actually, the 40 lbs / sq if the dead load of the building, not the carring capcity.
Actually the 40 lbs is live load, i.e. what the floor can handle in furniture, people, safes, interior non-load bearing stud walls, sheet rock,etc. The dead load used around here as referenced to the ICC (International Code Council) is 10 lbs per sq ft. without sagging beyond tolerable limits. The only time it really matters is with long spans plus the loads are spread out more than just the small area under a safe or frige due to plywood or other wood flooring, bridging and even non-load bearing walls to a certain extent.
I used floor trusses once on a house with a twenty foot span and L360 deflection factor which was code. It could sag 2/3" and be to specs. Naturally the people who bought the house put their frige right in the middle of the span. The 2nd floor span was also 20' but much less of a load. The 1st floor went down but the 2nd didn't. Pulled crown molding down and was a real pain to fix. One of those times when you do everything right and still get burned. I had to replace crown molding scrape old caulk, repair drywall and repaint all of the walls to get it looking right. After this L480 was minimum I would do and L720 with a span of 20' or more.
My wife's 286+ gal saltwater aquarium required a lot of shoring up as at 9 lbs per gal it was way over loaded.