Chris,

The glass passages at GCT are 1" thick unpolished slabs some four or
five square feet each set in steel frames.  A good solution similar
to the spaced window front wall at Cincinnati Union Terminal and
others.  And, yes, we did replace in-kind where needed.  And I have
seen some  library systems as well as the really dense sidewalk
varieties...like the floor of the old Ferry Terminal at Hoboken, NJ.

What I'm thinking about is how to provide a glass floor to pass light
in the spirit of an iron-and-glass vocabulary (aren't those old glass
canopies magnificent...see the Vanderbilt-42nd Street entrance to
GCT.  I even saw some glass and iron beauties in Buenos Aires.) which
will not scare the beJesus out of someone.  In the name of ADA, I
think it's only fair along a public way.  I suspect a THICK glass
block in a frame of some tight density would do better than the large
slabs of glass in slender frames.

On a related matter, last summer I went to the observation level of
the TV tower outside Moscow.  You get off the elevator a quarter of a
mile up and are distracted by the bright broad curved windows ahead
revealing a startling panorama of the city.  As you've walked
forward, what's really startling is to look down between your feet a
quarter of a mile down to the base of the tower!  (Large clear
panes.)  Some people literally hug the walls to get around.

--Jim