Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: QuantumClue on 11/01/2011 22:40:52
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It is believed that hollow pipes are stronger than solid ones... how is this so?
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That doesn't sound quite right. Perhaps it has a higher strength-to-weight ratio, but I doubt it has a higher overall strength.
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No, they're not. A solid pipe would be as much use as a chocolate teapot [;D]
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... hollow pipes are stronger than solid ones... how is this so?
Perhaps if each were constructed using the same amount of the same material, (i.e. both hollow and solid cylinders weighed the same), if both were the same length and made of the same material the hollow cylinder will have a bigger diameter.
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http://en.wikipedia.org/wiki/Area_moment_of_inertia#Second_moment_of_area_for_various_cross_sections
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ok. Geezer, who was that too? And in what reference is the solid pipe no good?
Supercryptid. It may be a myth then.... it's been one going about for a long time.
RD. So we need the same amount of the same material?
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Yes - I think RD is correct. Depending on the thickness of the tube wall of course, a tube will be able to support more weight than a solid round bar of equal diameter. This is because the closer the material is to the centre of the bar, the more "dead weight" it is.
A tube is not so different from an I-beam, but an I beam is a bit simpler to understand.
With an I-beam under load, the top plate of the beam is in compression and the bottom is in tension. The bit in the middle, the web, is really doing more to maintain the spacing between the top and bottom parts than anything else. In fact, at the middle of the web, there is no stress at all. You can drill a series of holes along the middle of the web, and you won't weaken the beam. But if you notch the top or bottom plates, look out!
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ok. Geezer, who was that too? And in what reference is the solid pipe no good?
er well, you QC. See, a pipe isn't a pipe if it's solid! A "pipe" usually has a hole down the middle. If it's not hollow, it's usually called a round bar or rod.
The "chocolate teapot" reference was intended to be a joke. Do you get it now?
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Yes - I think RD is correct. Depending on the thickness of the tube wall of course, a tube will be able to support more weight than a solid round bar of equal diameter.
I didn't said that, in my example the cylinders would have different diameters, but be of equal weight:
pound-for-pound a hollow pipe could be stronger than a solid rod, (both made from the same material).
[A pipe would have disadvantages over a solid rod: when it failed it would be catastrophic rather than a gradual failure:
once the pipe has a crease in it it's going to fail very quicky ]
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Yes - I think RD is correct. Depending on the thickness of the tube wall of course, a tube will be able to support more weight than a solid round bar of equal diameter.
I didn't said that, in my example the cylinders would have different diameters, but be the of equal weight.
[A pipe would have disadvantages over a solid rod: when it failed it would be catastrophic rather than a gradual failure:
once the pipe has a crease in it it's going to fail very quicky ]
Sorry. I missed that RD. I was assuming the diameters were the same, and you are quite right in saying that a rod would tend to bend rather than "fold up".
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ok. Geezer, who was that too? And in what reference is the solid pipe no good?
er well, you QC. See, a pipe isn't a pipe if it's solid! A "pipe" usually has a hole down the middle. If it's not hollow, it's usually called a round bar or rod.
The "chocolate teapot" reference was intended to be a joke. Do you get it now?
Oh yes yes, I know the jokey comment. It's like saying, it's as good as an ashtray on a bike.
Anyway, right. You mean you can't get a non-hollow pipe.
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Yes... Amount of material sounds reasonable.
So, ½" steel conduit has less material than ½" rebar. Perhaps similar to the amount of material in ¼" rod.
However, also consider different failure characteristics.
½" conduit will collapse when bent sharply... so once it is bent, there remains far less structural support. The ¼" rod can be straightened again and may have a large percent of its initial strength.
Some pipes such as thin-walled rigid copper plumbing pipe can tear when bent.
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Isn't a pipe so strong because it leads the pressure around it? And isn't it the same in a rod? That most of the force are redirected around the outer material of the rod?
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And if you mean that solid rod of the same diameter and type of material as the pipe would be weaker?
I don't think so myself. But I think you can take away a lot of the inner 'stuffing 'without weakening the rod very much, making it into a pipe.
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Isn't a pipe so strong because it leads the pressure around it? And isn't it the same in a rod? That most of the force are redirected around the outer material of the rod?
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And if you mean that solid rod of the same diameter and type of material as the pipe would be weaker?
I don't think so myself. But I think you can take away a lot of the inner 'stuffing 'without weakening the rod very much, making it into a pipe.
I believe so Yoron. A lot of the stress is supported by the outermost material. A rod of the same outer diameter will resist bending more, but it will be a lot heavier, which detracts from the load capacity. I also think you are correct in saying you can remove a lot of the material closer to the center because it is not stressed.
