Michael'
I like it - very impressive! 
You certainly haven't chosen a 'cheap' option!
Easy when it's only virtual(?).
Any grey hairs yet?
Regards Tim
Lost the last of my black hair a long time ago Tim, lucky to find much grey now either, all turned white.
With about 13 million cu/ft to play with, it will take me the biggest part of this year to complete the gallery and start filling it up with some of my work.
The spiral staircases are about 60" wide at the outside of the stringers, with the central open section being 20 feet diameter. the galleries that the staircases will be cutting through will be upwards of 320 ft and may as big as 400 ft square and about twenty feet high (was 25 feet, but cut it down as the distance to view higher up moves the viewer further back away from the walls). the floors shown are only temp floors at present which are about 80 ft square.
I intend to use every surface both interior and exterior, and also mosaic floors and maybe frescoes on the ceilings eventually, exterior sculptures on the fenced roof sections as well as whatever architectural features I can add, maybe some big glass shard ducts that break into the surface and down 80 ft through the subsurface galleries.
The domed roof on the very top of the gallery should pump lots of light into the bowels of the gallery through the center of the spiral staircases.
I will probably break the lower galleries down into separate drawing and attach them as xrefs to the main model as and when I use them, that way I can start filling the galleries from the top and start producing some prints along the way, attaching or detaching the xrefs as req'd.