huangyhg

buoyancy imposed at elevator pit
the site has excess water.
after completion of construction, there will be two feet of water above the bottom elevation of the elevator pad footing.
i need to perform a bouyancy calculation to determine the required gravity dead load to resist the upward bouyancy force imposed at the bottom of the elevator pad footing.
what is(are) the calculation(s)?
any other helpful comments or suggestions?
thank you in advance!

the upward buoyancy force will be equal to the weight of water displaced by the volume of the foundation and elevator core below the water line.
in my experience, the only case where this force has been an issue is with underground storage tanks. when the tanks are empty, the buoyancy force is much higher than the weight of the tanks and they must be anchored to a concrete footing.
anyway, you can check your structure and see. good luck.
also important for swimming pools..when empty for maintenance painting and such, they need to be heavy enough not to float.
the other two guys are right here, the principle is called archimeedies (not sure on the spelling), the buoyancy force (upward) is equall to the weight of the water displaced. it not uncommon for tanks to float out of the ground if they are not properly anchored.
in general, the uplift calculation comes for the design of liquid retaining structure which is partly / fully underground.
before starting of analysis/design you should have the following data -
1) size of tank/pit.
2) depth of tank/pit below g.l.
3) high flood level/maximum ground water table (worst for 50/100 years)
while analysis/design you have to consider the following load cases -
1) full water/liquid inside & no earth/gwt/surcharge outside.
2) no water inside the tank/pit & full earth/ground water table/surcharge outside.
while designing the base slab/wall you have to design it as a uncracked section for the moments/forces developed due to above load cases.
base slab should be designed as a uncracked section for the moments developed due to upward pressure of gwt.
the dead weight required to resist the uplift forces developed due to ground water table should be 20% more than the uplift pressure. (as per clause 4.2.b pp-7 of is-3370(part-i), 1965.)
i hope this will give you a very brief idea. if you need any help, send email
paresh
sounds to me like the base slab alone might be heavy enough keep it from floating since 1-foot of concrete @ 150 pcf = 2.4-feet of water @ 62.4 pcf
wouldn't you want to apply a safety factor to this calculation? i would say 1.5 off the top of my head since it is mainly quantifiable loads. you would do this on uplift on a footing due to wind or overturning on the toe.
paresh response is quite reasonable and logical.

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