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Hi I know this is a little bit of a strange one, but I’ve got a relatively cheap shelving rack that I bought to make good use of garage space.

I have included a pic,enter image description here It’s footprint is 1220mm x 610mm and it currently has the standard 9mm thick particle board shelves. Although it’s been ok so far, there’s probably about 50kg on the middle shelf now (3D Printer + Laser Cutter) and I’m starting to see it bend slightly. I’m also planning to get a rotary attachment for the Laser which will add another 10kg or so.

I want to strengthen it now so it’s not an issue long term. One idea I had was replacing the 9mm shelf with a 12mm MDF one as this is the thickest I could go while still having the shelf sit flush with the frame.

I know this will be significantly more rigid, but it will probably still sag over time and does not address the issue of the frame itself potentially bending as the shelves just sit into it. To counter this my plan was to have two sections of 2020 aluminium extrusion spanning the full width which would be positioned under the board about 1/3 in from the front and rear of the rack to evenly distribute the load. I would bolt these extrusions to the frame by tapping the ends. I would also add a couple of short extrusion lengths perpendicular to the long ones to tie them together for added stiffness.

Here comes the question: I want to use 2020 extrusion because is pretty cheap and easily available in the lengths I need. I believe I will provide plenty of strength/rigidity but my Dad doesn’t think it’ll make that much difference, and instead recommends 3030 extrusion. The problem is 3030 is much harder to get here and way more $$$.

How well would my 2020 solution work? Who is right?

enter image description here

NMech
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Billy
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  • Assuming you're talking about the shelves sagging, rather than the vertical parts of the frame. If you have some clearance to spare, a quick&dirty alternative may be to stiffen up the shelf surfaces themselves, by screwing in a couple of "beams" (material of your choice) to the underside of the affected shelves, going lengthwise. // Stiffening up the supports as NMech and r13 describe is also a really good idea – Pete W Mar 25 '21 at 14:32

3 Answers3

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I don't have idea about the material you are mentioning, so no comment there. Instead, I suggest to provide bracings and additional support beams, using shelve angles, as shown below. Also, adding 1/2" - 3/4" plywood board can stiffen the shelve bed.

Note, you shall confirm the shelve is rated for your application.

enter image description here

r13
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  • If I use Plywood would it need to be structural grade? When I bought the shelf, it claimed to be rated for 75kg per shelf, but it seems it was very over rated. – Billy Mar 25 '21 at 17:25
  • Any plywood that suitable for flat works can do, you might elect to chose a higher grade for better appearance. Important thing is to make sure the shelve is stable, either tie it to wall, or provide back bracing as suggested by NMech. – r13 Mar 25 '21 at 17:39
  • As you can hopefully see by the image I’ve added, it is unfortunately not practical for me to put bracing across the back due to the vent hose and power cables in the way. It is hard to accurately describe how I plan to attach the 2020 without showing it somehow, but I think we understand each other. The shelf isn’t bending much so it sounds like tapping the end of 2020 and bolting into it through the side of the existing frame them sitting the wood on top as planned should do it. I will consider bolting the wood down to the 2020 probably won’t need to – Billy Mar 25 '21 at 17:55
  • I think you are very clear on what you are doing, and know what need to be done. The shelve is slightly deflected, if the 2020 is stronger than the current beam, and easily attachable, yes I would go for it. But before putting extra money and energy, why not just try the plywood alone to have a feel, and make judgement from there. Good luck. – r13 Mar 25 '21 at 18:19
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According to this reference the second moment of area for the HFS series 2020 and 3030 is respectively (for each beam) $0.74\cdot 10^4 mm^4$ and $1.89\cdot 10^4[ mm^4]$. To give it some perspective the second moment of area you currently have with the MDF (610mm X9mm) is about is about $3.7\cdot 10^4 [mm^4]$.

enter image description here

This means that the 3030 will exhibit (on its own) only about 40% of the deflection of the 2020. However that is not the most deciding factor. The main problem I see in this image is the lack of bracing.

if you are ok with the current level of bending, then you could just use the 2020 extrusion and the added 20% percent weight (even if placed smack in the middle) would still result into less deflection. However, you'd have further gains (see below).

Mounting consideration

How you constrain the 2020 or 3030 beams will have a significant effect.

If you just put the 2020 on the shelf (without screwing it) and on top the MDF you should see a reduction is bending about 30%.

If you are planning to tap and screw the ends of the 2020 on the edges you should see a significant improvement (although I would place the beams slightly closer to the ends - about 20%-25% off the edge). Tapping as screwing the extrusions will add stiffness. If you could assume that the edges are fixed (you can't), then you'd have a reduction of bending by about 85%, just by using the beams.

enter image description here

recommendation.

Because you already have 50 kg on one shelf, and you are planning to put more, I would strongly recommended side and back braces on your shelves. Probably the back brace are more important.

enter image description here

Otherwise tapping and screwing, will create a inward bending moment of the side rails and without the bracing the shelves could collapse.

NMech
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  • Thanks for the detailed response. As you can hopefully see by the image I’ve added, it is unfortunately not practical for me to put bracing across the back due to the vent hose and power cables in the way. It is hard to accurately describe how I plan to attach the 2020 without showing it somehow, but I think we understand each other. The shelf isn’t bending much so it sounds like tapping the end of 2020 and bolting into it through the side of the existing frame them sitting the wood on top as planned should do it. I will consider bolting the wood down to the 2020 probably won’t need to – Billy Mar 25 '21 at 17:50
  • @billy if you can put the back brace in the middle section you can put it at the top and bottom. If you can't put it at the bottom you can use an extrusion along the back side and screw it on vertical columns. Additionally, bolting the mdf onto the extrusions will provide added stiffness. see the following [question](https://engineering.stackexchange.com/questions/23612/deflection-of-a-cantilever-beam-composed-of-separate-not-bonded-planks/37043#37043) for more details. – NMech Mar 25 '21 at 19:51
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If you place your printer on the sides of the shelf as opposed to the middle it will create much less moment, hence much less creeping and deflection.

The rack is stiffer 4 times depthwise, you shouldn't need any bracing on the sides. you can use small size cable for bracing the back panel using small cable locks, connectin the four corners of the 2nd and 4th shelf, if you want to keep a neat appearance.

kamran
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