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Calculating Uplift on pad foundation

Calculating Uplift on pad foundation


I am designing a concrete pad foundation 1m x 1m x 250mm thick.  The column (6.2m long) is centrally loaded onto the foundation with a total axial load from roof (SLS) of 27kN.  Wind load acting on column/walls which the foundation takes is approx 20kN.

I am aware that the load combination used is 1.4Wk + 1.0Gk  but I am unsure how to check if the foundation is adequate!

Do i work out a moment induced from the wind on the column into the foundation and check if the design moment of the foundation is greater than this.... or is there a different way of working this out like checking self weight of foundation is greater than 1.0Gk +1.4Wk?


RE: Calculating Uplift on pad foundation

Yes, you must consider the moment induced by the column, as well as the direct uplift on the foundation.  I'm not sure which code you are working with; however, you will need at least a 1.5 factor of safety for uplift or overturning due to wind loads.  This is taken into account by computing the overturning moment due to load, then providing a dead load resisting moment that is at least 1.5 times the overturning moment.

Then you check the footing for bending and shear due to the imposed loads.

Next you need to consider that under overturning loads, part of the footing might lose contact with the ground (or at the least decrease the bearing load on one side), thus increasing the bearing pressure on the other side.

RE: Calculating Uplift on pad foundation

Ron is right,

I would suggest to use following formula

M.ovrt=Fwind*(height of the column + 250mm + height of the plinth (if appears))

Then you have stabilizing moment which is:

M.stab=(Foundation selfweight+Axial force+ soil on the slab) x 1/2 of slab lenght

M.stab/M.ovrt > 1.5

I am currently designing similar foundations and our client's requirement is that at least 80% of fdn area should be in bearing.


RE: Calculating Uplift on pad foundation

250mm is very thin for a concrete pad.  

RE: Calculating Uplift on pad foundation

Thanks everyone for your help, got it sorted :)

csd72...thanks for notifying but that didn't help the initial question I was asking..but you are right!

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