box beam vs box girder

box beam and box girder both are nouns.

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

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

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

box beam (noun)	box girder (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