Tube Strengh / Rigidity

[GrahamGKD]

Hi,
Is there a table somewhere which will give me some idea of the strength / rigidity of various tube sections - e.g. For the sake of argument, if I have a 2 metre length of 304 ERW stainless steel tube, 35mm diameter and with a wall thickness of 2mm, how much weight can I hang in the middle of it before it will collapse? I have been asked to make a monkey bar for a gym and I need to decide what tube to source for it. Don’t worry though, once I can find some basic data I will add plenty on top (double it maybe) to ensure it is strong enough!

What I am looking for sounds like fairly basic data but I am struggling to find it - perhaps because I am not sure what 'terms' to search for?

Thanks,

 

[outofthefire]

This any good http://www.tbucketeers.com/threads/deflection-formula-for-comparing-different-tubing-sizes.6824/

 

[GrahamGKD]

Yes, I think I can have a go with that - the pictures seem to have been moved but I think I can manage without them. I'll have to find youngs modulus for the material but I don't see that being a problem. Thanks very much.

 

[KemppiFrog]

One of the oddest jobs I got down in the Algarve was to make sure a ss pole was OK for dancers to hang off, after one had flung herself into the crowd when the top fixing failed.
It was a question of flex on the thin walled tube, so I sleeved a steel tube inside the SS one and improved the mountings. I understand it stood up to further use...... No late night site inspections allowed, orders of Mrs KF.

For other calcs, I use 'The Engineers Handbook' by Hudson or 'Machinery's Handbook'. They will cover anything I am likely to come across. And what you need.
KF

 

[GrahamGKD]

Looks like the Hudson one may be out of print and the Machinery's one is £60 odd! It would be nice to thumb a copy before spending that much.

 

[Llama]

Assuming you want to keep everything elastic, what grade of stainless are we talking?

Assuming you were using 304SS, you could support a bending moment of about ~2900Nm (which gives you a reaction force of 2900N at 1 meter centre distance, therefore a total load of 5800N). Anyone know any ~600kg strippers?
I haven't calculated transverse shear yet, and his doesn't take into account stress changes over deformation, but that's a little more math than my head can handle on this lazy saturday evening

I've been working on a spreadsheet that can be used to calculate these values and things for my own projects, but it's far from being complete.

 

[GrahamGKD]

Hi Llama,
Thanks for the Reply - your comment about Saturday evening had me a bit confused until I noticed you are in NZ!

Yes, it will be 304SS. My application is for a horizontal gym bar above head height (the pole dance thing was Kemppifrog's story). The bar is to be 3m long with the centre section supported by brackets 2m apart (with an overhang of 500mm each end). It is intended to be used for training with those tape trainers where you use long tapes supported above and train against your own body weight but I think I have to make the bar strong enough for 2 or 3 people to hang their full weight on it at one time - it is bound to happen! I will make the bar so it can flex within its support brackets - i.e. it won't be welded, which I imagine could lead to stress problems where it is supported. Is that what you meant by keeping it elastic?

I guess what I was hoping to find is a spreadsheet or table that I could play around with different sizes and see what seems reasonable. I might try and work something up in Excel from the formula Outofthefire posted.

Thanks again for the reply.

 

[GrahamGKD]

Assuming you want to keep everything elastic

Thinking about that statement some more - I guess you mean not going beyond the elastic limit beyond which permanent deformation occurs? In which case - yes, I definitely want to keep it elastic!

 

[astro_al]

Stuff like this doesn't require any thought at all. Its easy to make a bar that'll support 10 people - let alone 3. Just pick a tube dia / wall thickness that is 'plenty'.

I have a barbell that is 50mm dia and about 10mm wall thickness. I could hang my car off it. I never bothered with any calcs for obvious reasons!

Have you found out what wall thicknesses are readily available in your chosen diameter?

Most of the calcs aren't going to give you specific value at which the tube collapses or crumples, it'll just be a loading at which some 'critical' bending criteria is breached.

I would urge you to use the largest diameter you can - not only for strength and rigidity, but also for the comfort of the users (having said that, 50mm is too big for most people to hang onto for long).

Al.

 

[chunkolini]

I would be wary of taking on jobs like this.
If somebody gets hurt using or misusing the equipment they might well come looking for your head with or without lawyers.

