In the Cambridge main library (about to be renovated) there is an interesting structural system for the Stacks. The space is divided into four floors with a shelf support system that provides the structure for the flooring. It is all iron work; you can see through the floor to the next level. I was told that this system was pioneered in the Harvard Gore library built in 1840 and razed in 1913 to be replaced by the Widener. This implementation will be destroyed soon; they considered using the flooring on the ceiling of the young adults section which will be in the space currently occupied by the stacks but structural difficulties have ruled it out.