"main supporting wooden beam that carries flooring," 1610s, agent noun from gird, on notion of something that "holds up" something else. Used of iron bridge supports from 1853.
Entries linking to girder
gird v.
Old English gyrdan "put a belt or girdle around; encircle; bind with flexible material; invest with attributes," from Proto-Germanic *gurdjan (source also of Old Norse gyrða, Old Saxon gurdian, Old Frisian gerda, Dutch gorden, Old High German gurtan, German gürten), from PIE *ghr-dh-, suffixed form of root *gher- (1) "to grasp, enclose." Related: Girded; girding.
Throughout its whole history the English word is chiefly employed in rhetorical language, in many instances with more or less direct allusion to biblical passages. [OED]
As in to gird oneself "tighten the belt and tuck up loose garments to free the body in preparation for a task or journey."