A Comparative Study of Imhoff Tanks 351 
The slab will therefore be designed as if composed of a series 
of horizontal beams, loaded uniformly by water pressure on the 
inside and earth pressure on the outside. 
The pressures will be as follows: 
Water p = wh id. = 62.4 pounds cubic feet 
pounds 
square feet 
4 feet depth 249.6 
8 feet depth 499.2 
12 feet depth 748.8 
16 feet depth 998.4 
Earth p = wh - id = 100 pounds cubic feet ^ = 30 
I — smcf) 
pounds 
4 feet depth 133.33 
8 feet depth 366.66 
12 feet depth 399.99 
16 feet depth 533.32 
When supported in the center of the long side, the slab has a 
span of 11 feet, and the beams are considered as fixed at both 
ends and the middle, so that a maximum negative moment of 
yV will be developed over the ends and a maximum positive 
moment of yt ihi the center. 
The reversal of stress due to earth pressure, when the tank is 
empty, will have to be taken care of independently by reinforce- 
ment placed on the opposite sides of the wall from that required 
for water pressure. While theoretical analysis would show a 
possible reduction in total amount of steel, due to the presence 
of reinforcement in both sides of the slab, it would seem unwar- 
ranted in this analysis, because of the large number of indefinite 
factors in the cases. 
Then for water pressure at the bottom w = 998.4 pounds 
square foot, I = 11, P = 121, M = yV X 998.4 X 121 X 12 = 
120,768 inch-pounds, fs = 15,000 pounds, fc = 600 pounds, n 
= 15 
then 
V = 0.0075 
k = 0.375 
i = 0.875 
M, = 15,000 X 0.0075 X 0.875 X 12 X 
