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Designing the Foundation

lb, load, ft, sq, weight, feet, wall and soil


Load to be Supported. The first step is to ascertain the load to be supported by the foundation. This load consists of three parts: (1) The structure itself, (2) the movable loads on the floors and the snow on the roof, and (3) the part of the load that may be transferred from one part of the foundation to the other by the force of the wind.

The weight of the building is easily ascertained by calculating the cubical contents of all the various materials in the structure. The fol lowing data will be useful in determining the weight of the structure.

Brickwork, pressed brick, thin joints 145 " ordinary quality 125 " soft brick, thick joints 100 Concrete 130 to 160 Granite or limestone, well dressed throughout 165 " rubble, well dressed with mortar . .. . 155 roughly dressed with mortar 150 " well dressed, dry 140 roughly dressed, dry .... 125 Mortar dried 100 Sandstone, less than granite Ordinary lathing and plastering weighs about 10 lb. per sq. ft. Floors weigh approximately: Dwellings 10 lb. per sq. ft.

Public buildings 25 lb. per sq. ft.

Warehouses . 40 to 50 lb. per sq. ft. Roofs vary according to the kind of covering, span, etc. Shingle roof weighs about 10 lb. per sq. ft.

Slate or corrugated iron 25 lb per sq. ft.

The movable load on the floor depends upon the nature of the building. It is usually taken as follows: Dwellings 10 lb. per sq. ft.

Office buildings. 20 lb. per sq. ft.

Churches, theatres, etc 100 lb. per sq. ft.

Warehouses, factories 100 to 400 lb. per sq. ft.

The weight of snow on the roof will vary from 0 in a warm climate to 20 lb. in the latitude of Michigan. The pressure of the wind is usually taken at 50 lb. per sq. ft. on a flat surface perpendicu lar to the wind, and on a cylinder at about 40 lbs. per sq. ft. over the vertical projection of the cylinder.

Bearing Power of Soils. The best method of determining the load which a particular soil will bear is by direct experiment and examination—particularly of its compactness and the amount of water it contains. The values given in the following table may be considered safe for good examples of the kind of soil quoted.

Bearing Power of Soils.

Bearing power, Kind of soil. tons per square foot.

Min. Max.

Rock, hard 25 30 " soft 5 10 Clay on thick bed, always dry 4 6 " " " " moderately dry 2 4 " soft 1 2 Gravel and coarse sand, well cemented 8 10 Sand, compact and well cemented 4 6 " clean, dry 2 4 Quicksand, alluvial soil, etc. 0.5 1 Area Required. Having determined the pressure which may safely be brought upon the soil, and having ascertained the weight of each part of the structure, the area required for the foundation is easily determined by dividing the latter by the former. Then,

having found the area required, the base of the structure must be extended by footings of concrete, masonry, timber, etc., so as to (1) cover that area and (2) distribute the pressure uniformly over it.

Bearing Power of Piles. Several formulas have been proposed and are in use for determining the safe working loads on piles. The three in general use are: Sander's formula.

Safe load in lb. ___ Weight of hammer in lb. X fall in inches.

8 X Sinking at last blow.

Trautwine's formula.

Extreme load in tons of 2240 lbs. = Cube root of fall in feet X Weight of hammer in lb. X 0.023 Last sinking in inches.

Safe load to be taken at one-half of extreme load when driven in firm soils, and at one-fourth when driven in river mud or marshy soil.

Engineering News formula is the latest and is considered reliable.

2 w Safe load in lb. = S in which w = weight of hammer in lb., h = its fall in feet, S = aver age sinking under last blows in inches.

Example of Pile Foundation. As an example of the method of determining the number of piles required to support a given build ing, the side walls of a warehouse are selected, a vertical section of which is shown in Fig. 15. The walls are of brick, and the weight is taken at 110 pounds per cubic foot of masonry.

The piles are to be driven in two rows, spaced two feet between centers, and it has been found that a test pile 20 feet long and 10 inches at the top will sink one inch under a 1,200-pound hammer falling 20 feet after the pile has been entirely driven into the soil.

What distance should the piles be placed center to center length wise of the wall ? By calculation it is found that the wall contains 1571 cubic feet of masonry per running foot, and hence weighs 17,306 pounds. The load from the floors which comes upon the wall is: From the 1st floor 1500 lb.

" " 2nd " 1380 " " " 3rd " 1380 " " " 4th " 790 " " " 5th " 720 " " " 6th " 720 " " " roof 240 " Total 6730 lb.

Hence the total weight of the wall and its load per running foot is 24,036 pounds.

The load which one pile will support is, by Sander's rule 1200 X 240 8 X 1 J 36,000 pounds.

By Trautwine's rule, using a factor of safety of 2.5, the safe load would be X 1200 X 0.023 2.5 X (1 + 1) -= 15 tons or 33,600 lb.

Then one pair of piles would support 72,000 or 67,200 pounds cording to which rule we take. • Dividing these numbers by the weight of one foot of the wall and its load, it is found, that, by Sander's rule, one pair of piles will support 3 feet of the wall, and, by Trautwine's rule, 2.8 feet of wall; hence the pile should be placed 2 feet 9 inches or 3 feet between centers.