What is the cheapest way to stiffen a tube/pipe?

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.

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.

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.

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.

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.

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)

I once read that a packed pvc pipe with sand worked very well.

how about polyurathane foam used to fill gaps around brick walls? it has a high expansion ratio that will easily fill your tubes, it grows in a quite stiff material and it also has great bonding qualities which means that it will stick all the way on the inside walls of the tubing so it will not allow sliding of one material on the other increasing its overall stiffness by working as a whole. I quess that it will not make the difference you require but some of that.

Assuming you can fix each tube end to a rigid frame, you could achieve what you want with help of tensile force. E.g. make each tube slightly short and ensure fixings at each end pull the tube apart. Tube should work sort of as string in a tennis racket.

I am facing a similar problem. I would suggest you try bending the tube virtically upwards by tieing its both ends with a strings as in a bow. This technique is similar to the basic design of suspension bridges. If you are using only tubes without fixing a flat sheet on them, you will have then to live with placing you stuff on two curved tubes.

Pipe in a pipe. Lightweight bicycles often have Balsa wood inside the magnesium pipe frames to add rigidity. Anything from wood to bar stock would work.

The steel tube would be strengthened a great deal by filling it with mortar (Type S might be best). This would add compressive strength that would stop the tube from collapsing under a load. Can't say whether or not it would fully meet your goal. You'd have to try it. Concrete contains larger aggregate, and would be too difficult to load into a small tube.

I have used concrete to fill vertical fence tubes , worked very well . It works by preventing collapse ( in the elastic stress range ) , ie .the concrete is in compression. This has been done in oil well casing that has exceptionally high collapse loads. A second casing is run concentrically and cement is put in the annular space. Also , I have put steel rebar into 1/2 conduit, you want a close fit, very heavy , but strong.

Scallop fishing, many years ago, we had a problem with the club stick bending. The club stick in question being a piece of 10 foot by 4inch schedule 80 pipe whose primary function was to help dump the bag full of scallops once brought aboard. We welded flat stock on the the outside of the pipe (opposing the direction of the bend). Sort of a variation of the truss system mentioned in previous comments. It did not work. We then tried welding a truss system using steel rod to the pipe hoping it would be more robust. Didn't work. Finally we simply filled the pipe with concrete and that solved the problem.

I'm facing a similar problem, and agree with the type-s suggestion. Have you seen barriers (often to protect things like gas pumps) made of steel posts filled with concrete? My theory is that when a tube bends, its internal volume is reduced. If you fill the tube with a material that can't compress, it will resist bending. My situation involves a patio chair made of 1" square tubing, which rusted through and finally bent. My plan is to fill the void with either anchoring epoxy or JB Weld and then insert a steel rod.

First, I would switch to 3/4" rigid conduit or black pipe. If that isn't stiff enough, then consider using 1/8" (or 1/4") thick x 1/2 width flat metal welded as an 'I-beam'. flat cold rolled bar is inexpensive and as an I-beam would have good span characteristics. See http://iris.nyit.edu/~maltwick/BC2/Steel%20Rules%20of%20Thumb.pdf for span strength estimates.

I have read all the questions and answers and have come to the conclusion which I don’t think anyone has thought of which is using a combination of re bars and expanding foam to hold it in place inside the tubes

Fill it with concrete - here's a scientific paper about doing that: https://www.hindawi.com/journals/amse/2018/9065378/

They were getting less than 1mm flex from 4000kg on their 1.2meter span. (see table: https://www.hindawi.com/journals/amse/2018/9065378/fig7/ )

Concrete is not compressible, and since it's inside a steel tube, the only way that combination can flex is for the concrete to crack and for all the steel on the sides and bottom to stretch as well - and since I'm assuming you're probably not putting 4 tons on the middle of that shelf, that's never going to happen.

OP inquired about stiffening a thin wall tube. It was flexing too much. Much of the discussion went into how to strength the tube which is not the answer he was after. The formula for the center displacement of a simple supported beam with a center load:

Y = P*L^^3/(48*E*I)

where

• Y is the displacement
• P is the center load
• L is the total span
• E is the elastic modulus
• I is the moment of inertia of the section

Looking at the formula will give you the solution. Reduce the span will do a lot of good. Changing to titanium will increase E more than 3 folds, but hardly a cheap solution. A tubular section is most efficient in getting a high I value. Thicker wall and bigger diameter put you in the right direction.

The most popular suggestion here is filling the tube with concrete. This no doubt will get you a stronger member that is capable in flexing more and take more load without bucking the top or compression side of the tube.

What about displacement? It is inversely proportional to E*I. The E value of steel is about 10 times of concrete. Let's say the I value of the tube is roughly the same as the hollow space or the concrete fill. The increased E*I is therefore 10% after the concrete fill, and reduction in displacement is roughly 10%. It will not be a reduction of from 40 mm to 5 mm OP was looking for.