EE AND CO-WORKER WERE INSTALLING A CATWALK STRUCTURE APPROX 8' ABOVE GROUND LEVEL. INJURED EE WAS CLIMBING DOWN TO THE GROUND LEVEL TO OBTAIN ANOTHER COME-A-LONG WHEN HE LOST HIS GRIP ON THE I RON, THEN LOST HIS FOOTING, THEN FELL FORWARD STRIKING HIS KNEE ON A PIECE OF CHANNEL IRON. EE'S FALL PROTECTION LANYARDS WERE SECURED TO 2 SEPARATE ANCHORAGE POINTS ABOVE HIM.