This is an interesting thread. Although I studied engineering and
mechanical structures, I simply can't remember much of it. I want to
look at a loft conversion for our mid terraced house and will need to
consider strengthening the ceiling joists. I have a feeling that this
is a little more straight forward than you problem and has been done
100's of thousands of time across the UK so there should be lots of
info out their.

At the moment all I want to consider is strengthening the
floor/ceiling. The full conversion with stair will come a little later.