12 ft/sec is 720 ft/min, which is approximately the rate of descent of a stable 3-degree approach. A landing without flare (which is effectively what you're talking about) would certainly rattle your teeth, and would have O2 masks deployed, passengers rather upset, and the galleys in a total mess!
I can't remember the specific RoD on landing that requires a heavy landing check (the aircraft would prompt the engineers in any case), but I'll check at work tomorrow.