Yep, "bottom chord of an attic truss" is what I am referring to. Your
link is precisely what the contractor is using. After reading these
posts, I obtained the diagrams from the truss company, and it appears
they can bear 60 psf (40 live load, the rest I'm assuming dead load).

Thanks.

The OP used the term "floor joist", but I suspect that he is looking
at the bottom chord of an attic truss. He needs to clarify this.
Since most attic trusses will have a spliced bottom chord, I suspect
that is what he has, and his contractor hasn't given him a very good
answer.