Here are a few pics of the Vampire stage that I have tracked in my last few entries. In the top right corner is one of the frame for the second story deck half way completed. (sorry uploaded in the wrong order) The 2x8 beam in front is backed up by a 2x4 between the columns which provides a sort of tray for the platforms to nest into. Top left is the second story deck complete from the front, bottom right is from the rear
(notice there are only three columns in the front rank allowing some playing space underneath the platform) bottom left is a view of the top of the deck taken from above in the up right corner of the stage. Note that the metal roll-around stairs seen here will provide a more conventional means of entry and escape from the second story, while the Vampire will use the trap door in the DR column (the hole is visible in the top view) there is a pipe and frame ladder that allows him to "appear" among guests in the lower playing area without using the fire escape.
The deck on top is made up of 6 4x8 wagon platforms (stock in most theatres) the outside beam is bolted to the columns and the platforms and the platforms are bolted together underneath. The structure is completely stable and will hold well over a thousand pounds without any danger.
For theatre companies who are interested a complete set of drawings is available. The columns can be collapsed after disassembly allowing this entire structure to be stored in a very small amount of space. The total cost of construction (starting from scratch including platforms) is under $2000 for materials, including the back wall of flats not shown here.