How to support a turn in a staircase?
Dog-Leg stairs


  Structurally, the flights of a dog-leg stair are usually supported by the half-landing, which spans the adjoining flank walls.



Image: Bill Bradley. CC by SA 3.0

In this arrangement, the stringers are joined to a newel post. The lower landing is also partly supported by the newel post. The upper by only the flanking walls and, I guess, effectively braced by lower stringers.