if a file get corrupted, drag the file from explorer into a new 3ds session and merge the files. it won't always work but does from time to time.
as for your buildings using too much ram, the short answer is use less verts. also if you have very tiny detail in a very large object, it may cause memory issues. I'm sure this is not what you wanna here, but either try just making the model as a cube plus any additional detail for doorways or building tops and put a material over the faces of the buildings. or if you really need holes in the object, say to have a view inside the window, only do it for the floors you really need it for. a trick I've used for flying in or out of a window was to run two passes. one with an opaque window and one with the window removed and only having the details inside the room when all you can see is that one single building.