Footing depth for grain storage bin?

Daughter just bought a used grain storage bin for 4H breeding swine project.. Not sure if that is what it is called or not. It is round metal corrugated pipe tank 6' diameter on 4 - 6' legs. used & all steel . Legs are about 6' long Unit is about 12 high off the legs so height will be about 18' . Filled with grain I expect it could weigh 4800 lbs but I estimate she will only be buying a ton at a time. Not sure what the unit itself weighs. Hoping you guys have more knowledge or have usd one. Best on a concrete pad (depth ?) or would 4 concrete footings be adequate ? if footings, how deep ? Thanks !!! Soil is cl 1 clay loam . not sandy. In northern California . Never gets below 20 degress. rarely freezes. Wind load 70 m.p.h. in area & used for roof permits. oh, & think a permit will be needed ?