hi, tom24.
You use a worrying phrase "I
think the foundation is a 12*12*12 slab". Do you perhaps mean that you really don't know what size it should be, but that someone may have suggested such a size to you, or that you know of a similar tower that has such a foundation block?
For what it's worth, I
don't think that 12*12*12 is anywhere near enough unless the soil on the sides is strong enough to prevent overturning of your foundation block, together with the grain tower, and that ground water is never closer to the surface than about 15 feet (= 12 feet plus an Austim safety factor
).
You refer to an 108 kip uplift force, which I have taken to mean that someone has determined that to be the maximum uplift at each of two corners at the bottom of the windward face of your tower. Such a force seems reasonably consistent with your stated wind velocity of 85 mph applied to an empty grain tower, so I have no problem with that.
However... your 12*12*12 ft block of concrete only weighs about 260 kips (assuming that ground water is always more than 12 ft below the top of your foundation).
If you apply a static load of 2*108 = 216 kips upwards close to the edge of such a block, you will lift up that edge, and you have a global stability problem. If ground water can rise above the bottom of your foundation, then your foundation block will lose about 40% of its effective mass, and your stability problem would be very much worse.
To calculate your factor of safety against overturning, I would proceed thus:
(a) Assume a reasonable bearing area required to take the full weight when it is concentated at the leeward edge of the base. - as a guess, say that you resultant upward reaction is about 1 foot inside the edge.
(b) Your overturning moment is then 216 * 10 = 2160 kip ft.
(c) your stabilising moment is 260 * 5 = 1300 kip ft (even less if ground water may come into play), which is only 60% of the overturning moment, and you are well on the way to having a seriously leaning tower.
(d) for an adequate factor of safety against overturning, you should have a stabilising moment at leat 20% more than the overturning moment (unless your local code of practice requires more than that, of course)
There are a couple of factors that could help to avoid total collapse (though I would ignore them both were I the designer of your foundation).
Firstly, before your block would overturn completely, horizontal earth pressures would develop. These would provide some degree of stabilising moment. Unless you have some very sound geotechnical advice, it would be very risky to rely on that.
Also, our simple use of quasi-static wind loads is a very primitive approximation for what really happens in the very dynamic situation of wind loads. So
perhaps the wind load will not act long enough to overturn tower plus foundation block. I would not take the risk.
As for reinforcement - you must have at least enough to hold the block together when you are pulling upwards with a force of 216 kips on any one edge.
I hope that this will help you.
