While cutting a haul truck frame with liquid oxygen/propane torch, EE was instructed to cut the piece 75% through and then wait for the foreman to secure the piece before the final cut. EE cut 100% through, at which point the 6000lb started to fall. EE was pinned less than 5 min. as foreman placed bottle jack, to extricate EE. As soon as EE free, immediately called ambulance.