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I plan building things with aluminum extrusions.
On this website I can calculate the deflection depending on the profile length and load. https://8020.net/deflection-calculator
I.e. if I select a 20/20 (mm) profile with 500mm length and 150kg centered load then the calculator shows 2 mm deflection. So far I understand this. Let's say if I would build a chair and I would sit on that profile (to make it easy just pretend there is just this one 500mm profile). And the profile would deflect by 2mm. Is that acceptable in a way that the chair won't break? Will the profile go back to the original position when I don't sit on it anymore? Would 5mm or even more still be acceptable? How do I know how many mm deflection are acceptable?
I saw a similar question and it was asked what does acceptable mean. For me it means that the construction is not permanently damaged. I.e. in this case a heavy person could sit on that chair for hours, then don't sit on it, then sit on it for hours again and the chair would still work after years.
Here is a sample with data from above website:
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