This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
Footings should be designed for the load they are to carry, with the object of producing a uniform settlement in all parts of the building. They evidently should not be as wide under an opening as under a solid wall, and when the openings form a considerable proportion of the wall area, that part of the footings under them should be omitted, the weight being transmitted by arches or beams to the footings under the sides of the openings. This caution is very important, as a majority of cracks in masonry are probably due to continuous footings where little or none are needed. If one portion of the foundation, as, for example, that under a tower, carries much more weight than another part, its width should be proportionately increased. In designing footings, the center of the wall should be placed vertically over, or, preferably, a little inside the center of the base, as sketched in Fig. 2; then the walls will be slightly tilted inwards, but will be kept in place by the floor joists, etc. Where there are interior piers, these should have a somewhat greater load per square foot - that is, less area per ton of load - than the walls, so that the latter will be pressed together, thus preventing cracks; also, as the piers usually support iron columns, to allow for the compression in the brickwork joints, the iron being practically incompressible.
Footing courses should be battered or stepped up, making the angle a b c in Fig. 3 (a), about 60°. The load then becomes well distributed over the base. If the footings are laid as shown at (b), the projections are liable to break off at the edges of the wall, and the load will be unevenly carried by the soil.
To show the method of proportioning footings (and of figuring loads in structures), the area of the footings for a 45'X60' brick warehouse, shown in Fig. 4, having five stories and basement, a tar-and-gravel roof, tile arch floors, and without partitions, will be determined. There are two rows of columns, spaced 14 1/2 ft. apart longitudinally and transversely. The walls of the building are 75 ft. high, 25 ft. being 20 in. thick, and 50 ft.. 16 in. thick. As the basement floor rests directly on the ground, its load will not be consid-ered. The floor loads on the first and second stories will be taken at 200 lb. per sq. ft., and on the others at 150 lb.
Assume that brickwork weighs 120 lb. per cu. ft.; the tile floor 80 lb. per sq. ft.; the tar-and-gravel roof 10 lb. per sq. ft.; and the snow load at 12 lb. per sq. ft. Then, for each foot in length of the side walls, the load is:
1 ft. 8 in. X 1 ft. X 25 ft. = 41.7 cu. ft.
1 ft. 4 in. X 1 ft. X 50 ft. = 66.7 cu. ft.
108.4 cu. ft. X 120 lb. =
(80 1b. per sq.ft. X 1ft. X 7 1/4 ft.) X 5 =
10 lb. per sq. ft. x 1 ft. X 7 1/4 ft. =
On Floors -
(200 lb. per sq. ft. X 1 ft. X 7 1/4 ft.) X 2 = 2,900
(150 lb. per sq. ft. X 1 ft. X 7 1/4 ft.) X 3 = 3,262
Wind, neglected on nearly flat roof
12 1b. per sq.ft. X 1 ft. X 7 1/4. =
Total dead and live load =
Assume that, upon testing, the soil has been found to be moderately dry clay. By reference to page 74, the average safe load for this soil is found to be 3 tons per sq. ft. Hence, dividing the total load. 22,229 lb., by 6,000 lb., there results 3 2/3 ft. as the approximate width of the footings for the side walls. If the foundation walls are made 6 ft. deep, and battered 1 in. per foot, the top width will be 2 2/3 ft. Thus far the weight of the foundation wall has not been considered. Having obtained the approximate width of the footing, its weight can now be computed. The average width of the foundation wall is 3 ft. 2 in., and it is 6 ft. deep; the contents will be 19 cu. ft. per foot of length; the walls are good limestone rubble, weighing 150 lb. per cu. ft., so that the total weight will be 2,850 lb. Adding this to 22,229 lb. and dividing by 6,000 lb., the unit load, the final result is about 4 1/4 ft. as the width of the footing. The footings for the end walls and piers may be similarly figured.