box girder vs box beam

box girder and box beam both are nouns.

Word	Noun	Adjective	Verb	Adverb
box girder	Yes	No	No	No
box beam	Yes	No	No	No

As nouns, box girder and box beam are synonyms defined as:

box girder and box beam: a beam built up from boards; has a hollow rectangular cross section

box girder (noun)	box beam (noun)
a beam built up from boards; has a hollow rectangular cross section	a beam built up from boards; has a hollow rectangular cross section