I'd go for girder, as someone who "girds their loins". A girder can be a person, but a garter is an object, rather than a person, and the clue specifies "someone."
I've just had a thought (don't get many, these days).
"Someone who belts" = someone who girds i.e. fastens a belt around;
"strong member" = a (steel) girder is a strong member in a building/bridge or the like.