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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.

1. 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.
2. 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.
3. 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.
4. 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.

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.

1. 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.
2. 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.
3. 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 there can also have be other impacts.impacts of the position.
4. 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.