A door structure is described which includes a door frame having a threshold. The door frame encloses an upper door section and a lower door section. An upper door guide track guides movement of the upper door section between a closed position enclosing the upper portion of the door frame and an open position moved upwardly clear of the door frame. A lower door guide track guides movement of the lower door section between a closed position enclosing the lower portion of the door frame and an open position moved downwardly into the threshold clear of the door frame. The upper door section and the lower door section are independently operable to enable the lower door section to remain in the open position while the upper door section remains in the closed position.