One viewer who caught my camera in a renovation store asked what to do about the 20 x 20 ceiling in his condo that was dipping down in the centre.
A serious visible dip must be caused by someone having removed a support wall from the original construction. Each floor is designed to remain flat and vibration free, but that is a combined calculation of how far it is between supports and how thick and rigid the floor is to span that distance. In modern houses we tend to go much further than in the old days because of the strength of glued and screwed plywood sub-floors, or in the extreme, the use of any of a wide variety of engineered floor joists.
A quick look at the Span Tables tells us that his 20 foot span would have required 2x12 joists on 12 inch centres using glued and screwed 5/8" subfloor, as one possibility. This is probably much bigger than is in this old condo. This case requires a structural engineer or a qualified contractor to evaluate if the existing floor can be strengthened, or does he need to put back some support under the centre of that ceiling.
The Canadian Wood Council in Ottawa publishes the Canadian Span Tables.