Its bottom area is 30*30*3. 14=2826 (square millimeter).
Its volume is: 2826*20=56520 (cubic millimeter).
The section radius of round steel is 40/2 = 20 mm.
The sectional area of round steel is: 20*20*3. 14= 1256 (square millimeter).
The required interception length is: 56520/1256 = 45 mm.