Torsion Box Question

**Chris Parks** · 31st October 2022, 12:47 PM

I have built torsion boxes before, one was 3.6 metres long but I have never built a torsion box with unequal top and bottom thickness. I need to make some shelves which will be heavily loaded and about 1.8 metres long and I am trying to limit the thickness so I have the idea of a 3mm MDF lower and 12mm upper skins with 3mm internal grid 9mm deep. I would make the top plate out of 6mm but 6mm melamine does not seem to exist. The question is how do unequal top and bottom skins work in practise or has some here done it and can shed any light on how it went. My gut feeling is there should be no problem but if I am not mistaken the majority of the load is taken by the bottom skin.

**Bernmc** · 31st October 2022, 07:42 PM

I can't answer your question, but I believe torsion boxes are based on Sandwich Theory (and not the cheese and ham kind) - there's a lot of complex maths involved in calculating the stresses, but you may be able to do some googling and find something that simplifies it.

Fusion 360 has stress modelling capabilities, so that would help... or do it the easy way and make a prototype and try it.

**Chris Parks** · 31st October 2022, 11:58 PM

Bernard, generally the skins are very thin and the deeper the internal grid the stronger it becomes, making the upper and lower skin of thick material is defeating the whole premise of why they are used. The 3.6 metre one I made was easily lifted into place by two people and I built it on my own easily moving it without help during the build.

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