How to Strengthen Tubing Mechanically

I smell homework.

If you can convince me that this is not a homework problem, I'll divulge some secret information to you.

Hey RVZ717,

He's for real. Help him out.

Lyn

4" od 11 gauge tube that spans 28' with a 1100 lb load. no mid support? christ, that sags with its own weight on the span.

1 st what type of material?

I would download a shareware software called beam boy (google it), and apply the geometry, supports, material property and loads. to see where you stand with deflection.

if the ends are open may be think about some type of internal truss or bridging to slid on the inside. but with they information you have giving does not leave us much to work with.

phoenix911

GA. That was the Secret, oval... I still think it smells of homework...

hit it with a hammer

Good idea DKWarner!, however it increases the risk of buckling for any side loads. I guess it all hangs (forgive the pun) on how the tube is supported on either side. If it is possible to constrain it on both ends it will artificially increase the resistance to bending. Whatever way you look at it however you will always end up adding material to the tube to strenghen it (apart from deforming the tube). If you are able to add material then you could weld a plate segment to the centre section of the tube.

Foam fill would be my system but that adds material. Would grooving by extruding it through somehting that would shift tube material into ridges beat the no additional material ban.

Of course if you cut out the middle bit and put it on the floor it wouldn't sag, and you could reduce gravity. I agree it sounds like homework, or someone as broke as I am trying to use materials completely unsuitable for the job, but why the no additional material rider.

The oval suggestion is a definite GA

Simon

can you 'preload' the tube, like they do with prestressed concrete? That might add some rigidity...

You don't really want to strengthen the tube, you want to stiffen it. Since the deflection is inversely proportional to the moment of inetia, you can reduce the moment past the load and increase it near the support end. The easyest way to accomplish this is by use of a tapered tube. Before you laugh at this solution, look to see how many traffic lights are supported. There are other methods, but this would be my first approach.

GA! It is not at all clear in the original post whether it is required to modify the existing tube (which is how I interpreted it), or whether a replacement tube is acceptable.

Now it sounds even more like homework!

The point load alone causes a stress of 67ksi and a deflection of 10.5". The self weight deflection is 0.84", based on mild steel, the most likely material. There would be a hinge develop in the middle and the ends would try to pull away from the wall.

Can you bend it down in the middle and accept horizontal loads at the supports?

Squeezing it into an ellipse, as has been suggested, might work if the load was hung from the bottom, that would reduce the tendency to buckle, but the deflection and stresses would still be high.

If you form the tube in an arch it will support quite a bit more loading provided it does not move out of vertical. The tube is not realistic for the job, I thnk I have seen this question before in CR4 certainly seems like homework.

Please excuse my lack of engineering experience or knowledge if I am way off base. Would filling the pipe with highly pressurized gas of some sort help to strengthen it to resist bending? Just a shot in the dark.

Ed

In a tube with such a thin wall, I believe the limiting factor is the compression of the top of the tube. This will depend a great deal on how the load is supported. If the load is supported by a cable looped over the tube, your pressure might help a bit, but it would increase the tension in all parts of the tube, including the bottom, which might then become the limiting factor. As someone pointed out earlier, filling the tube with rigid foam would help prevent collapse, but the OP said no weight added.

Hopefully the load is supported by some kind of a 'saddle' to distribute the weight over a broader area.

Thanks to all! I am a part time student working on my degree in Mech Eng Technology, but this isn't homework. I work for a company and I'm part of a team that is trying come up with a new design of overhead support, and I thought that I might try here for some ideas. Typically, there is bracing welded to the top part of the pipe, which helps with the deflection. We want to eliminate that bracing.

You have given me some great ideas, that I can take back to my team...Keep'em coming!!

Thanks!

I hope this beam your working on designing is not headed for any type of consumer use, Cause I have saftey concerns if it is. From the sounds of it, its sorely under designed. What is this companies specialty, most likely not the overhead supports?

will prestressing the same way they do concrete help? Pass a cable through the tube that has eye bolts on each end and tighten them. This will put the tube in compression. Perhaps a cable at the top and bottom where you have max stress in bending.

If this is diagonal bracing in a vertical plane, you should understand that the bracing is almost certainly directly supporting almost all of the point load and 5/8ths of the weight of the pipe itself.

What about using proper structural materials in the first place?

heat treating might be a possibility.

pressurizing the tube is another. Atlas missles relied on a very thin outer metal skin for structure. the skin was only about the thickness of a dime, but pressurized to keep walls from collapsing.

roughly, a structural metal tube with a certain outer diameter will exhibit similar stiffness initially, over a wide variety of wall thicknesses..... just until the wall collapses on the thinner wall specimen.

- similarly, the most significant effect of pressurizing the tube will be that the preesurized tube will be significantly more stiff, after the point that the unpressurized tube wall has collapsed.

if you can reshape the tube to strengthen and stiffen it, it would be advantageous to use a tube with a varying diameter, such that the widest diameter of the tube is at the center and supporting the load, and the diameter of the tube decreases as it approaches the supports.

it would also be advantageous to increase the wall thickness at the connection to the support and at the connection to the load.

benbenben

p.s. ....also without adding any material, if a surface for support existed in close enough proximity below, then one part of the tube could be relocated to support the load from directly below. or even above if suitable. this would certainly reduce deflection. :-P

Sure. Turn the tube vertical.

Make an arch....

Good answer, except that you need to mention the requirement for massive supports for the arch ends, as shown in your photo.

