Tributary Area and concentrated load question

I have question related to tributary area and concentrated loads. If you have a concentrated rooftop load on a metal building, AC unit for example, positioned between two intermediate frames how do you allocate that load to each frame? Do you use a ratio of the distance from each frame? Or if it falls within the tributary area for one frame do you assign the whole concentrated load to that frame?


Some would go to each frame.

The bigger question is whether the purlins (z-shaped beams between rafters) can take the load. These can be 8" deep and 20-some feet long–very flimsy.

Above is a snippet.