The Collaborative International Dictionary of English v.0.48:

Hammer-beam \Ham"mer-beam`\ (-b[=e]m`), n. (Gothic Arch.)
   A member of one description of roof truss, called hammer-beam
   truss, which is so framed as not to have a tiebeam at the top
   of the wall. Each principal has two hammer-beams, which
   occupy the situation, and to some extent serve the purpose,
   of a tiebeam.
   [1913 Webster]