Compressible soils are often met with which will bear from 1 to 2 tons per square foot with very little settlement, and, as a rule, this settlement is uniform under the same unit pressure (pressure per square foot). In such cases it is often cheaper to spread the foundations so as to reduce the unit pressure to the capacity of the soil than to attempt to drive piles. "Spread" footings may be built of concrete with iron tension bars, of steel beams and concrete, or of timber and concrete.

48. Concrete with Iron Tension Bars

When the necessary height can be obtained, spread footings composed of Portland cement concrete, with iron tension members, have more qualities to recommend them than any other construction. Such footings are easy of construction, they are cheap, and their durability is everlasting. The iron being so completely imbedded in the concrete it cannot rust,* and hence there is no possibility of deterioration in the footings.

Masonry is undoubtedly the natural material for foundations, and the author believes that it should be preferred to iron or steel wherever practicable.

By the use of twisted iron rods the concrete footings may be made of equal transverse strength as footings of steel beams, but they require more height.

48 Concrete with Iron Tension Bars 10014

Fig. 13.

Fig. 13 shows the most economical section for a concrete and twisted iron footing. In building the footings with steel beams, the strength of the concrete is practically wasted, while in this method of construction it is all utilized. It has been proved that the entire tensile strength of the twisted bars can be utilized, and, being held continuously along their entire length by the concrete as a screw bolt is held by the nut, they neither draw nor stretch, except as the concrete extends with them.

In building concrete footings, as shown in Fig. 13, a layer of concrete from 3 to 6 inches thick, made in the proportion of 1 to 3, should first be laid, and the iron bars laid on and tamped down into it. Another layer of 4 inches, mixed in the same proportion, should then be laid, after which the concrete may be mixed in the proportion of one to six. Each layer should be laid before the preceding layer has had time to harden, otherwise they may not adhere thoroughly.

* In cutting through a portion of a foundation built of concrete and iron, and submerged in salt water, ten years after the work was done, no deterioration to the iron whatever was found. Iron imbedded in concrete, with the end projecting, has been found bright and clean after the projecting end had completely rusted away.

The author has prepared Table III., giving the strength and proportions of footings constructed in this way, which he believes to have a large margin of safety. Two sizes of bars are given, with the corresponding safe loads for the footings, the other measurements applying to both cases. The measurements in the third column refer to the width of the brick or stone footing resting on the concrete. The greater the width of this footing in proportion to the width of the concrete, the less will be the strain on the tension rods.

Table III. - Proportions And Strength Of Concrete Footings With Twisted Iron Tension Bars

WIDTH OF FOOTING IN FEET.

THICKNESS OF CONCRETE.

WIDTH OF

STONE FOOTING.

DISTANCE

BETWEEN

CENTRES

OF BARS.

SIZE OF

SQUARE

BAR.

SAFE LOAD

PER LINEAL

FOOT.

SIZE OF

SQUARE

BAR.

SAFE LOAD

PER LINEAL

FOOT.

Ft.

In.

Ft.

In.

Inches.

Inches.

Tons.

Inches.

Tons.

20

3

6

6

0

8

2

78

1 7/8

66

18

3

3

5

6

8

2

76

1

56

16

2

10

5

0

7

1

73

1

50

14

2

8

4

8

7

1 5/8

70

1 3/8

49

12

2

6

4

4

6

1 3/8

65

1

48

10

2

3

4

0

6

1

65

I

42

8

2

0

4

0

6

1

60

40

6

1

8

3

6

6

55

29

Piers. - Footings for piers may be built in the same manner, with two sets of bars laid crossways of each other, and also diagonally, as shown in Fig. 14. In the case of piers the corners should be cut off at an angle of 45 degrees, as shown. The same size of bars should be used for a pier as for a wall, whose footings have the same projection beyond the masonry, and the depth of the concrete should be the same.

Example. - What would be the safe load for a pier footing 14 feet square, with a stone footing on top 6 feet square, the corners being cut off, as in Fig. 14?

Answer. - The area of the pier footing would be 196 - 72 = 124 square feet, and the projection of the footing beyond the masonry would be 4 feet. In Table III. we find that the projection of the 12-foot footings is 3 feet 10 inches, and that the safe load for this.

footing (with 1-inch bars) is 48 tons, or 4 tons per square foot. If we make our pier of the same thickness and use 1-inch bars we would have the same strength per square foot, which would give a total safe load on the footing of 496 tons.

Unfortunately this method of construction, including all forms of concrete construction with twisted tension rods, has been patented by Ernest L. Ransome, of San Francisco, Cal., and the rights are now owned by the Ransome & Smith Co., of New York, Chicago and San Francisco, to whom a royalty must be paid when twisted bars are used; but even after paying the royalty it is much the cheapest footing for the strength obtained.

Fig. 14.   Plan of Pier.

Fig. 14. - Plan of Pier.

This form of construction has been used to a considerable extent in San Francisco.