interlaced alternating stairway
Two alternating stairways interwoven within one stairwell, with adjacent straight flights rising in opposite directions.
Interlaced alternating stairways are two alternating stairways interwoven with each other in a single stairwell or vertical space. Each flight rises straight from one floor to the next, while another one traverses an adjacent space in the opposite direction. The two stairways generally serve the same floors but take turns accessing points on either side. Each of the two stairways can be walled off from the other so that the person inside may be unaware of the other one.
This type is used sometimes in high-rise buildings as a means of concentrating maximum stairway access into a limited space. Its space efficiency is even greater than a hairpin stairway's because it does not require intermediate landings, and it allows better transportation of large objects because there are fewer turns. However it is generally practical only when there is a need for multiple stairs within a small space.
The routes can be separated by walls, but enclosure is optional. The interwoven straight flights define the arrangement; neither route needs to be visible from the other.