jjezmarlo3
Structural
- Aug 9, 2012
- 13
I'm trying to figure out how to calculate the factor of safety of a tank floating. Stupid question, right? Ignoring soil friction it is just empty tank weight plus soil weight over slab extension divided by the buoyant volume x 62.4 pcf. However, I'm double checking some calculations of a vendor, and I'm not so sure any more. As an example, I have uploaded an example drawing to demonstrate. The point isn't that the example doesn't have enough factor of safety, but that the factors of safety can vary so significantly depending on the way it is calculated. I would appreciate all of your thoughts.
Option 1 (Which I have generally used):
(Tank weight + soil weight) / Total buoyant volume x 62.4
Concrete Weight = .150 pcf x (196 ft3 + 144 ft3) = 51.0k
Soil Weight = .110 pcf x 384 ft3 = 42.2k
Buoyant Force = 0.0624 pcf x 980 ft3 = 61.2k
Factor of Safety = (51.0 + 42.2) / 61.2 = 1.52
Option 2 (Which the vendor is using):
(Buoyant weight of tank + buoyant soil weight) / Inside tank volume x 62.4
Concrete Buoyant Weight = (.150 pcf - 0.0624 pcf) x (196 ft3 + 144 ft3) = 29.8k
Soil Buoyant Weight = (.110 pcf - 0.0624 pcf) x 384 ft3 = 18.3k
Inside Tank Buoyant Force = 0.0624 pcf x 256 ft3 = 16.0
Factor of Safety = (29.8 + 18.3) / 16.0 = 3.0
Option 3 (Which the American Concrete Pipe Association uses for Manholes):
(Tank weight + buoyant weight of soil) / (Concrete volume + empty tank volume) x 62.4
Concrete Weight = .150 pcf x (196 ft3 + 144 ft3) = 51.0k
Soil Buoyant Weight = (.110 pcf - 0.0624 pcf) x 384 ft3 = 18.3k
Buoyant Force = 0.0624 pcf x (256 ft3 + 196 ft3 +144 ft3) = 37.2k
Factor of Safety = (51.0 + 18.3) / 37.2 = 1.86
Option 1 (Which I have generally used):
(Tank weight + soil weight) / Total buoyant volume x 62.4
Concrete Weight = .150 pcf x (196 ft3 + 144 ft3) = 51.0k
Soil Weight = .110 pcf x 384 ft3 = 42.2k
Buoyant Force = 0.0624 pcf x 980 ft3 = 61.2k
Factor of Safety = (51.0 + 42.2) / 61.2 = 1.52
Option 2 (Which the vendor is using):
(Buoyant weight of tank + buoyant soil weight) / Inside tank volume x 62.4
Concrete Buoyant Weight = (.150 pcf - 0.0624 pcf) x (196 ft3 + 144 ft3) = 29.8k
Soil Buoyant Weight = (.110 pcf - 0.0624 pcf) x 384 ft3 = 18.3k
Inside Tank Buoyant Force = 0.0624 pcf x 256 ft3 = 16.0
Factor of Safety = (29.8 + 18.3) / 16.0 = 3.0
Option 3 (Which the American Concrete Pipe Association uses for Manholes):
(Tank weight + buoyant weight of soil) / (Concrete volume + empty tank volume) x 62.4
Concrete Weight = .150 pcf x (196 ft3 + 144 ft3) = 51.0k
Soil Buoyant Weight = (.110 pcf - 0.0624 pcf) x 384 ft3 = 18.3k
Buoyant Force = 0.0624 pcf x (256 ft3 + 196 ft3 +144 ft3) = 37.2k
Factor of Safety = (51.0 + 18.3) / 37.2 = 1.86
