The pilot was performing a solo flight in preparation for a commercial rating. After three uneventful touch-and-go landings, the number of airplanes in the pattern increased from two to four or five. The pilot sequenced himself between two other training flights and inadvertently failed to lower the landing gear prior to landing. Substantial damage to the wing strut crossover bulkhead resulted. The chief flight instructor for the university flight training program reported that the accident scenario will be incorporated into the curriculum for all students.