This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
65. It is very important that the foundations, whether continuous, as in a foundation wall, or isolated, as when divided into piers, should have the footing courses proportioned to the weight they will be required to carry, and to the bearing capacity of the soil.
The pressure on the soil from each square foot of the footings should be the same, where the soil is uniform, and at no place must the bearing power of the soil be exceeded. To secure the most satisfactory results, therefore, the footings must be proportioned to properly distribute the weight they are to carry over sufficient areas of ground, to secure uniform settlement in each case. If these conditions were always properly considered, there would be few cracks in the mason work, as such cracks are caused usually by unequal settlement. A uniform settlement even of an inch or more would in most buildings pass unnoticed.
66. In order to proportion the area of the footings, the weights coming on each pier, and the weight of, and loads carried by, all the walls should be computed, and entered in a memorandum book for reference. The ground should be examined, and by means of Table 1 - the bearing power of different soils, see Art. 19 - the load per square foot which it is deemed advisable the footings shall carry, may be determined. The load on the various footings divided by this unit load, will give, as results, the proper area of each, in square feet.
The pressure under a brick pier which supports a tier of columns may be assumed at 10 per cent. less than the calculations show, when the exterior support of the building is a brick wall; for the joints in the brickwork will close slightly under the weight and cause about 10 per cent. more settlement than will exist in the columns, each of which, being one piece, is practically jointless, and hence will settle less.
67. One of the objects in proportioning the footings is to provide for uniform settlement in all parts of the building, so that the floors may remain level, and that no cracks may occur in the walls. Therefore, the loads for which the footings are proportioned should be as near the actual conditions as possible, or, as stated in the Chicago building law, "Foundations shall be proportioned to the actual average loads they will have to carry in the completed and occupied building, and not to the theoretical or occasional loads."
Thus it will be seen that the dead load under the walls of a five-story building would be a considerable item, while the dead load under a tier of iron columns would be much less in proportion to the floor area supported; and as the dead load is always constant, and the live load may vary greatly, only the amount of live load that will probably be supported by the footings should be considered.
68. For warehouses, stores, etc., 50 per cent. of the live load that the floorbeams have to carry should be added to the dead load carried on the footings. For office buildings, hotels, dwelling houses, etc., the weight of the people occupying them need not enter into the calculations for proportions of footings, and only from 25 to 80 pounds per square foot of floor need be allowed for the weight of furniture, books, safes, etc. It has been proved by statistics that the average permanent loads do not exceed the above limits. For theaters, halls, etc., a larger allowance should be made for the weight of people, but even a densely packed crowd of men will not weigh more than 100 pounds per square foot of floor.
69. Fig. 22 represents a building, the footings of which are to be proportioned. At (a) is shown the plan; at (b), the longitudinal section; and at (c), the transverse section of a six-story warehouse and its basement or cellar, supposed to be built on an ordinary sand and gravel soil. The building is 50 feet wide, with a double row of longitudinal columns a supporting iron girders b. What will be the dimensions of the footings under the walls and the columns? The load on 1 lineal foot of the side walls will be about 140 cubic feet of brick and stonework, weighing about 17,160 pounds; 1 foot of wall has 8 square feet of each floor, shown at c, and an equal area of the roof d to support. The floors are assumed to be constructed of iron beams filled in between with hollow terracotta tile, with cement filling on top. This floor will weigh, altogether, 75 pounds to the square foot of surface. The roof, shown at d, is also of fireproof construction, but with lighter iron beams, and weighs 60 pounds to the square foot. Thus the dead load from the six floors and the roof would amount to 8(6 X 75 + 60) = 4,080 pounds. The first, second, and third floor are supposed to carry 150 pounds to the square foot, and the fourth, fifth, and sixth floor, 100 pounds per square foot. The weight of snow on the roof is taken as 12 pounds per square foot. The total live load on the footings amounts to 8(3x150 + 3x100+12) = 6,096 pounds per square foot. If the three loads - the wall, floors, and live load - are added, we have 17,160 + 4,080 + 6,096 = 27,336 pounds on each lineal foot of the footing.
The soil will safely carry 6,000 pounds (3 tons) per squarefoot; dividing the load by 6,000, we obtain 4 1/2 feet as the required width of the footing, as shown at e on the plan (a), Fig. 22. To obtain the load on the footings under the columns, we must take the weight of the floors and the roof, together with the live load, the weight of the columns themselves being so little, in proportion to the other loads, that it need not be considered in the present example.
Supposing the columns to be spaced 14 feet apart longitudinally and 16 feet transversely, each column would support 224 square feet of floor; so we have a dead load on the footings under the columns amounting to 224(6 X 75 + 60) = 114,240 pounds, and a live load of 224(3x150 + 3x100 + 12) = 170,688 pounds, a total of 284,928 pounds. Dividing by 6,000, gives us 47 1/2 square feet as the area of the footing, or nearly 7 feet square. The footings of the front and rear walls may be Figured similarly. These footings are shown at g, plan (a), Fig. 22.
70. A memorandum should be made of the calculations for the foregoing weights, etc., as follows:
Data for Footings of Warehouse for Thomas Tucker, No. 1941 Main St., Tuckerville.
Under One Foot of Side Walls.
Cu. ft. of brickwork 108 @ 120 =
Cu. ft. of stonework 28 @ 150 =
Total weight of wall..........
Floor area supported, 8 sq. ft.
Weight of floor per sq. ft., 75 lb.
Weight of roof per sq. ft., 60 lb.
Total for six floors and roof
510 lb. X 8 sq. ft. =
Live load per sq. ft.
1st, 2d, and 3d floor, 150 lb.
4th, 5th, and 6th floor, 100 lb.
Weight of snow, 121b.
Total live load......8 X 762 -
Assumed bearing load of soil 6,000 lb. per square foot. Width of footing course under walls 4 ft. 6 in. X 2 ft. thick. Under columns 47 1/2 sq. ft., 7 ft. X7 ft. and 2 ft. thick.