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The simplified method I've used when modeling commercial buildings involves assigning ground-contact surfaces the 'Ground' outside boundary condition (which you can do from the Skechup Plugin, or in the OS application) and an 'always run' EnergyPlus measure to add a Site:GroundTemperature:BuildingSurface object to define monthly ground temperatures. From the input description for that object: "a reasonable default value [of monthly ground temperature] is 2C less than the average indoor space temperature."

To make the ground temp measure flexible in case you do have monthly temperatures, you can write the measure to take a user argument for each monthly temperature, like:

#repeat for each month
jan_temp = OpenStudio::Ruleset::OSArgument::makeDoubleArgument("jan_temp",true)
jan_temp.setDisplayName("Ground Temperature in January")
jan_temp.setDefaultValue(20)
args << jan_temp

and in the run section, get the argument value(s), create the object and pass into the workspace:

#repeat for each
jan_temp = runner.getDoubleArgumentValue("jan_temp",user_arguments)

#create ground temp idf string
groundTemps = "
Site:GroundTemperature:BuildingSurface,
    #{jan_temp},
    #{feb_temp},
    #{mar_temp},
    #{apr_temp},
    #{may_temp},
    #{jun_temp},
    #{jul_temp},
    #{aug_temp},
    #{sept_temp},
    #{oct_temp},
    #{nov_temp},
    #{dec_temp};
    "
#creat object and add to workspace
idfObject = OpenStudio::IdfObject::load(groundTemps)
object = idfObject.get
wsObject = workspace.addObject(object)

This method should allow some heat transfer through ground-contact surfaces to be calculated by the program, although in simplifying the temperature inputs, the magnitude of that heat transfer might be over- or under-predicted. How significant these effects are on your model will depend largely on the number and type of spaces with ground contact surfaces, and how loads in these spaces compare to the rest of the building's envelope and internal loads.