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By siggy_7
#1322057
I've not seen torsion boxes built with the internal ribs in only one axis, but my intuition tells me you're right in saying that the lengthwise ribs are the most important for your application. For the dimensions you're looking at, I would suggest in addition to a rib along the front and the back of the shelf you put one down the middle of the shelf - that's what I would build anyway, but it may be unnecessary. If you're going to include front to back ribs (again, I would if only for belt and braces comfort), I would space them at something like 200mm intervals - over a 900mm span, one each end plus three in between at 225mm centres would be fine I should think.

Regarding how to install the shelves, my first choice would usually be the little metal shelf supports that fit into 5mm holes in the carcass sides. If you want fixed shelves, then dowels (or dominos if you have the machine) would be the way I would do it - bump up the rib thickness on the end of the shelves to 18mm so the fixings have plenty of purchase. Housing joints would also work well, but fit and finish may be a bit harder to get a really tidy look.

I've not used MDF like this in a structural application, but I imagine it will be fine. I've had bad experiences with solid MDF shelves sagging where I've under-specified it, but for a torsion box of this thickness I can't see you running into issues. Someone more familiar with MDF may be of more use here.