tank bottom in submerged tank - rising water level
tank bottom in submerged tank - rising water level
(OP)
this may be a mouth full but here goes.
Your designing a tank, smallish size. Using plastic soil conditions.
The tank imposes a surcharge on the soil.
For the structural design you would tank the weight of walls spread over the base area which say produces a pressure of 15 kn/m2 (again just pulling figures out of air).
Therefore the pressure is applied over slab area and forces calculated.
Now if the water table rises above base and provides uplift of 10kn/m2.
Now is the pressure used to calculate forces 10 + 15 or 15 as the water buoyancy takes some of the pressure off soil.
Or two load cases. 1) wall weight over slab 2) force due to uplift of water.
For a continuous base and wall things will get more detailed.
So how would people approach? I'm having doubts i do it the most economical and no one seems too convinced here.
Your designing a tank, smallish size. Using plastic soil conditions.
The tank imposes a surcharge on the soil.
For the structural design you would tank the weight of walls spread over the base area which say produces a pressure of 15 kn/m2 (again just pulling figures out of air).
Therefore the pressure is applied over slab area and forces calculated.
Now if the water table rises above base and provides uplift of 10kn/m2.
Now is the pressure used to calculate forces 10 + 15 or 15 as the water buoyancy takes some of the pressure off soil.
Or two load cases. 1) wall weight over slab 2) force due to uplift of water.
For a continuous base and wall things will get more detailed.
So how would people approach? I'm having doubts i do it the most economical and no one seems too convinced here.






RE: tank bottom in submerged tank - rising water level
Normally, I would not assume that the weight of walls was spread over the bottom unless it was a very small tank or intentionally had the bottom stiffened to accomplish that.
With water around the tank, the downward force on the top of the foundation would be the same. You would need to check for uplift of the tank and check for overturning stability including the uplift effect. Also most tank bottoms are not designed to resist uplift forces.
RE: tank bottom in submerged tank - rising water level
I dont get what you mean by tank bottoms are not designed to resist uplift. The tank itself would resist uplift (i.e. not move) but the external water pressure will put forces on the bottom and walls.
RE: tank bottom in submerged tank - rising water level
Joe Tank
RE: tank bottom in submerged tank - rising water level
RE: tank bottom in submerged tank - rising water level
Joe Tank
RE: tank bottom in submerged tank - rising water level
RE: tank bottom in submerged tank - rising water level
Pressure on the base is due to the weight of the walls.
A rising water table will reduce the weight of the tank.
Once the water table is high enough and the tank has floated you have a boat, subject to pure hydrostatic pressure.
To my mind as the water table rises your 15kN/m^2 reduces (eventually to zero when the tanks becomes neutrally buoyant) but the uplift pressure (hydrostatic) increases to a maximum of 10 kN/m^2 assuming this is depth of water required to displace the weight of the tank and make it float.
So everywhere in between is going to be a summation between the two cases.
As the weight of the base is not included when calculating the pressure (15kN/m^2) and the water table has rose the depth of the base you would have the 15kN/m^2 + the uplift component from the depth of base?
Maybe do a plot of one against the other?
I dont know if this is correct but it seems to be they way my mind thought about it.
RE: tank bottom in submerged tank - rising water level
RE: tank bottom in submerged tank - rising water level
On larger flat-bottom tanks, the bottom is a flexible thin steel plate or concrete slab that is supported by the subgrade. It acts as a membrane to retain the fluid, and does not act like a blind flange retaining the pressure.
For a cylindrical tank with flat bottom, partially submerged, the hydrostatic force upward on the bottom is exactly the same as the hydrostatic force downward on the part of the foundation directly under the tank. So you should never have a net increased load from surrounding water. You would have an uplift or partial uplift that would need to be accounted for.
RE: tank bottom in submerged tank - rising water level
Question is why would the groundwater rise? Why would you let the bottom of your tank (and some height above the base) be continually under water? - assuming that your tank is one 'above' ground and not a buried tank.
RE: tank bottom in submerged tank - rising water level
BigH, the tank in my theoreical situation is buried. I would think that ground water can 'rise' due to seasonal variations in railfall etc.
Jstephen I have already stated that the tank is suitably small for the bottom to act structurally, not be a thin 'membrane' connected to a thicker edge and wall detail. A 'retaining wall analogy' is what you are referring to.
RE: tank bottom in submerged tank - rising water level
RE: tank bottom in submerged tank - rising water level
RE: tank bottom in submerged tank - rising water level
Please see "Design Analysis of Beams, Circular Plates and Cylindrical Tanks on Elastic Foundations" By: E.S. Melerski
Taylor & Francis Group Lindon, UK, 2006
This book has CD RAM to solve your problems/concerns
RE: tank bottom in submerged tank - rising water level
Design Force = Total Weight - Uplift Force + Uplift Force