JFYI (and it may depend on each country's building regulations) -anything (being it slabs, rooms, corridors, terraces, staircases) in "public" buildings is required by norm to be calculated for a "dense crowd" (cannot say if that is the proper term in English) which equates usually to between 400 (static) and 600 Kg/m2 (dynamic).

This roughly equates to 4-6 people per square meter.