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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:

enter image description here

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In general the acceptable deflection has to do with perception. What I mean by that is, if it is possible to perceive the deflection of the structure. Therefore it is usually presented in building codes (usually that's where you most see it), as a percentage of the span of the beam.

However, there is no universally accepted allowable deflection. For example:

Code maximum allowed deflection as a % of beam span
International Building Code (IBC) $\frac{1}{360}$
International Residential code (IRC) $\frac{1}{360}$
AS 1170.1 $\frac{1}{240}$ or $\frac{1}{300}$

Additionally, the type of materials usually has different acceptable allowable deflections. For example, the recommendation for more rigid/brittle materials is to have less deflection than more flexible/ductile materials.

Materials maximum allowed deflection as a % of beam span
tile and stone flooring $\frac{1}{720}$
flexible materials $\frac{1}{240}$ or $\frac{1}{180}$

Ultimate Strength and Permanent deformation

Although not explicitly stated above, when the allowable deflection limit is set, it implicitly imposes constraints on the maximum stresses on the structure (this is why flexible material generally have higher acceptable deflections).

Of course there are exceptions.

Example

Excerpt from rmit.edu

Deflection in beams is a major issue in structural design. Engineers adopt deflection limits which suit the nature of the structures. For example, according to AS 1170.1 Minimum design loads on structures (known as the SAA Loading Code):

maximum allowable deflection = span ÷ 300

This means that to calculate the deflection in a beam which spans 6,000 mm, divide 6,000 by 300.

6,000 ÷ 300 = 20

So a span of 6,000 mm has a maximum allowable deflection of 20 mm.

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  • $\begingroup$ Thanks for your detailed answer. I upvoted it already. I guess but I don't know if this is the correct answer. For that reason I wait a little before I click on accept. Thanks! $\endgroup$
    – Edgar
    Jun 29 at 23:51
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What's acceptable will depend on your application. For a chair, 1mm of deflection will not be noticeable. For a CNC machine not so much.

If you really care about optimizing the design, I would start by designing the object with narrow extrusions, and then see if/were the deflection will be too great, then bulk it up as necessary.

This calculator unfortunately does not tell you when you've left the extrusions elastic region (the region where it returns to it's original shape once bent).

Metals have an addition property where cyclical loads can weaken the metal over time. This property is called the fatigue strength. This is a load where the material will not be damaged even by cyclical application of force. Unfortunately there is no load where aluminum will last forever. ..but for something like a chair I wouldn't worry about it, because we're talking millions of cycles.

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