stone of having considerable transverse strength, so that when fully hardened it is much like a wide beam laid on top of the ground under the walls; and should a weak spot occur in the ground under the footing it would probably have sufficient transverse strength to span it if the spot were not very large. Concrete must also necessarily bear evenly over the bottom of the trenches, so that there can be no cavities, as is sometimes the case with stone footings. In localities where large blocks of granite or flagging cannot be cheaply procured, concrete makes much the cheapest footing.
In stiff soils trenches for the concrete footing should be dug below the general level of the excavation and of the exact width of the footings, so that when the concrete is put in and tamped it will bear against the sides as well as the bottom of the trench. In sandy soils this of course cannot be done, and planks must be set up and held in place by stakes to form the sides of the trench. After the cement has set, but not before, the planks may be removed.
Concrete for footings should be mixed in the proportion of 1 part cement to 2 of sand and 4 of stone for natural cements, and 1 to 2½ and 5½ for Portland cement. The thickness of the concrete should be one-fourth of its width, and never less than 12 inches, except under very light buildings. The concrete should be put in in layers about 6 inches thick. If the footing is considerably wider than the. wall it may be stepped in by setting up plank to hold the upper layers of concrete, or a stone footing of proper width may be placed on top of the concrete, as in Fig. 27. The latter is apt to give the best results.
For the manner of mixing the concrete see Section 142 and specifications in Chapter X (Iron And Steel Supports For Mason Work. - Skeleton Construction). For width of offsets see Section 70.
For buildings of moderate height stone footings are generally the most economical, and if they are carefully bedded, answer as well as concrete.
If practicable, the bottom footing course should consist of single stones of the full width of the footing, and the thickness of the stones should be about one-fourth of their width, depending much, however, upon the kind of stone. If stone of sufficient width cannot be obtained, the stone may be jointed under the centre of the wall, and a second course consisting of a single stone placed on top, as shown in Fig. 28.
For light buildings of only one or two stories, used for dwellings or similar purposes, irregular shaped stones, called "heavy rubble," are generally used, as shown in Fig. 29, which represents a plan of the footing course, the spaces between the larger stones being filled in with smaller stones. Each stone should be laid in mortar and the spaces between the stones solidly filled with mortar and broken stone.
Under heavy buildings the footing stones should be what are called "dimension stones," that is, they are roughly squared to certain dimensions, Dimension stones for footings may be obtained from 4 to 8 feet in length, according to the kind of stone. The width of the stones, measured lengthways of the wall, should be at least 2 feet, or two-thirds the width of the footings.