The numbers I am giving you are based on my experience as a Residential & Commercial designer. The 15lbs/ft2 is dead load, which is based on the material itself.
The 40lbs/ft2 is the live load, which is people, furniture, fish tanks, etc.
Consider live load to be anything that can be moved, and the dead load the structure it self.
If you need more info I will pull out my building code to verify.
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