interlaced helix stairway
Two or more helical stairways interwoven around the same axis within one shaft or vertical space.
An interlaced helical stairway is a set of helical stairways (usually only two) interlaced with each other in the same shaft or vertical space. This form of stairway is sometimes used to accommodate bi-directional travel within a narrow space, such as a tower or steeple with an observation deck on top. In such cases it is common for one of the stairways to be designated for climbing and the other for descending.
Each route remains a helix while sharing the vertical space with the others. Two routes are usual, but the definition allows more.
Assigning one route upward and another downward is a possible circulation arrangement. Opposing one-way use is not mandatory for the interlaced geometry.