 

[astro_al]

That logic can apply to just about anything which isn't purely decorative.

Get an engineers report and price it in if you're worried.

Al.

 

[GrahamGKD]

I would be wary of taking on jobs like this.
If somebody gets hurt using or misusing the equipment they might well come looking for your head with or without lawyers.

Chunko, I know exactly what you mean and I am wary but the thing is just about any job can lead to similar problems can't it? E.g. a sculpture can fall off the wall or fall over and hurt someone; a small sculpture can be played with by a kid and poke them in the eye and so on? The concern you have raised is one reason I want to make sure it is amply strong and I would like to back that up with a bit of science. I'm not taking responsibility for bolting the thing to the wall - someone else is doing that and specing what size bolts etc..

Astro_al, I would normally do as you suggest but I want to be sure this thing can't be broken. I think a length of 304 50mm diameter with 10mm wall might blow the budget and would be dangerously heavy if it ever did come down - that's virtually a solid bar!

I've got a piece of 1 and a quarter inch 304 with 1.5mm wall and that is definitely a bit dodgy when I bounce my weight on a 2 metre span (as you might expect). The only other thing I've got to hand (for a test) is 40 by 40 by 2.5 steel box but the customer has decided on stainless.

 

[astro_al]

Hmm. If you're not even mounting it, the only risk to you is under-speccing the material.

Maybe this will help - see below for a list of stainless pipe (not tube) sizes.
I anticipate the patch of 9 you're looking at on the chart are from 1" schedule 40 pipe up to 1.5" schedule 160.

Obviously there are tube sizes to check too.

If the client knows stainless costs, then its up to him to pay for it, but don't get squeezed into thin walled crap just because he wants to cheap out - its a decent thickness or he can't afford it and he has to use mild steel.

Could chrome it in mild - might be cheaper?

Or maybe you can find an offcut of 304 pipe in the right size?

Al.

 

[GrahamGKD]

Hmm. If you're not even mounting it, the only risk to you is under-speccing the material

Thanks for that info Al - there is slightly more to it than that as I will make some brackets to support the beam off the wall but I'm confident of my ability there - the brackets will be erring well and truly on the heavy side in good old painted steel box!

I've 'coded up' the formulae suggested in the link posted by Outofthefire in a spreadsheet - seems to work reasonably (it reproduces the examples given in the link). I've got it so I can enter parameters like length, load, tube OD and wall thickness and then it gives me a deflection. It's not quite what I set out looking for but it gives some idea. At least it's given my brain a bit of exercise this afternoon!

 

[Llama]

I guess the good thing about steel (I'm assuming this also applies to 304, but I couldn't see why it wouldn't) is that unless the gym is kept at Antarctic temperatures, it is not going to shatter or break, just bend into a wonky shape and not drop anyone.

 

[GrahamGKD]

I guess the good thing about steel (I'm assuming this also applies to 304, but I couldn't see why it wouldn't) is that unless the gym is kept at Antarctic temperatures, it is not going to shatter or break, just bend into a wonky shape and not drop anyone.

Yes, you are right Llama, good point, but I'd rather it didn't come to that.

 

[Llama]

Had made some errors in my calculations before (I blame it on excel )
(all in Newtons)
max load due to shear = 87587.60318
max load due bending = 1327.393806
max load due to transverse shear = 15047.19856

so a 135kg stripper then.

 

[GrahamGKD]

Hi Llama,
That does sound in the right ball park against my gut feel.

Is there any chance you could share your formula with me please - I'd like to see what effect fiddling with the various parameters makes.

I will PM you my email address on the off-chance you are able to.

Many Thanks,
Graham.

 

[GrahamGKD]

Well, I finished the gym bar and delivered it a while ago now but it took a while for the Customer to put it up. It is up now though, and I went and had a look at it today – I used 60mm diameter 304 stainless steel tube with a 2.5mm wall thickness in the end and it is rock solid – astro_al could hang his car off it! Thanks to Llama for providing the formula – one thing I did learn is that tube diameter has a much bigger influence on rigidity than increasing wall thickness. Thanks All.

 

[Llama]

Good to hear, happy to provide advice. I had a few ideas of making a full-fledged program, but then university/work/car/sport everything else kicked in and it's on a loooong list of back burners!