Thanks you for asking this question. You are correct, you do not want to model all of the floors; it would take long to model and a ong time to run. The thermal zone multiplier is exposed in the thermal zone tab of OpenStudio 1.5. It is also in earlier versions of OpenStudio but is a little more hidden. If you are using pre-1.5 version I can provide specific instructions.
- On a smaller building you could geometrically model your building as three stories, and then set the zones on the mid-floor to a higher zone multiplier than the default value of 1.
- You would also want to set the interior floor and surfaces to adiabatic boundary conditions. I would first run surface matching, assuming your three stories were modeled adjacent to each other. Then I would use surface search in SketchUp to find all floor and ceilings that have a surface boundary condition. Run the user script to change the boundary conditions to adiabatic, it will allow you to choose a construction. You want to make sure you pick a construction since adiabatic surfaces in OpenStudio do not inherit from a construction set.
- Optionally you can move your top floor and mid floor to a different height. If nothing else this make it obvious to someone else opening your model what the intent is, but can also have other impacts.
- Since your building is so tall, you may want to consider modeling more than one mid floor. I'll let others chime in on how many stories is appropriate for a mid-floor multiplier on high rise buildings, but maybe something around 10-20 floors each? Two factors that would impact this decision is shading by adjacent structures, and effect of wind at different heights above the ground.