Machine foundations

It sounds like you're dealing with a machine that moves or vibrates quite a bit. One rule of thumb, for geotechnical considerations, that always sticks with me is that the machine pad (typically reinforced concrete, but I suppose steel would work too) should have a weight equal to the weight of the machine or greater. This is to prevent excessive foundation vibration which could lead to settlement of any underlying loose to moderately loose sandy soils. With a pad of similar weight to the machinge, the chance that resonant frequencies being set up in the whole system (machine and pad) are reduced. This reduces vibration amplitudes. Most of the machine pads I've seen are below the surrounding floor slab, maybe 12" below the floor slab, so this would prevent the machine from "walking." If this machine is being set up over an existing slab, it might need to be secured to the floor slab, if the slab can handle the loading. You could always try it and see if it walks. Check the weight of the steel pad you are talking about and compare with the machine weight for starters.

I would think your best approach would be to contact the machine manufacturer and see what their recommendation is.
Generally, they will not design a base for your, but they do know if one is needed. Don't try to run the machine to see what happens first, that is the best way to do a LOT of expensive damage to it.

The rule of thumb I've heard most often is the machine foundation weight should be 2-3 times the weight of the moving parts. But, there is a lot more to machine base design that a rule of thumb! To prevent damage to the machine and possiblly your building, get information from the manufacturer, soil bearing information and then contact an engineer that specializes in machine base design. It will save you a lot of grief later on.

I agree with jheidt, the numbers I have seen are the base mass two times the equipement. Consideration to dampen the vibration, react the load, and attach the machine should be given. Some of the bases we installed were even installed on pile caps.

In addition to the mass considerations, the pad foundation for the machine should be structurally separataed from the rest of the building where grade supported.