EE WAS WELDING CHECKER PLATE IN THE RAWMILL FEED STRUCTURE. CO WORKERS WERE WORKING APPROX. 10' ABOVE HIM MOVING OTHER SECTIONS OF CHECKER PLATE WHICH HAD BEEN STACKED ON THE FLOOR OF THAT LEV EL OF THE STRUCTURE. AS THEY PULLED A SECTION OFPLATE FROM THE STACK, A PIECE OF STRUCTURAL STEEL FELL THROUGH A SMALL FLOOR OPENING AND STRUCK THE INJURED EE WORKING BELOW ON THE FOOT.