Can't help with the calcs but I believe if it will be occupied space ie office then the steel needs to be coated with intumescent paint.
I use 2 local to me, will pass you details if you like.Need some drawings and calculations for a mezzanine in a warehouse to be used as office space.
Need some calculations and drawing for three beams
I use 2 local to me, will pass you details if you like.