I've encountered severe turbulence twice in my career... once was in a Piper Seneca flying over Appalachia... we hit mountain wave and it was enough to pick my flight case and some of the cargo in back up and smash it into the ceiling hard enough to break the plastic covering...
The other was at 29,000' working our way around imbedded thunderstorms... we hit an updraft that didn't paint (and had small hail in it)... that one tossed the folks around in the back pretty good and put a negative G load on us for about 2 seconds followed by a positive 2-3 G load for another couple of seconds. Fortunately nobody was hurt.