A problem with pipes is that they may not resist deformation very well, meaning it's not too difficult to make them lose their circular form. If the application can cause that condition, it results in a weak point where the pipe may "fold" under heavy load. That's why plumbers put steel springs inside copper pipes while they are bending them.
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I have seen on TV the 'Myths busters team' constructing a chocolate teapot and actually using it to make tea.
They were quite successful but no doubt the tea came out more like cocoa
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I notice that imperial notation is used here - Is the USA the only country to refuse the adoption of the metric system?
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In the UK we use an annoying mixture, we buy beer in pints but petrol in liters, we weigh people in 'stones' and babies in pounds and ounces but our car engines use liters to generate horsepower, while we insist on having a different currency to the rest of Europe.
When I was at school in the fourties we worked in metric (CGS) and was assured the country would be moved over by 1960.
PS our builders use a metric foot what ever that might be!
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I have seen on TV the 'Myths busters team' constructing a chocolate teapot and actually using it to make tea.
TNS kitchenscience also did that ...
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I notice that imperial notation is used here - Is the USA the only country to refuse the adoption of the metric system?
What do you mean "to refuse the adoption". Obviously, its the ROW that refused to adopt the perfectly good imperial system.
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I think the USA was one of the first to refuse "to adopt the perfectly good imperial system" - Boston 16th December 1773
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I think the USA was one of the first to refuse "to adopt the perfectly good imperial system" - Boston 16th December 1773
Perhaps, but Ulstermen were the first to make it "official".
http://en.wikipedia.org/wiki/Mecklenburg_Declaration
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Isn't a pipe so strong because it leads the pressure around it? And isn't it the same in a rod? That most of the force are redirected around the outer material of the rod?
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And if you mean that solid rod of the same diameter and type of material as the pipe would be weaker?
I don't think so myself. But I think you can take away a lot of the inner 'stuffing 'without weakening the rod very much, making it into a pipe.
I believe so Yoron. A lot of the stress is supported by the outermost material. A rod of the same outer diameter will resist bending more, but it will be a lot heavier, which detracts from the load capacity. I also think you are correct in saying you can remove a lot of the material closer to the center because it is not stressed.
A problem with pipes is that they may not resist deformation very well, meaning it's not too difficult to make them lose their circular form. If the application can cause that condition, it results in a weak point where the pipe may "fold" under heavy load. That's why plumbers put steel springs inside copper pipes while they are bending them.
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Why is it so Geezer?
that the force of the pressure follows it around? Has it to do with the graining of the material, or is it something else doing it? It's a weird one. Heh, got one for you, how to build a forcefield in your toilet :)
Easy, we all know that the toilet paper never breaks of where the perforations are in rolls, don't we? So to build that forcefield you just need a never ending supply of toilet rolls and then start to perforate them. When the rolls are gone you will have your impregnable forcefield where those rolls was. Then just add more perforations around you :)
Yes, it is scientific.
It's a principle.
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Isn't a pipe so strong because it leads the pressure around it? And isn't it the same in a rod? That most of the force are redirected around the outer material of the rod?
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And if you mean that solid rod of the same diameter and type of material as the pipe would be weaker?
I don't think so myself. But I think you can take away a lot of the inner 'stuffing 'without weakening the rod very much, making it into a pipe.
I believe so Yoron. A lot of the stress is supported by the outermost material. A rod of the same outer diameter will resist bending more, but it will be a lot heavier, which detracts from the load capacity. I also think you are correct in saying you can remove a lot of the material closer to the center because it is not stressed.
A problem with pipes is that they may not resist deformation very well, meaning it's not too difficult to make them lose their circular form. If the application can cause that condition, it results in a weak point where the pipe may "fold" under heavy load. That's why plumbers put steel springs inside copper pipes while they are bending them.
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Why is it so Geezer?
that the force of the pressure follows it around? Has it to do with the graining of the material, or is it something else doing it?
Well, I'm not very sure about the toilet paper exactly (I think that's worthy of a new topic in the TNS Bathroom forum) but, if you are referring to the sudden collapse of a pipe, here's one explanation:
Ahem, as I mentioned earlier, if you think about what's going on in a loaded I-beam, it's not so hard to understand.
The web (the vertical bit) of an I-beam does a very good job of keeping the top and bottom plates at a fixed distance from each other. This is because the web is pretty good at resisting the compression and tension forces it experiences as the I-beam is loaded.
On the other hand, a tube does not have much to prevent the top and bottom surfaces from getting closer together as the tube is loaded. So, a tube is like an I-beam with a "soft" web. When the width of the web decreases a bit under load, it reduces the load carrying ability at that point, which results in a cascade effect that reduces the "web" dimension to nothing.
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Could you say that the 'force' of pressure seeks to spread over all possible area. And that the way you design your 'bar/rod/pipe/I-beam" is a redirection of that force, where it will meet the most resistance. And that the pipe is the best example of a redirection of force as it has no end, well, sort off?
