To elaborate further on CR/RTs posts, and feeling a tad pedantic today, an example of the weight limit imposed by floor strength (or lack of same) could be our A300s which, in the rear lower-deck, allows for a maximum weight of either 1588 (R1L/R) or 1285 kg (R2-4L/R).
Generally speaking, ULDs will accept a higher weight than the aircraft and hence the ULD limit could, in most cases, be described as "theoretical" - ish. Now try to explain that to your commercial department
I'll get my coat.