Hi Jon,
pretty straightforward to make your own main steelwork (if you've got a big welder) , and use pre-bought purlins/cleats.
Steelwork sizes for a standard 'strong' mezz
Columns: 100x100x5mm SHS
Top plate: Can't remember but it's about 100 x 250 x 10mm (cut them from 100 wide flat strip)
Base plate 250x250x12mm squares cut from 250mm flat strip
I-Beams 254x102x22 KPM
Use M16 bolts to hold I-Beams to top plates
Keep the purlin length to around 12' max for a strong mezz. Column spacing on the main steels can be a bit longer .
welding the steelwork . I use a 270 amp MIG turned nearly all the way up , with a large torch and 1.2mm wire. Up to three passes as you see fit. (especially on column bases)
Folded steel purlins - 65x170 C-section. probably get them made new pre-cut to your length and pre-drilled. Not expensive . Buy the L-shaped cleats from the same place.
All the info you need is here
http://www.ayrshire.co.uk/C%20Sections.pdf
use M12 hardware on purlins/cleats. Be sure to plan/install the purlins on centres to match your chipboard lengths. 600mm centres are normally good.
Chipboard 38mm Tongue & Groove P5 Grade from your timber wholesaler. Use nice big self-drilling screws through the purlins. Stagger the joins.
Staircase - pre-made treads available from
http://www.fhbrundle.com/flor-st1.htm
drilling the steel: It's possible with a pillar drill, but a bit tedious. Magnetic broaching drills go through like a knife through butter .
https://www.machinemart.co.uk/c/magneti ... g-systems/ . Make the holes 2mm bigger than the fasteners.
Any other questions - just let me know !
Pic attached. Make it look like this .

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