The stresses on the supports could be reduced considerably by having one or more cables stretched between the ends of the arch.

Greenhouse not yet assembled.

The wall thickness of the frame tubing is 0.04 inch, and 0.98" in diameter. I want to make it stronger by adding to the hollow area of the tube using a mortar mix injected. The structures are prone to collapse due to snow load issues and strong winds. Would my idea work to help the original posters question ?

WOW! Only 0.04" wall thickness, and less than an inch OD! That's clearly NOT designed for snow country. While filling the arches with mortar will prevent the tubes from collapsing, you'd have to fill them upside down and already assembled (I'm assuming that the arches are delivered in sections). Unless the mortar contained a significant content of some kind of fiber, it would not add significant strength to the arches (beyond collapse prevention), and it would make them several times as heavy. Depending on the density of the mortar, it could make each arch weigh around 5 or 6 times as much as the hollow arch. ...and that's assuming they are steel tubes; if they are aluminum, they could easily weigh 10 times as much.

Also, to get even close to complete fill, you'd have to use a very thin (low viscosity) mortar mix. I don't know what may be available now, but the mortars I've used in the past became quite weak when mixed thin. You'd have to fabricate a support system to hold the arch while being filled and until the mortar cured. Once cured, I suspect you would not even be able to put the filled arches in place without breaking the mortar.

Here is an end view of my suggestion:

Securely tie a wire/cable from each longitudinal tube to the next (Black lines in the illustration) at each arch. This will stabilize the structure significantly, while adding very little weight.

Excellent ! I think I see how it works. The downward pull pre-stresses the middle of the arches forcing the top and mid sides to use up/ shorten flex area. The resulting strength should be between 1/3 to 1/2 as much greater. Economical too.

Getting the right tension will be a little tricky but doable. Some cheap ratcheting load ties should work and remain adjustable.

Thank you !!

Good. I would have the wires just barely taut in calm conditions. I forgot to mention: If you or anyone likely to enter is very tall, it might be a good idea to hang some flags from the top wires.

Actually, I was thinking of wires/cables with turnbuckles. If you are thinking of the nylon straps with ratchets that are used to tie down loads, I'd say NO. Nylon stretches too much.

Something like this:

What I know as "aircraft cable" in something like 1/8-3/16" diameter. There are crimp-on cable clamps that are probably cheaper than the setscrew type shown here. You could also use guy-wire, but it would stretch a lot more than the "aircraft cable" under heavy load. I'd use two of the crimp connectors at each cable end.

I'm short 5'9", and the short ceiling will bring things like that to focus anyway.

I had made it to aircraft cable already. Economics of project. Harbor freight has it 5/32 x 100 feet for $13.99 and a max work load of 600 lbs. Coupling nuts to connect to original frame bolts, then a threaded, forged eye bolt to run the cable through.

I will be having treated 4x4's attaching bottom rails of the frame to. Corrosion prevention. Plus it gives a good structure to anchor it all down with. Ground screw anchors with lags to attach them.

I'm wondering if it might be beneficial to run the cables on down to the treated 4x4's. Then tie off the cables with lag eye bolts secured in the wood 4x4's ? Wind comes from all directions plus it needs to be out in open as much as possible to get max sunlight in shorter day months of year.

The unit comes with 2 rope ties that go over the fabric. Intended to hold it down. Plus some inside ties connected to fabric which tie off on the frame.

I intend to lay down heavy ground cloth, frame up the structure on the treated 4x4's all on top of ground cloth. Have the frame all tight and attached to the wood. Then apply the outside cover. Align as much as possible, and stretch lightly against frame. Use the extra tail they left at bottom to bring to inside around the bottom of the treated 4x4's, staple and use wood weather stripping nailed on to keep it tight. I will need to reinforce the ends with some wood frame, as a 10' x 7' area makes a great sail or rip point.

Bring the cables on down or no ?

"I'm wondering if it might be beneficial to run the cables on down to the treated 4x4's."

Good idea, IFF the cables are attached to the bottom of the arc in a way that they can't slip there. Best would be if the 4x4s extend up to the bottom of the arc, and the eye bolts were very near the outside top of those posts.

Local Lowes has treated 4x4x10 for $20.48 each. The 8 footers are $12.48 each. I will need some verticles to build shelving on. Likely the ends and middle on each side but 2x4x6's cut to go up from bottom runners to first lateral bar 3 to 4 per side. Simple truss supports 2x4 size nailed to verticles for outer shelf support 2x4 for inner shelf support. Conserving space inside as much as possible for bedding plants. Vegetables and herbs needing early planting. Plenty of scrap lumber from old garage falling in for shelving. The lateral at 5.5' level use for hanging plants.

I could just put the brace cable as you original design suggestion shows. Keeping it simple, easy and economical.

Thank for the help. I enjoyed sharing too.

Sounds good.

I forgot to mention: short is always relative. I was the tallest member of my family (and now the last/only one), but you're almost half a foot taller than me. I'd have no problem with those wires!

The shorter profile helps with having less sail for the wind to blow. But being able to walk and work inside is yet to be seen. I'll still need a ladder to work on the higher parts.

I'm the oldest of a family with 8 kids originally. 5 boys and 3 girls. 2 brothers now on the other side, and 1 sister. One younger brother is taller than me by a head still here, the other same height as me. The girls are all shorter. Parents and grandparents have all passed over too. It's not so bad to be average when you see the problems taller people have.

Closest weather station: https://www.wunderground.com/weather/us/mo/hannibal/KMOHANNI13

thank you dkwarner