Cable Tension 2

[engpes]

I am trying to analyze the deflection of a cable strung horizontally across a span with a force applied near the middle.

Can I treat this as a simple beam to calculate the deflection at a point near the center of the beam?

Does the cable tension prior to applying the vertical force need to be analyzed?

Will the horizontal reactions at the end restraints be calculated from the pre-tension and added to the horizontal components of the deflected cable?

Thanks in advance.

Regards,

 

[KENAT]

This question, or variations there on have been asked before. A quick google search of this site (box at top of window) turned this up: thread194-233220; and there were others.

You might want to give some more application information since this almost sounds like a homework question which isn't allowed so some folks might not respond.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MintJulep]

No, you can't treat it as a beam.

The analysis you are trying to do is well documented.

 

[zekeman]

Google "catenary"

 

[PetieSmo]

This problem will not be accurately modeled with beam equations. The deflection will be heavily dependent upon the tension in the cable. As mintjulep said, this problem is pretty well documented.

Because of the force applied at midspan, the cable will not assume a catenary curve.

 

[engpes]

Thank you. This has been very helpful.

If I pre-tension the cable to 2000 lbs prior to applying my force (P) at the center of the cable, and I use the equation T = P(1-x/s)/(sin(theta)) to determine theta, can I calculate the vertical component of the deflected sling to determine my new tension after the force (P) is applied to the center of the cable?

I have attached a sketch to clarify.

Thanks a lot for the help.

 

[rb1957]

does the able have weight ? well, of course i does, i mean are you accounting for the effect of weight ? if you want to, you aren't with the equations presented and you need to read up on "catenary", as posted above.

if you aren't, then the basic approach is right ... the tension in the cable reacts the load. But there's a "2" missing ... draw a full free body to see.

btw, are you sure this isn't a student post ?

 

[engpes]

Thank you very much.

For this case, I will be assuming the cable to be weightless.

And, no I'm afraid this isn't a homework problem. It is for a rigging support system.

Thanks again.

 

[rb1957]

did you get the "2" comment ?

 

[engpes]

Yes I did. It is the tension in the other half of the cable.

Between the 2 of them, the vertical component will equal the downward force applied to the sling.... Right?

Thanks a lot.

 

[rb1957]

yeah, there are two tension forces reacting the load

 

[Compositepro]

As theta approaches zero the cable tension becomes infinite. So do not try to keep theta too small.

 

[desertfox]

Hi engpes

Have a look at this site its run by another member here and a very useful site:-

http://www.slideruleera.net/USSWireRopeEngrHandBook.zip

go to page 44 which shows a single span rope anchored with a single point load in the centre, there are lots of other cases to.

desertfox

 

[chicopee]

You will have an initial stretch during pretension, however the cable should have virtually no sag if you neglect its weight because it has a small diameter and is of fabric composition. Note fabric rope (polyesther, nylon,etc...) will stretch about 3% to 6% prior to its breaking strength, so do some research.

If the cable is steel rope, there will be a slight catenary under pretension, therefore there will be a sag without anyload and that should be calculated using the catenary equation. I don't remember the percentage of stretch prior to its breaking strength so also check that out.

When you apply the load, there will be an additional stretch and that additional stretch can be used to calculate the additional sag.

The total deflection will be additive.

 

[engpes]

The cable is steel cable. Now to complicate the situation a bit.

I have added a vertical choker cable (assumed rigid) to attached to the horrizontal cable at the midpoint. Nothing is keeping the choker cable from translating horizontally (simple shackle connection).

See the attached sketch.

I am trying to calculate the max deflection and the tension as each point in the horrizontal cable.

Thanks

 

[kelowna]

I do not understand this post. The problems described are very simple and you should be able to solve them without any help.

Are you an engineer? Why do you need to solve this problem? If you are not an engineer and this is a real structure/problem, you should consult somebody. Responses on an internet forum are all great, but you should not rely on them when the safety/security of people is at risk.