Smashing a punchbag seems an exercise in futility: they're built for that sort of thing!  ;-P

ST 15 at minimum, I'd think?

(I have a 50-lb canvas heavy bag in storage.  Came in handy from time to time: much stronger than a pillow if you feel the need to punch something during high school.  You can shear off a cheap metal hook if you hit it right--I have--but a reputable gym would mount 'em properly: reinforced swivel.)