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I have some steel tubes (18 mm OD) which I wish to use as horizontal members in a storage rack (a bit like shelves). However, the tube flex and bend quite a bit when I put heavier boxes on them. The tubes flex about 40 mm over a 1000 mm span. Ideally I would like the flex to be <5 mm.

Is there any cheap(~$0) & readily available material I can put into the tubes which would stiffen them significantly.

I was thinking about filling them with concrete, but that seems a bit messy and I don't have cement/sand to hand. Would concrete work?

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Steel tubing does not have much lateral stiffness; it's not really a good fit for this type of use. –  TomG Jan 21 '13 at 23:33
    
Are you referring to conduit tubes? If so, perhaps swap them out for thicker walled steel pipe instead. –  DA01 Jan 22 '13 at 3:43
    
from all the answers, it sounds like there is not that much that can be done within my constraints. I was hoping to use the material I had lying around, but I guess I'll have to spend $ on stiffer tubes or some other beam solution. –  Ken Jan 22 '13 at 13:50
    
Have you tried Sildenafil citrate? –  Chris Cudmore Jan 22 '13 at 19:07
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5 Answers 5

You could truss the tubes as shown in the picture below. The center block helps to support the tube when the truss member is placed in tension.

enter image description here

The truss could be constructed from heavy wire or threaded steel rod. The concept here would be similar to the scheme used on the wider steps of a wooden step ladder.

I would have suggested the possibility to place the truss wire / rod inside the tube but I suspect that the 18mm diameter of the tube is too small to let the center support in such scheme be large enough to make a significant impact.

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Thanks @Micheal, good idea, but I don't have the clearance, below the tube to truss it. Clearance is part of the reason in need to stiffen the tubes. –  Ken Jan 21 '13 at 14:52
    
@Ken -- You would then likely get a lot better result by removing the tubes entirely and then laying in pieces of 19 or 20mm thick plywood that spanned the same distance as the rods. Another alternative would be to simply lay the plywood on top of the tubes. If doing that make sure the plywood is at least the full 1000mm dimension so it overlays the same supports that hold up the tubes. –  Michael Karas Jan 21 '13 at 16:39
    
@Micheal, part of the reason I'm using tubes is to minimise the visual appearance of the shelves when unloaded. The plan is to use just two tubes (e.g. EMT) across the span Positioned to support fixed sized boxes at 1/3 and 2/3 "box widths" from the wall –  Ken Jan 21 '13 at 17:00
    
Your next best bet may be angle aluminum or steel about the same size as the tubing; it may make the tubing redundant, though; it may be more visually appealing than plywood. –  TomG Jan 21 '13 at 23:36
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If you can't use the truss technique described by @michaelkaras, the only real solution would be to move to thicker-walled tubing or larger diameter tubing.

Anything you fill the tube with is not likely to make much of a difference at all. All the bending strength of the tubular member comes from its topmost and bottommost elements. The center of the tube does very little to resist bending.

And as others have pointed out, concrete is lousy in tension, which is present on the bottom half of the core, so you'd be likely to develop cracks in your concrete core, rendering it useless.

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I think plain concrete would work, but whatever you end up using, you could fill the tubes with a caulk gun and fillable caulk tube to cut down on the mess.

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good idea, I'll do that –  Ken Jan 21 '13 at 14:54
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I'm not too sure about concrete. It's pretty crappy stuff in tensile loading or bending (which is half tensile). –  Chris Cudmore Jan 21 '13 at 15:53
    
I am also fully uncertain about the idea of filling the tube with concrete. It will do next to nothing to make the tube sag less under load. –  Michael Karas Jan 21 '13 at 16:27
    
My (naive) thinking was that the package would form a steel reinforced concrete beam. –  Ken Jan 21 '13 at 16:55
    
Epoxy might be a better choice. –  The Evil Greebo Jan 21 '13 at 17:44
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Expanding on The Evil Greebos' idea, I would try aerosol expanding foam (aka Great Stuff). You may have to extend the the little extension tube with some tubing to reach the center. It cures to a somewhat ridgid state. It is also lighter than chaulking.

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The aerosol foam will do next to nothing for making the tubes more rigid in the application described by @Ken. BTW, EvilGreebo was not suggesting to fill the tubes with caulk. Instead he was suggesting to fill re-usable caulking gun tubes with wet mortar mixture to then be pumped into the tube with a caulking gun tool. –  Michael Karas Jan 21 '13 at 16:32
    
@MichaelKaras is probably correct here. However, the foam WOULD serve SOME purpose, by reducing the tendency of the tubing to flatten under load, which would greatly reduce its load carrying capacity. This trick is how one can bend copper tubing (without causing the tube to flatten) by filling it with ice. –  user558 Jan 22 '13 at 13:39
    
@woodchips - The expanding foam I've played with was easily compressible. It is after all full of bubbles. I cannot agree that this would do all that much to keep the OPs EMT conduit from flattening. Also consider that by the time his conduit has bent so far that it has started to flatten his support concept has already failed. --- On another point you do kind of place doubt on your point by using the scenario of bending copper tubing simply by noting that it can bend without flattening!! –  Michael Karas Jan 22 '13 at 13:53
    
@MichaelKaras - yeah, I was going to add that point about being able to bend the tubing at all, and I agree that adding foam is of little value. I was merely saying that it would help a little. Not much though. –  user558 Jan 22 '13 at 15:37
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I don't think there's really a solution that's cheap and will be acceptable. You say you're looking to reduce the deflection to about 1/8th, which is a large difference. Filling the tubes with concrete will increase their stiffness and therefore reduce deflections, but not 8x. Michael Karas's truss idea is also not going to make an 8x difference.

I think you're going to need to be creative / look further to find a solution to this problem:

  • Deeper / thicker walled tubes
  • Shorter span (add intermediate supports, hang wires from the ceiling, etc.)
  • Reduce the load on the shelves (you haven't described the project, but maybe you could make one shelf "heavy duty" and the others lightweight)
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I would beg to differ with your opinion regarding the trussing of the tubing. If the truss is made directly from the bottom of the tubing it would surprise you how much stiffness that would put into the tubing span of one meter. The greater the height of the center support block the more support you can achieve within the limits of the tension strength of the truss rod or wire. I would invite you to take a look at how strong various types if welded wire shelves are when triangulated with rather narrow vertical height. –  Michael Karas Jan 22 '13 at 3:20
    
I do think the tension wire/rod could add a lot of strength — maybe doubling or tripling it. But he's looking for almost an order of magnitude increase in stiffness. Even if he cut the span in half I don't think he'd get the deflection he desires. –  Henry Jackson Jan 22 '13 at 18:39
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