EE WALKING OR GRATING AT ELEVATION 719 TO RETREIVE TORCH SET. EE WAS NOT TIED OFF WHILE EXPLOSED TO FALL HAZARD. EE WAS WEARING PERSONAL FALL ARREST SYSTEM HARNESS AND 2 LANDYARDS. EE STEPPED ON GRATING THAT HE INSTALLED THE DAY BEFORE BUT WHICH HAD NOT YET BEEN WELDED TO THE STRUCTURE. THE GRATE 107 X 24 SHIFTED AND EE FELL 48"" TO CONCRETE BELOW.