Modulus of Elasticity for guy wire 1

[transmissiontowers]

ASTM A475 lists the properties of several types of galvanized stranded cables used for guy wire. I was looking for a reference for the modulus of Elasticity (MOE) of these cables. It should be less than solid steel rod since as you load the cable it unwinds the individual strands a little and it probably depends on how much load is applied.

To further complicate matters the ASTM A475 lists the diameter of the strands with galvanizing and calculates the cable area including the galvanizing thickness (I think). Since zinc has a different modulus than steel, will the effective cable modulus be dependent on the steel area or the total stranded area?

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[oldrunner]

Check page 30 of the USS Tiger Brand Wire Rope Engineering Handbook published by United States Steel. This page provides approximate moduli of elasticity for a number of wire ropes. Anywhere from 12E6 to 25E6.


My edition is October 1968.

 

[transmissiontowers]

I remember that book and will look for it around the office. I wonder if it is still in print?

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I have been called "A storehouse of worthless information" many times.

 

[transmissiontowers]

I found a PDF copy of in on slideruleera's web site.

http://www.slideruleera.net/

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I have been called "A storehouse of worthless information" many times.

 

[dozer]

I've got a manual called "Technical Data" from Wireco. It list moduli from 8E6 to 15E6 psi. It also says you have to use something they call "metallic area" instead of the nominal area when computing rope stretch.

I glanced around their web site a little to see if you can order or download it but I didn't see it. I got this some time ago from them by snail mail. There web site is

http://www.wrca.com.

You're asking the right question though. I suspect many people don't model the correct stiffness of cable in structural models.

 

[transmissiontowers]

Thanks for the link, I'll take a look there.

We are currently using 28,900,000 psi but I think that is too high. It might be correct if the cable was a solid steel rod. Someone said 23,500,000 psi was correct but they can't cite a source for the data and I wanted to have some verification before I switch to something else.

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I have been called "A storehouse of worthless information" many times.

 

[Denial]

A cable's axial stiffness should always be obtained from the manufacturer, or determined from tests on a sample of the particular cable.

However for the purposes of preliminary design, or if manufacturer's data is truly impossible to obtain, you can use some rough rules to get an even rougher estimate:
(1) The effective cross sectional area of actual material in a twisted wire rope will usually be between 0.48 and 0.76 times the gross area, where the gross area is calculated on the basis of the nominal diameter. The lower values tend to apply to hollow-cored cables, while the upper values tend to apply to strand-cored cables.
(2) The effective Young's modulus will usually be between 0.41 and 0.72 times the material's actual value. This factor allows for the fact that the individual strands of material are twisted into a spiral pattern rather than running directly from end to end. The actual value depends upon the winding pattern that is used.

The product of these two "effective" values is an estimate of the axial stiffness, ie the "AE" value that you would use in the PL/(AE) formula.

 

[transmissiontowers]

Someone pointed out that the minimum elongation is specified as 5% in A475 for HS grade wire. If I remember the numbers right, 1/2" 7 strand has an area of 0.1497 square inches and a RBS of 18,800 pounds. Since delta/L is 0.05, if I solve for E, I get:
E = 18800/((.05)(0.1497))
so E is 2,511,690 psi which I think is about 10% of the value that it should be. Are my figures correct? Why is E so low.
TIA

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I have been called "A storehouse of worthless information" many times.

 

[Denial]

My factor (2) above could explain things if you were calculating E_effective at 50% of E_actual. The best I can suggest to expain the rest of the discrepancy is that the 5% elongation and the 18,000 pound RBS do not correspond directly.

 

[transmissiontowers]

The only thing I can think of is that the equation for Young's Modulus is Stress/Strain and it applies to a solid rod and is probably not applicable to a stranded cable. The cable will deform partially due to PL/AE and more to the twisting of the strands.

If I know that E=(P/A)/(dL/L) and for a elongation of .05 we get E = (P/A)/0.05 and if we say that E is reported as 25,000,000 psi, then the stress in the cable is (0.05)x(25,000 ksi) or 1250 ksi which is very strong cable.

So there must be some empirical formula to give the elongation for the cable unwinding in addition to PL/AE elongation.

As a side note, does anyone know how to type a capital Greek Delta in this forum? I used dL to represent delta L but was trying to put in a small triangle.

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I have been called "A storehouse of worthless information" many times.

 

[Tibby]

In the AISC steel design guide,the E for cables is listed at 14000ksi

 

[civilperson]

The manufacturer will list the Modulus of Elasticity (tangent for 0.2% offset) on each heat of steel fabricated into stranded cable and wire rope. It varies as stated in other posts. 27000 ksi is the highest I have seen, (270 ksi yield low relax pre-stress seven strand).

 

[btrueblood]

tt,

The 5% elongation is a maximum plastic strain for the material, so it's no wonder an elastic calculation is not giving you the right modulus.

Further, cable is a machine, not a material. Each time a cable is loaded in static tension to a new, higher value, the wires in the cable will stretch/slide/unwind and in general redistribute. This is called "taking a set" in the parlance. It looks like a plastic strain, but the material in the individual wire strands may not have actually reached its yield point. Once you've stretched a length of cable to some new maximum load, it will generally follow a linear load/displacement curve from there on, provided it never gets stressed beyond that original proof load, and unless the cable is also rolling over sheaves or pulleys, in which case further motion/redistribution of the wires will occur. Aircraft control cables have take-up mechanisms to account for the relief that occurs over time and use.

 

[transmissiontowers]

We use the cable for guy wire on utility poles so unless it is pre-streched, it will sit there at 2% of ultimate for many years until a hurricane comes along and loads up the system. I contacted a couple of cable manufacturers and they say that the MOE is between 23,000 ksi and 25,000 ksi for the 1/2" wire we use.

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I have been called "A storehouse of worthless information" many times.