I’ve got a barn that currently has a suspended ceiling (used to be a farm shop before farm shops were a thing).
The ceiling is basically some form of fibre board nailed underneath 3×2 joists at 40cm centres and then insulation on top. I’m getting rid of the ceiling because it’s manky, but I’d like to have a bit of a mezzanine floor to store some fairly light stuff on (spare dining chairs, boxes of xmas decorations, etc.)
The problem is that the span of the joists is about 4.5m and 3×2 isn’t strong enough to carry mush weight. Might it be OK to double up the joists using ones that I’m removing completely to effectively make 3×4 joists? Or should I just replace with 6×3 and then chipboard on top?

can you suspend the beams?

Hmmm. Well the roof trusses are made of angle steel and there are some (fairly feeble) hangers already there so I guess I could.

Are the existing ceiling joists bearing on a wall plate either end? If so and you have the room you may be better off fixing some 5″ or 6″x2″ alongside them over a 4.5 m span keeping the same ceiling height but a higher loft floor. I’m no structural engineer but would certainly feel happier walking about up there rather than on doubled up 3″x2″, even if you have to mitre the ends to get under the pitch of the roof.

For a floor you will need 47x220mm at 400mm centres.
I would say worthwhile checking the ‘fibreboard’ is not asbestos.
Have you considered applying for planning to turn said barn to a house? Permitted Development now with Prior Notification. Loads of farm buildings being converted round here and making the owners a more than tidy sum.

I’m afraid that the doubled up tiddly joists would not be much good. It would be the deflection that would be high rather than actually failing. Maybe 30mm in the centre which would be too much if you had a plasterboard ceiling. 50x150s would be much better. But if you wanted Building Regulations you would be looking at 50×200! And if you wanted it to be habitable then 50×220.