vertical 1/2-inch round rods in the correct positions, the next step is to place the horizontal reinforcement for the walls. This we have previously seen consists of 1/2-inch round bars spaced 10 inches apart. Where the bars lap, they should be firmly wired together.

TABLE XXVIII.-SHOWING SIZE AND SPACING OF RODS IN WALL.

We will illustrate the method of using Table XXIX in building a tank 15 feet deep and 10 feet in diameter. From the table we see that the thickness of concrete in the walls of the tank is 10 inches; that the size of reinforcement to be used is 3/8-inch rods, that is, round rods 3/8 of an inch in diameter; for the first foot these rods should be spaced 4 inches apart, and the vertical rods should be

placed 30 inches apart. The table calls for the spacing of the horizontal rods 18 inches apart at the top of the tank, and the intermediate horizontal rods will therefore be spaced distances varying from 18 inches to 4 inches; thus in the second foot from the bottom the horizontal rods will be 5 inches apart; in the third foot, 6 inches apart; in the fourth foot, 7 inches apart; in the fifth foot, 8 inches apart; in the sixth foot, 9 inches apart; in the seventh foot, 10 inches apart; in the eighth foot, 11 inches apart; in the ninth, 12 inches apart; in the tenth, 13 inches apart; in the eleventh, 14 inches apart; in the twelfth, 15 inches apart; in the thirteenth, 16 inches apart; in the fourteenth, 17 inches apart; and in the fifteenth, 18 inches apart.

CONCRETE DAMS

Concrete is now being extensively employed in the construction of dams of every conceivable shape and size. They are all, however, of three general types, the solid or gravity type, the arched type and the hollow reinforced type.

The fundamental principles in the design of gravity dams are much the same as those underlying the design of retaining walls, the main difference being that the dam must not only be strong enough to be safe against a full head of water in the reservoir, but also in the case of very high dams it must be safe against the weight of masonry in the structure itself. Furthermore the external pressure of the water can be determined with scientific exactness while the pressure of earth on walls is subject to many uncertainties. The amount of water pressure per square foot against a dam at any depth is found by the simple rule

31.25 H2

P=62.5 H. and against the surface one foot wide. P = P = Pressure in lbs. at any depth H in feet. Table XXX gives the pressures at different depths: Gravity dams may be constructed of solid concrete or of concrete in which is embedded large blocks of rubble. The latter type which is called "Rubble Concrete," or "Cyclopean Masonry," is by far the most economical as the amount of cement required is reduced to a minimum.

Small Dams. The construction of small dams under six feet

high may be undertaken without special engineering advice as follows:*

"If possible, dig a temporary trench so as to carry the water around the dam while it is being built. If this cannot be done, run the water through a wooden trough in the middle of the dam, and after the wall each side of it is finished, carry the forms across the opening, and make these tight enough so that the water is quiet between them; then place the concrete.

"Dig a trench across the stream slightly wider than the width of the base of the dam, carrying it down about 18 inches or 2 feet below the bed of the brook, or if the ground is soft, deep enough to TABLE XXX.-HYDROSTATIC PRESSURES.

reach good, hard bottom. In case the earth is firm enough for a foundation, but is porous either under the dam or each side of it, sheet piling consisting of 2-inch tongued-and-grooved plank can be pointed and driven with a heavy wooden mallet so as to prevent the water flowing under or around the dam. Build the forms so as to

*From "Concrete Construction About the Home and on the Farm," published by Atlas Portland Cement Co.

make the wall of the dimensions shown in the table. Wet them thoroughly, then mix and place the concrete.

"Use proportions one part Portland cement to two parts clean, coarse sand to four parts screened gravel or broken stone.

"Take special care to make the concrete water-tight by using a wet mix. If possible, lay the entire dam in one day, not allowing one layer to set before the next one is placed. If it is necessary to lay the concrete on two different days, scrape off the top surface of the old concrete in the morning, thoroughly soak it with water, and spread on a layer about 1/4 inch thick of pure cement of the consistency of thick cream, then place the fresh concrete before this cement has begun to stiffen.

"If the forms on the lower side of the dam are well braced, the forms on the upstream side may be removed in three or four days, and the pond allowed to fill. The forms on the down-stream face should be left in place well braced for two or three weeks. No finish need be given to the surface."

Reinforced-Concrete Dams.†-Reinforced concrete is particularly adapted to the construction of dams. When so used there is a

TABLE XXXI.-DIMENSIONS FOR SMALL DAMS AND QUANTITY OF MATERIALS FOR DIFFERENT HEIGHTS OF Dams. Proportions:

1 Part Portland Cement to 2 Parts Sand to 4 Parts Gravel or Stone

* Make deeper if necessary to get a good foundation.

Note.-A large single load of sand or gravel is about 20 cubic feet.

A large double load of sand or gravel is about 40 cubic feet.

†This discussion is arranged from "Concrete, Plain and Reinforced," by Homer A. Reid.

great saving in material, and on this account a reduction in cost of, in some cases, as much as 20 per cent. Again, the space under the apron may be utilized for storage or power-house purposes, as for the location of turbines, electric generators, etc. Another advantage is that of securing a practically impervious curtain face wall, without any of the dangerous leaks so troublesome to locate in some masonry structures. If sufficient number of reinforcing rods are used and run in every direction there will be little or no danger of cracking in the deck concrete.

The design of steel dams is that of a triangle with the upstream face so flatly inclined that the water pressure is made to give increased stability by its weight, and this basic principle has been the leading feature in the development of dams of reinforced concrete,

T

H

G

FIG. 100.-Design for Small Dam.

which were first introduced in the Eastern States about the year 1902 by the Ambursen Hydraulic Construction Company, of Boston.

About 30 dams varying in height from 10 to 80 feet, some over 1,000 feet long, have been erected during the last 8 years, many of them attracting marked attention by the engineering profession.

The design of these dams illustrates very strikingly the adaptability of reinforced concrete to new conditions. The principle followed in the design is that the vertical pressure of the water is utilized to firmly hold the dam down on its foundation.