(Source: Harlan Brumm, The Revit Clinic Blog. http://revitclinic.typepad.com/my_weblog/2011/04/the-case-of-the-disappearing-partsfloors.html)
The new Create Parts tool in 2012 allows you to select a floor, wall, ceiling, or roof (flat) and divide the system family into its sub layers, creating parts. If you haven’t tried it yet, check it out. It's intended for construction uses to make dividing your model into constructible pieces easier without having to duplicate and recreate the original geometry. However, you may notice that if you are in the floor plan view that has a floor and you select the floor and make parts from its layers, the floor and its parts seem to disappear from the view.
Here one minute
Gone the next
The issue here is that floors are a little special when it comes to displaying in a floor plan view and that same “specialness” doesn’t apply to parts. Floors in Revit are typically created below the Bottom of the view range (they start from their host level and go down) and in normal visibility and graphic terms, shouldn’t really display in a floor plan anyways. However, due to architectural convention and our drafting needs, Revit displays floors in a floor plan view even if they are not technically within the view range of the view (and instead are at the bottom).
Parts don’t get this special treatment. There are a few ways to have the floor or the parts display in the floor plan view again.
There is a new parameter on views called “Part Visibility.” Changing this parameter to “Show Original” will display the original floor object. Changing the parameter to “Show Both” will slow the floor and the parts, but the parts will still not be shown in the view as they are outside of bottom of the view range. “Show Parts” will hide the original object (the floor in this case) and only show the parts (which you cannot see in the view yet).
To get the parts from the floor to display, you will need to adjust the bottom of the view range and the view depth so that the parts are included in the view range. In my case, simply dropping the bottom offset -1.0 mm and the offset for the view depth the same amount results in the parts from the floor being able to be displayed in the view. As long as the parts exist at all between the Top and Bottom of the Primary View Range, the parts will display. (They don’t have to be fully within the top and bottom, just a sliver needs to be within the Range).