Guy Wire Tension and Shear 2

[Dick44]

I'm having a discussion with other people at my company on how much load a continuous wire rope can hold.

Example:

A continuous wire rope is threaded thru a pulley attached to a pole. They are attached at the ground at the same point. For a load of, say 4000#, each leg takes 2000#. The rated breaking strength of the wire rope is 4000#.

Question: Since each leg is taking one-half of the load, can the 4000# load be increased to 8000#. Is the tension at the point of contact with the top of the pulley 4000# or 8000#, thereby exceeding the rated breaking stength.
Also, will the shear at this point be 4000# or 8000#.

Thank you for your help.

 

[rb1957]

if the rated strength of the rope is 4000#, call it that. trying to change the rated strength due to geometry is TOO confusing. what you could say, with being confusing IMHO, is that 4000# rope in this geometry means that the allowable load (applied at such-and-such a point) is 8,000 lbs (or whatever), and the applied load is Y lbs ...

i think the tension in ths cable is 4000# throughout the length of the cable. At the top, by the pulley block, one 4,000 lbs is reacting the other, rather than doubling the load in the cable. same for the shear load.

 

[DaveAtkins]

And be careful about using the word shear when talking about a cable--a cable cannot take any shear, but only tension.
The shear in the "pole" (again, this is a poor choice of word--if it is a horizontal member, it is a beam) will depend on where the supports for the "pole" occur, relative to the location of the pulley.

DaveAtkins

 

[Dick44]

Thank you for your comments. Just to clarify a couple of things. The pole is a vertical utility pole. It is guided at the top to stabilize the pole. I'm not saying to increase the rated breaking strength, but to increase the load. The guy is actually at a 45 degree angle, however, I was trying to be less confusing so I didn't mention it.

 

[rb1957]

you could, for very simple strcutures, apply an increased external load (in the direction of the service load) and show that this load is marginally acceptable (ideally MS = 0.00), once you'd considered all the possible failure modes. Then the margin for the structure under the service load is calculated from the applied loads.

but caution, some failures are not linear with applied load.

i'd prefer to apply the load required, and calculate MSs accordingly ... seems more direct and takes no longer to do.

 

[DRC1]

Assuming no friction in the pulley, each leg will take 1/2 the total load if guys are symetrical and in line load will be 1.41 times the horizontal (or vertical) load. the load in the pulley axial (which will tranfer to the pole) will be twice the pull in either cable. Cable will act in tension only, pulley will induce shear, bending and pullout at the connection. Rated breaking strength does not change with load conditions, only applied load

 

[Dick44]

Thank you DCR1. You are consistent with myment, however, I need something in writing to prove it to the others. Do have something. I've checked statics books, mechanics of materials and other civil books. Nothing has any thing that shows what we are saying for the point of the connection. Any help will be appriciated.

 

[SlideRuleEra]

The application where multiple part lines (usually on cranes) are used to pick up loads that exceed the cable strength is very similar to your situation, and is well documented. Download "US Steel Wire Rope Engineering Hand Book" from the home page of my website (link below) and see pages 57, 58 & 59.

http://www.SlideRuleEra.net

 

[Dick44]

Thank you SlideRuleEra for the information. One more question. With the guy going through the pulley and then both parts anchored to the ground, would you call this a one or two part line?

 

[SlideRuleEra]

Dick44 - If you mean like this...

then I would certainly consider that situation a two-part line.

An analogy is to look at an upside-down similar situation. At steady state (no movement of the line) the load on this platform is equal to the weight supported by the pulley (not counting the weight of the "stick-person" )...

http://www.SlideRuleEra.net

 

[Dick44]

Thank you.