We designed a long span stair with the appearance of a folded plate.   No stringers!  (picture below courtesy of Diamond Iron, the fabricator and erector)

The stairs were actually designed with thin saw tooth vertical plates hidden within the thickness, here is an FEM screen shot...

The stair was not governed by strength (only about 11ksi) but by live load deflection (1/4") and vibration.  We wanted to keep the vertical frequency above about 8Hz without adding to the steel weight.  This was set to limit affect of exciting the second harmonic when descending stairs quickly.   We did this by trimming down the thickness of the tread and risers, and increasing the fin plates thickness.  Also, we reduced the landing hangers from 6 to 4 and cantilevered the corners of the 3" platform over 4 ft.