When I perform concrete design on an element, the output reports reinforcement in terms of "SQ.MM/MM". Can you please explain why?
When you ask for an element design or a slab design using the commands
DESIGN SLAB ..
STAAD designs the element for the moments MX and MY at the centroid of the element. By definition, MX and MY are termed as Moments per Unit width, since that is what they are. They have units of Force-length/length, as in 43.5 KN-mm/mm, or 43.5 KN-m/m. In other words, if you take a one metre width of the slab at the centroid of the element in question, the moment over that one metre width on that element is equal to 43.5 KN-m.
The design of that element hence has to be done on the basis of a unit width. Thus, in order to design an element for a 43.5 KN-m/m moment, one needs to use a one metre width of slab. The reinforcement required for that element is thus reported in terms of unit width of the element. The results are hence in the form Area of steel/unit-width of element, as in, "SQ.MM/MM".