Exposed beams (fig.) over doorways is an ancient Chinese practice that indicated the status of the resident. The number of beams over the doorway increased as the occupants' status increased.

The lowest ranking person would have no beams and the emperor would have the maximum of 12 beams. And, the higher the number of beams was the higher also the threshold would become.

Raising the foot very high in order to step over the threshold, the knee would bend and that is symbolically the same as kneeling, and thus showing respect. to the occupant of the household.