Basement Uplift Design 2

[pappyirl]

Hi, I'm looking at designing a RC raft slab in a basement with thickenings under the columns. The basement slab is below the water table.

I'm thinking of designing the raft as two separate models. One as a suspended slab subjected to the uplift pressure in the negative gravity direction and one as a typical raft with point loads on springs ignoring the uplift. I think the uplift load will cause issues with the zero tension springs used in FE models, hence the two models.

Does anyone have experience of the above, useful references or threads welcome. I looked but could not find much in the line of the above.

 

[r13]

Your overall dead weight should exceed the uplift with a safety factor, then just model as usual with compression only spring supports.

 

[pappyirl]

Thanks for the quick reply.

In general it does have a FOS.

The building over is a U shape, the basement is rectangular, so there is a podium area in the middle of the U where the dead load does not balance the uplift, although it is close. My thoughts are to span the slab at basement from one side of the U to the other to carry the uplift to where the line of the building columns over is located. Tension piles in this area is another option.

 

[r13]

If all parts are linked together, then uplift wouldn't be a problem, although piling may be required for over turning stability. Seems you are in a correct path with well thought handle.

 

[SlideRuleEra]

....designing a RC raft slab in a basement with thickenings under the columns. The basement slab is below the water table.
Tension piles in this area is another option.

Seems you are designing the basement slab for minimum allowable thickness.
Unless the basement is really large, make the slab as thick as necessary to provide adequate dead load. If this means the slab has to be double or triple the design thickness, so be it. For the cost of mobilizing a pile driver for a few tension piles, a large quantity of "extra" concrete can be placed.

With a "thin" slab, you need to be concerned about hydrostatic uplift cracking the slab in addition to overall buoyancy.

http://www.SlideRuleEra.net

 

[pappyirl]

Update - I've thickened the slab at basement to counter the uplift in the area noted. The weight of the podium slab is also included, together they provide the FOS. Additional concrete but a foolproof solution. They can go with tension piles or ground anchors depending on the economics.

Thanks for the help retired13.

 

[r13]

I think SRE has a very practical point, and glad you followed it. Good luck.