An employee was installing decking (steel grates) in a new construction steel framed building. The employees were working approximately 22' above the next lower level, which was a concrete ground floor. The decking was pre-cut with the floor opening prior to installation. Shortly after installing the floor opening, the employee fell through the floor opening, approximately 18' onto a section of metal ductwork, resulting in fractures to the skull, ribs, thorax, and lumbar vertebra, as well as an ear laceration.