The profile resulting from this method of design is somewhat irregular and may be modified by fitting it with more uniform batters and smooth curves, thus giving a more pleasing appearance and better profiles for construction purposes, without appreciably affecting its stability.
Vertical Water the water face of the dawn is nearly vertical, it is usual to disregard the vertical component of the water pressure, which is of small consequence in clams of less than about 180 to 200 feet in height. This component has the effect of diminishing the stress upon the outer edge of the section while somewhat increas ing the total pressure. Its neglect is therefore a small error on the safe side until a depth is reached at which the slope of the inner face may make it of more importance.
The shape of the profile depends upon the top width given to the dam, and the weight of the masonry used.
The top width must be sufficient to resist any probable wave action and ice pressure, and should usually he made greater for high dams than for low ones. This is a matter of judgment in each case, about one-tenth of the height of dam being frequently used, with a minimum of about 5 feet and a maximum of 20 feet where no roadway is carried on top of the darn.
The dam should always extend to a sufficient height above the normal water surface to prevent water passing over the clam due to waves of floods for which wasteways might not be quite sufficient. This may require the dam to be raised 5 or 10 feet above the eleva tion of the expected water surface. In designing the dam, water should be assumed level with the top.
The weight of masonry used in dam construction commonly varies from about 135 to 150 pounds per cubic foot. The heavier the masonry is assumed to be, the less the required width of section until a depth is reached at which the width is determined by the necessity of pro viding sufficient area to carry the weight of masonry above. Below this point, usually about 200 feet below the water surface, the width required is greater for the heavier masonry if the same unit com pression be allowed.
Uplift and Ice Pressure.—If water under hydrostatic pressure has access to the interior of the dam, the upward pressure will tend to lift the masonry and diminish its effective weight in the moment which prevents overturning. In this discussion it has been assumed
that the dam is constructed water-tight, but as this is not altogether possible, in many instances it may he necessary to allow for upward pressure in designing the profile, or make special provision for drain age—a topic discussed in Section 140.
If ice forms on the surface when the reservoir is full, a consider able pressure may be brought against the top of the dam, which should be considered in its design. This will be a concentrated hori zontal thrust at the surface of the water equal to the crushing strength of the ice, and has been assumed in a number of important dams at from 2500 to 4500 pounds per linear foot of dam. In storage reser voirs generally, heavy ice is not likely to occur with full reservoir, and if water be low when freezing occurs no special allowance for ice pressure is necessary. Local conditions must determine the necessity of allowing for ice pressure in each instance.
138. Diagonal Compressions.—The common method of analysis, already described, considers only the stresses upon horizontal sec tions and resolves the diagonal thrusts into normal compressions and parallel shears upon these sections. This method does not give the actual maximum compressions, but by using proper unit stresses has seemed to give satisfactory results in use. Several methods have been proposed for computing more accurately the maximum unit compressions.
Diagonal Compression upon Horizontal Sectinn.—In 1S74 Bou vier I used the actual diagonal pressure (I?, Fig. 72) in computing the maximum unit compression upon a horizontal section, claiming that the unit compressive stresses produced by R parallel to its line of action are greater than those normal to the section. He con sidered 1? to be distributed along so as to act upon successive small sections normal to its direction as shown in Fig. 73. If 0 is the angle made by R with the normal to section A —B, and b is the width of section, the area upon which R acts is AC= b • cos • 0, and the maxi mum intensity of the compressive stresses is in which is the unit compression at the outer edge of the section parallel to R, and is that normal to the section at the same point.