The Collaborative International Dictionary of English v.0.48:

Superstructure \Su`per*struc"ture\, n. [Cf. F. superstructure.]
   1. Any material structure or edifice built on something else;
      that which is raised on a foundation or basis; esp.
      (Arch.), all that part of a building above the basement.
      Also used figuratively.
      [1913 Webster]

            You have added to your natural endowments the
            superstructure of study.              --Dryden.
      [1913 Webster]

   2. (Railway Engin.) The sleepers, and fastenings, in
      distinction from the roadbed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

superstructure
    n 1: structure consisting of the part of a ship above the main
         deck