Advantages/disadvantages of twisted wires VS wires bundled next to each other

[Femme_physics]

I saw "Oresund Bridge - Megastructures" in national geographic, and that bridge is made out of wires bundled next to each other, rather than twisted wires. I wonder, since twisted wires are stronger, what is the advantage of wires bundled next to each other?

 

[dlgoff]

If you are asking about twisted insulated wires like these:

Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally. The noise thus produces a common-mode signal which is canceled at the receiver when the difference signal is taken.

http://en.wikipedia.org/wiki/Twisted_pair"

 

[Studiot]

Why do you think twisted wires are stronger?

They are certainly longer.

 

[Femme_physics]

Because I asked that question in http://answers.yahoo.com/question/index?qid=20110222034117AA17vlH" and two seemingly reliable and confident sources told me "twisted wires".

I'm not talking about electricity here. Again, this entire question came up from the Megastructures series about Oresund Bridge. I just don't remember if they provided an explanation about why they preferred stretched wires bundled next to each other rather than twisted ones to hold the bridge, and being an overly-curious individual and a fan of mechanics and materials, I decided to ask to see where's the logic.

 

[Studiot]

I'm not convinced.

I think there are other reasons involved.
The correct term for twist is the lay of the rope. Ropes are laid.

Here are some safe load tables for wire rope.

http://www.engineeringtoolbox.com/wire-rope-strength-d_1518.html

As you can see from the pic they are laid (twisted).
You can buy these from the factory ready made.
The lay (twisting) of the rope holds it together as a single entity. If the rope was not laid you would have to have wrapping wires to keep the bundle together.

Now imagine the cable for a large suspension bridge. You can't go down to the local supermarket and buy something like this.
And even if you could how would you put it up?
These ropes are actually spun in situ. How would you achieve a lay in something you are actually sitting on to extend high up over water?

Here is some useful information about bundled wire ropes.

http://www.tpub.com/content/engine/14081/css/14081_359.htm

 

[dlgoff]

Sorry Femme_physics. I missed your question completely. That will teach me to reply before my morning coffee when my eyes are still sealed shut.

 

[Femme_physics]

Heh, np dlg. And thanks Studiot. So the answer why they those bundled wires is that it's impossible to put twisted wires over a suspension bridge? ..hmm... not sure how convincing is that...but I do trust you...

 

[vlado_skopsko]

Rope needs to resist tension and torsion.
The tensile strength will be the same on both types of rope.
But the rope with twisted wires can resist torsional forces too.

 

[Studiot]

You might like to look through this site about the Forth road bridge.
Press button 3 for information about the cables. It explains quite clearly why the cables can't be of 'laid' construction. It's a practical thing.

We are obviously going to have to have a discussion about torsion as people keep bringing the subject up.

go well

 

[Femme_physics]

Rope needs to resist tension and torsion.
The tensile strength will be the same on both types of rope.
But the rope with twisted wires can resist torsional forces too.

So why did the use bundled ropes in the Oresund bridge? Because of the mechnical difficulties to input it twisted due to size/location?

 

[vlado_skopsko]

Yes, I guess it has to do something with that, although there must be some other torsional strengthening added.
I couldn't find this tread about Forth road bridge.

 

[Studiot]

I am most terribly sorry. I omitted the link.

http://www.forthroadbridge.org/thebridge

I also posted the following, but PF or my link went down and the post was lost.

Big bridge cables weigh between 10,000 and 20.000 tonnes.
Not the sort of thing you find on the shelves of your local supermarket.
Imagine the transport and lifting gear required for this job if they were assembled elsewhere, brought to site and lifted into place.

Furthermore they are draped on tall slender masts, pylons or towers. Imagine trying to place such a weight without pulling the tower over.
When in place they are in balance and very stable.

As described in the link, the cbales are actually made by taking thin strands across the gap one at a time.
Now imagine this process.
You string the first across. Then how do you twist the second, third, 533rd roudn each other to achieve the lay?

Like I said it is question of practicability.

go well

 

[AlephZero]

Big bridge cables weigh between 10,000 and 20.000 tonnes.

That number is the right order of magnitude for the tension in the cable of a long suspension bridge. not its weight.

Now imagine this process.
You sting the first across. Then how do you twist the second, third, 533rd roudn each other to achieve the lay?

The Forth bridge may not have been built that way, but most large suspension bridge cables are now "spun" that way. The technique was first developed for long American suspension bridges in the early 20th century, and has been developed further since then.

Like I said it is question of practicability.

I was living only a few miles from the Humber suspension bridge when it was being built, and I saw the spinning process being carried out many times over the several months that it took to complete the cables. The main cables were spun from about 15,000 separate 5mm diameter wires, divided into about separate 40 "sub-cables". There is nothing complicated or impractical about the process, but there were a few unexpected risk factors, like when a couple of fast military jet pilots decided to fly UNDER the cables, just for a laugh. (They probably stopped laughing when they were court-martialled afterwards).

http://www.humberbridge.co.uk/media/Engineering_The_Humber_Bridge_e-book.pdf