EE was helping tack weld a brace support on a dragline. Standing on fold down step and stepped down onto pony winch gear box. His foot slipped off and fell on right side, landing on the gear box. Taken to ER where his injury was diagnosed as a contusion in his rib area. He returned to work the next shift. CT scan on 9/19/11, was notified that he fractured several ribs, T8-T12.