The fifth variation, increasing the area of the section, is required to thicken
and strengthen the outer toe.
We now come to the last cause of change from the theoretical section, at the
top of the dam. This sixth increase of section and weight on the diagram (shown
at A k t) is required to resist ice, wind and wave action.
The addition of this weight enables us to encroach upon the theoretical section
at t,-t',-t' -without reducing the several factors of safety below the limit fixed ; w^hile
it adds to the required breadth of section throughout the lower portion of the dam.
The strain sheet, and its tables, exhibit how evenly all the factors of safety are
maintained in the final section, under consideration, providing for all the above-
mentioned disturbing conditions.
As no demonstration could be found of Sazilly's trapezoid of pressure for great
dams, I have developed one and obtained a formulated and graphic method of
calculating the pressures on the outer and inner toes of joints of high m.asonry
dams, and used them to re-calculate the pressures on the strain sheet obtained by
the formula of moments, as a check on those results.
By this method, as in all others, we must neglect the factor of elasticity in
masonry, which affects the results but slightly, and introduces a factor of safety.
The demonstration is as follows : (See Fig. 5).
