Interesting handle.
Not necessarily the weight of the device, but the tendency of the structure itself.

Unless the building is on the verge of falling down it shouldn't be an issue. An average 500 lb upright piano is a load of around 50 lb/sf, but only over 10-12 sf in a 100-200 sf room. 40 lb/sf is an accepted average loading, so unless you have the room packed with pianos you should be fine.

Does the piano sit flat on the floor or stand on 4 rollers? Can you see the floor underside from the basement?

mxyplx, I'm looking into it--the piano is at someone else's house right now and I can't remember. Pianos are usually on rolling casters, aren't they?

I can see most of the underside of the living room floor from the basement, though not the spot right where the piano will sit. But I can see what kind of framing was used in general for the floor. What should I be looking for?

lalala:

You're worrying about nothing.

I wouldn't assume anything. Our previous house was built in 1963 before I was in the family. Stud spacing was totally random in the bedrooms. Spaced in odd feet not inches. One of the carpenters had the nickname "One Nail." Can you imagine why?

A quick look costs $zero.

See if the floor joists are regularly spaced and relatively close together and if there is subflooring so the load is distributed. Wide spaces like 3-4 feet might indicate a possible problem with possible roller breakthru if construction involved shortcuts on sub flooring. Sub flooring in the '30s would probly have been 2X8. With adequate sub flooring - no problem.

If the piano is on rollers the floor load at each roller would be locally concentrated and quite high. Like 100 lbs on each roller over maybe 1/2 inch is 200psi. Not psf - psi.