All that said, in Edwardian times it wasn't uncommon to have a piano in the house. One of my great-grandfathers was a professional piano-mover (there were enough of them that that was a full-time job in a market town) and he used to say that they were quite often upstairs (and a nuisance to get there - or words to that effect
).
Our local building inspector once told me that modern floor joists are spec'd at least to support bags of cement laid uniformly, one layer thick, covering your whole floor. That may not be much help though if your joists a) were laid before building regs came in or b) have been degraded by time e.g. by having the ends go rotten where they are set into damp external walls or by not-so-professional plumbers and electricians cutting excessive slots and/or boring excessive holes in them.
Cheers,
GJ