This section is from the book "Building Construction And Superintendence", by F. E. Kidder. Also available from Amazon: Building Construction And Superintendence.
We will suppose that a six-story and basement warehouse is to be erected on an ordinary sand and gravel foundation. The building will be 50 feet wide, with two longitudinal rows of columns and girders. What should be the width of the footings under the walls and columns?
Answer. - For the load on one lineal foot of footing under the side walls we will have about 140 cubic feet of brick and stone work, weighing about 17,000 pounds.* One lineal foot of wall will also support about 8 square feet of each floor and the roof. We will assume that the floors are of iron beams and terra cotta tile, with concrete filling, weighing altogether 75 pounds to the square foot, and the roof of the same material, weighing 60 pounds. Then the dead load from the six floors and roof would amount to 4,080 pounds. The first, second and third floors are intended to support 150 pounds to the square foot, and those above 100 pounds per square foot. The possible weight of snow on the roof we will not take into account. There might then be a possible live load on the footing of 6,000 pounds, but as it is improbable that each floor will be entirely loaded at the same time, and as some space must be reserved for passages, etc., the actual live load would probably not exceed for any length of time 50 per cent, of the assumed load, or 3,000 pounds. Adding these three loads together (the wall, floors and live load) we have 24,080 pounds as the load on one lineal foot of footing. We will allow 6,000 pounds (3 tons) for the bearing power of the soil, and dividing the load by 6,000 we have 4 feet as the required width of the footing. The load on the footings under the columns will consist only of the weight of the floors and the roof and the live load, plus the weight of the tier of columns, which would be so small in proportion to the other loads that it need not be considered. If the columns were 14 feet apart longitudinally, each column would support 224 square feet of each floor, so that the total dead load on the footing under the columns will amount to 114,240 pounds, and the possible live load to 168,000 pounds. As it would be scarcely possible for every foot of floor on every floor being loaded to its full capacity at the same time, we would probably come nearer the actual conditions if we take only 50 per cent, of the total live load, or 84,000 pounds, making a total load on the footing of 198,240 pounds, which would require 33 square feet in the area of the footing. But as there will be no shrinkage or compression in the iron columns we had better reduce this area 10 per cent., making 30 square feet, or 5½ feet square.
* For weight per cubic feet of materials, see table in appendix.