[QuantumClue (OP)]

It is believed that hollow pipes are stronger than solid ones... how is this so?

[Kryptid]

That doesn't sound quite right. Perhaps it has a higher strength-to-weight ratio, but I doubt it has a higher overall strength.

[Geezer]

No, they're not. A solid pipe would be as much use as a chocolate teapot  []

[RD]

... 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.

 [ Invalid Attachment ]

http://en.wikipedia.org/wiki/Area_moment_of_inertia#Second_moment_of_area_for_various_cross_sections

[QuantumClue (OP)]

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?

[Geezer]

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!
 

[Geezer]

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?

[RD]

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 ]

[Geezer]

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".

[QuantumClue (OP)]

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.

[CliffordK]

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.

[yor_on]

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?
==

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.

[Geezer]

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?
==

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.

[syhprum]

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

[Foolosophy]

I notice that imperial notation is used here - Is the USA the only country to refuse the adoption of the metric system?

[syhprum]

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!

[RD]

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 ...

 [ Invalid Attachment ]

http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/how-useless-is-a-chocolate-teapot/

[Geezer]

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.

[imatfaal]

I think the USA was one of the first to refuse "to adopt the perfectly good imperial system" - Boston 16th December 1773

[Geezer]

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