Boundary conditions

[Ingseven]

Hi
I have a structure 100ft deep into the ground, 9 ft square shape, and water is at 20 ft below surface. There is nothing inside the structure (only self weight). What would be the boundary conditions in the bottom if I want to calculate (Staad) buoyancy? I I considered it not supporte at the bottom, what friction at the walls could I assume? Suggestions please.

Thanks

 

[civilperson]

Uplift of 80 feet of hydrostatic pressure, (5000 psf), minus self weight of base/structure.

 

[Ingseven]

Yes it is clear. But what could I use for friction between structure and soil, which will really avoid the structure from uplift.

 

[DRC1]

Typically friction against bouancy is not used, only self weight. What depth of groundwater is present?

 

[civilperson]

Some engineers use the weight of the soil prism above the foundation area for resistance to buoyancy, I think this a very risky assumption since the soil is not present during construction or may be remove at a later time. I would only use the volume of the limits of the concrete in calculating buoyancy, since the base uplift pressure is counteracted by the weight of the water on the top of the structure. If the top is above the water table, use only the volume below the water table for buoyancy. Safest method is to increase thickness of concrete to achieve neutral buoyancy, (use of high specific gravity aggregate can minimize this increase in thickness).

 

[Ingseven]

What if the self weight is not enough? In the case I presented it seems to be enough, but if the section is bigger (that could be the case) the uplift pressure would be more than the self weight.

Thanks

 

[civilperson]

Happens all the time! I had to put a five foot thick floor, (and two foot thick walls/roof), in a 15 foot square box culvert in Louisiana to achieve neutral buoyancy.

 

[casimmons]

or you can use ground/rock anchors to resist uplift.