Employees instructed to use JLG lift to install and tighten bolts on expansion joint. Employee untied safety harness, climbed out of JLG basket & climbed over onto an unsecure platform to allegedly look for an air hose. Employee did not tie off and fell through a floor opening. He fell approx 3-5 feet onto ""X"" bracing and slid down to top of classifier another 4-5 feet.