that stairs are helicoil type too, but, they are half sided helicoil indeed.
The stairs go down in a 3/4 rectangle opening from one floor to another and are in the center "crowned" by a big self supporting stainless steel tube artwork.
That stairs are in the center tower of the main building of the European Parliament in Brussels and you climb here from the ground entrance level to the big hemicycle on the third floor.