WHILE REPAIRING A BROKEN CONVEYOR BELT THE INJURED ATTACHED A 3/4TON CHAIN HOIST TO A 6""X 6""X 7'I-BEAM THAT WAS MOUNTED VERTICALLY ON A CONCRETE PAD. WHILE ATTEMPTING TO PULL APPROX. 20'OF SLA CK BELT WITH CHAIN HOIST THE BEAM BECAME DISLODGED AND TOPPLED OVER STRIKING THE INJURED MAN IN THE LT. SHOULDER AND LT. SIDE OF THE HEAD.