:)
Sort off.
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I dunno! Try this.
Here is a square pipe. If we apply force as shown on the left, it will resist bending quite well. If we rotate it through 45°, bet you can't guess what's going to happen [:D]
When loaded, a round pipe is a lot more like the right hand sketch than the left hand sketch.
[diagram=616_0]
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The force becomes stronger (that is more defined) and applied on one single point. But as you rotated it will get deflected more easily I guess, making the square pipe stronger too? Sh* this is tough thinking Geezer :) and to early in the morning for me ::))
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But you better pray you made good 'joints' when you rotate it..
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Naah, looking at it again it looks easily compressed as the joints will take all the pressure. That is the two at the middle. If you look at it as a flow the flow will separate at the top, run down to the middle but, well there, find no easy way to follow the 'structure' . That should mean that the 'forces' will accumulate at those two joints.
Does that make better sense.
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This message is brought to you by the coffee drinkers association, now in full flow.
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It is not round. Da*n it :) In a round pipe the forces will have no 'grip'. they will have to spread over the whole surface, maximizing the pipes strength per volume and mass of material. And this have to be true. Or I will ...
Drink more coffee.
Heh.
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Yup! The top and bottom right angles wil tend towards a straight line, while the right and left ones will close up. If you add a horizontal or vertical member to convert the square into two triangles, it will be able to resist bending much better.
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You know what, this goes back to a question I have. Would you agree to that a circle can be seen as an infinite collection of 'straight lines' slightly angled against each other?
If you do look at the pipe that way you could call each 'thought up' angle a 'joint' and then see that as they are so many and following each other so smoothly they will all share that 'force' applied at the pipe. And so the force becomes 'evened out' over the whole surface. And it also means that with a round pipe, when it 'compress' the force that does so will have to start at the sides of the pipe.
Yep..? :)
No, I'm sure I'm right here..
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Heh, we made a pretty good intuitive picture of it here, didn't we Geezer? Tout sweet as the Froggies say. And stop throwing things at me :)
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"And so the force becomes 'evened out' over the whole surface." is not perfectly correct as the most pressure per joint will be at the middle, but as a flow it will find its way around easier than when meeting that 90 degree turn you gave in your rotated square pipe, as I think of it. Seems you need to think of it both as a flow and then of the geometry as well as of the 'force direction'.
All as i see it :)
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You know what, this goes back to a question I have. Would you agree to that a circle can be seen as an infinite collection of 'straight lines' slightly angled against each other?
If you do look at the pipe that way you could call each 'thought up' angle a 'joint' and then see that as they are so many and following each other so smoothly they will all share that 'force' applied at the pipe. And so the force becomes 'evened out' over the whole surface. And it also means that with a round pipe, when it 'compress' the force that does so will have to start at the sides of the pipe.
Yep..? :)
No, I'm sure I'm right here..
Yes. I think that's quite right. The diamond shape on the right is a very crude approximation to the pipe. The diamond will only hold up as long as the angles don't alter, but there is a lot of force trying to alter the angles.
With the square on the left, there is no force tending to alter the angles.
The round pipe will only hold up as long as the material it is made from is rigid enough to maintain it's curvature (which can be considered an infinite number of angles).
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I think this one should be a sticky :)
The next endeavor Geezer, will be to apply it on SpaceTime :)
Let's make a wiki ::))
ahem :)
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And to make it perfectly correct.
When you look at a pipe you would do well to both consider it consisting of an infinity of angles (joints) and also 'split in the middle'.
Split in the middle because any force applied will according to Newton meet a reaction in the opposite direction. so when you put pressure on the top of that pipe resting, let us say, on the ground (for the visual effect) then the 'reaction' will be an equal force meeting it from the earth, both applying the most pressure in the middle.
If the pipe is freely mounted in the air you still will need a foundation for it, ending in the ground somewhere, so the same reasoning should apply, I think? Maybe it will differ slightly though depending on the 'joints' connected to that piece of pipe we see the pressure applied on.
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It would not totally surprise me if there is some connection here with curvature and stress in space, but I haven't the faintest idea how [;D]
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I reckon you're perfectly right Geezer.
We only need to find out where SpaceTime is mounted.
That is, where is the foundation?
Then we can follow the stress tension all from there. I will start immediately. Me and me pal Jules will build ourselves one of those bullets and fly of in search. It can't be that big, can it?
SpaceTime?
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But seriously, I do think you're right.
And the 'foundation' will be a interesting thing to see.
If we find it.
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PS our builders use a metric foot what ever that might be!
30 cm it is: just enough to save 1% and to make joints in replacement timber very loose!
In Australia we are almost completely across to the metric system (SI) by now, but there are a few glaring anomalies!
Ceramicists write their recipes in "gram per pint".