a 20' span is damn near the limit of standard 2x12 dimensional lumber. Standard building construction is figured on 50psf (pounds per square foot) live load and only 30psf dead load. your saw is considered dead load....and is above and beyond that 30psf the construction was most likely rated for.... also consider that in home construction the floor is reinforced under bathtubs to be able to support the weight of the tub when filled with water....a gallon of water weighs just over 8lbs, and a standard tub holds about 60-70gallons of water for a bath scenario....so that's about 550-600lbs of dead load and they reinforce for THAT.
While the 2x12 framing will "hold" you will certainley cause excess sag in the joists over time.....how much and over how long is obviously tough to say, but the BEST way to deal with it is to pour a concrete footing in the basement roughly 18x18 and 36-48" (depends on your lcal municipalities requirements) deep, then put a column on it with a small beam centered on it to support the area of the floor directly beneath this monster of a saw you have.
for piece of mind so you don't think I'm just some know-it-all on the internet, I hold a degree in architecture with 5yrs experience and currently work as an engineer in process of that degree as well, and additionally have been certified as a building code enforcement official for the state of NY.
