Calculating amount of sag and tension for 1/8 steel cable

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    Cable Steel Tension
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Discussion Overview

The discussion revolves around calculating the sag and tension in a 1/8 steel cable suspended horizontally between two concrete pillars. Participants explore the implications of pre-tensioning the cable, the weight of the objects being hung, and the use of online calculators for determining sag and tension. The context includes practical applications and technical considerations related to the setup.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant provides details about the setup, including the dimensions, weight of the objects, and cable specifications, seeking help with calculations.
  • Another participant suggests using an online catenary calculator to assist with the calculations.
  • Some participants emphasize the importance of pre-tensioning the wire and inquire about the intended pre-tension level.
  • There is discussion about the relationship between pre-tension and sag, with specific sag values provided for different pre-tension amounts.
  • One participant notes that safe working tension should be considered, suggesting a conservative approach based on the cable's breaking strength.
  • Another participant raises the idea of trial and error in calculations, questioning whether simultaneous equations could simplify the process.
  • A later reply cautions against attempting to eliminate all sag through excessive tension, warning of potential structural failure.

Areas of Agreement / Disagreement

Participants express varying views on the importance of pre-tensioning and its effects on sag, with no consensus on the best approach or specific calculations. The discussion remains unresolved regarding the optimal tensioning strategy and its implications.

Contextual Notes

Limitations include potential missing variables in the calculations, the dependence on specific definitions of tension and sag, and the lack of a straightforward method for determining the appropriate pre-tension level.

Cjbsurf
Thanks for reading, hopefully I am posting my first question in the correct area of the forum!

I will list as many details as I have in hopes that someone will know how to calculate the amount of sag I will have and tension/force there will be.
I am suspending a 1/8 steel cable horizontally (like a zipline) between two concrete pillars that are 32' across and will sit at 16ft high. I will be hanging stationary objects 16' centered across this 32' cable, the 14 objects weigh 3.5 ounces each so combined they will be just over 3 pounds total hanging from the cable. The cable has a breakstrength of 2,000lbs.

With this information I am in need of finding out how much force or tension I am causing between the two pillars I am suspending the cable from and what the sag in the cable will be. Thanks in advance for any information or help you can offer, it is appreciated!
 
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Depends largely on how much you pre tension the wire . What do you intend to do about this ?
 
Nidum said:
how much you pre tension the wire
Surely there would be a length / tension adjustment to allow for the bends in the wire to be taken out, after the wire is installed and loaded.
 
Doesn't matter that much when the wire is actually tensioned - what matters is that it is a user controlled input ?
 
Nidum said:
a user controlled input
Meaning a design variable?
 
Nidum said:
Doesn't matter that much when the wire is actually tensioned - what matters is that it is a user controlled input ?

anorlunda said:
https://www.spaceagecontrol.com/calccabl.htm

You need a catenary calculator. There are
Nidum said:
Depends largely on how much you pre tension the wire . What do you intend to do about this ?
Nidum said:
Depends largely on how much you pre tension the wire . What do you intend to do about this ?
I have not calculated the pretension. I have only gathered the information I listed along with the weight of the 32' 1/8 steel cable (1.344). I wasn't sure where to go with the information I have to calculate the sag. Thank you for your guidance and input! There doesn't seem to be a simple online calculator for this, I seem to be missing variables that I am unaware of.
 
OK, I used the online calculator for you using your numbers.

If you pre-tension the wire 1 pound, the sag is about 15.7 feet.
If you pre-tension the wire 10 pounds, the sag is about 1.3 feet.
If you pre-tension the wire 100 pounds, the sag is about 0.13 feet.

Safe working tension is about 300 pounds.
 
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If necessary there is a relatively easy calculation possible if we simplify the problem a little .

To some extent it is trial and error . Let's start with a safe tension for the wire and see where that gets us . 2000 lb breaking strength is quoted so let's say no more than 10% of that as first guess .

Will your calculator accept that as an input or do we start working this all out ourselves ?
 
  • #10
Nidum said:
To some extent it is trial and error .
Why is it not just a matter of some simultaneous equations?
 
  • #11
Wow, thank you anorlunda, nidum and everyones input! I just got back home and I was pleasantly surprised to see your useful replies. I am extremely grateful for your willingness to help in an area that I have no skill in yet!
 
  • #12
In case it's not obvious, the key is not to try and tension the wire in an attempt to remove all of the sag. It will break or pull your pillars over.
 

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