EMPLOYEE WAS CUTTING A SECTION OF BEAM OUT OF THE FLOOR STEEL FOR REPLACEMENT. WHEN THE CUT WAS COMPLETED, THE BEAM SHIFTED. HE JUMPED BACK, LOSING HIS BALANCE, FALLING APPROXIMATELY 4' TO THE FLOOR BELOW. HE WAS WEARING A HARNESS THAT WAS CONNECTED TO THE STEEL BUT AT FLOOR LEVEL. THE LANYARD WAS NOT CONNECTED HIGH ENOUGH TO PREVENT THE FALL.