lift girders

[Ya Sam]

Was going to ask this long ago. Just now remembered. I couldn't find the 
term "lift girder" in any dictionary, even google didn't yield anything 
coherent. So what do "lift girders" (in the beginning of GR) exactly mean, 
and in which way are they different from girders proper? (Sorry if that was 
asked before, but I couldn't find anything in the archives as well).

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