I will cut the floor joists in the old stable to install a glulam beam in the middle to support the upper floor without posts and with improved ceiling height. Without going into details: the existing joists will be attached with joist hangers to the beam, and they are from the time when building was done with what worked last time, rough planks that are about 180 mm high and 110-120 mm wide. Where can you buy such "roomy" joist hangers? XL and Beijer don't seem to have them in their range and the load isn't as enormous as the timber dimensions might lead one to believe, it's just floor joists.
I'm happy to order online if there's a shop with good prices, as I need quite a few.
