Can a mezzanine be enclosed completely under it yet open to the main room?
The area underneath the mezzanine can be enclosed; however, the floor area of the enclosed space cannot be included as part of the floor area of the room to which the mezzanine is open. In other words, when determining the 1/3 maximum area for the mezzanine, you could not include the enclosed space under the mezzanine.
Otherwise, there would be nothing to stop someone from considering all enclosed spaces as part of the area to which the mezzanine is open to determine the allowable mezzanine area.
Now picture this. You also enclose the mezzanine level by installing two exits which allows you to do this. Now that sure looks like a two story and not what I believe the Codes intention was. That is why I am thinking you cannot enclose the area below
The issue is confusing at best. Going through the "openess" required for mezzanines (505.2.3), it appears that Ron is correct. The cause of enclosing the area below a mezzanine is reduction in allowed mezzanine size. Being able to enclose a mezzanine above the mezzanine floor has further limits and conditions that are trade offs for sprinklers and exit arrangements. If all conditions are met, then a mezzanine does not constitute another "story".
Sorry I am still not seeing it. By enclosing the first floor area AND the floor above this the so called mezzanine is not within the room of the adjacent space that it is suppose to be a mezzanine within. I see it has just a two story adjacent to a tall one story space. Here in Calif the definition of mezzanine is;
MEZZANINE. An intermediate level or levels between the floor and ceiling of any story and in accordance with Section 505. [DSA-AC] An intermediate level or levels between the floor and ceiling of any story with an aggregate floor area of not more than one-third of the area of the room or space in which the level or levels are located. Mezzanines have sufficient elevation that space for human occupancy can be provided on the floor below.
The italic is text added by our State
To help explain the issues involving mezzanines, I've created the attached sheet.
WOW, thanks rlga, you're the man. BUT you left off the one that applies to my question. See attached
It is a combination of my diagrams 2 and 3.
Per your diagram, C can be considered a mezzanine, provided the area of C does not exceed 1/3 the area of A, and C has an occupant load of 10 or less or has two exits or access to two exits.