Reference: Total Warfare, 9th printing
The rules for infantry sharing a building hex with other units are quite clear.
Page 167: if the total tonnage of units on any single level of a building hex (other than level 0) exceeds the hex's current CF, the building collapses.
Page 176: Use the Tons of Cargo Space Occupied column on the CI/BA tables to determine infantry weights for this purpose.
However, the rules are less explicit on what happens if infantry is mounted in or on another unit. Do they still contribute their weight for the purposes of building collapse?
Speculation: It seems reasonable that Mechanized BA would contribute weight, since they are very much additional weight on top of the original unit. Infantry in cargo space or infantry compartments are a little more ambiguous. Officially, vehicle weight defined during construction is its weight whether any such compartments are empty or not, which might imply that infantry mounted inside a vehicle would not contribute their weight to collapse. However, the same logic would lead to infantry dismounting inside a building hex suddenly increasing the total weight in the hex. I suspect a fully logically consistent solution is not possible, and we will have to go with "the game rules say it works this way" instead. Pick whichever you think works best.
