EMPLOYEES WERE INSTALLING STIFFNER CHANNELS IN THE WEB OF STEEL COLUMN. THE UNINSTALLED CHANNELS WERE LEANING AGAINST THE HANDRAIL NEAR THEIR REQUIRED AREA. THE VIBRATION OF THE PREP PLANT FLO OR CAUSED ONE OF THE CHANNELS TO FALL, STRIKING AN EMPLOYEE ON THE BACK, BRUISING HIS BACK & RIB AREA.