30 ft moment frame on top of 100ft structure SDC D

I am designing a 30 ft high metal building on top of another steel structure that is 100 ft tall (by others),the site is located on SDC D, The baseplate will be pinned and bolted to a steel beams of the 100 ft structure, the purpose of the structure is to enclos machinery. According to Chapter 11, this structure can be designed under Chapter 15 for nonbuilding structures. However, there's an exception in Chapter 12 allowing the use of single-story OMF with unlimited height for this type of building. Additionally, Chapter 15 permits OMF with unlimited height but with seismic coefficients equal to 1.

Some questions I'm grappling with:

-Which chapter is more suitable for this structure?
-Does the upper portion qualify as a single story and therefore be designed according to Chapter 12?
-Designing it as a nonbuilding structure will result in a seismic load 3.5 times higher than designing it as a building structure (R=1 vs R=3.5). Does that seem accurate?
-This is my first time designing a structure with this scenario. Are there any special design considerations I should be aware of?

 

[ENGIRL]

I think I would look at finding the floor spectrum for your superstructure. Basically, you apply the ground motion on your steel structure find the acceleration response of your steel structure (on top) and use that acceleration as ground motion for your superstructure. I would build a 2D model for this analysis.