Reaction force at top of column from removing sag in cable

[Charred]

I've got a project where I have been asked to provide a design for a series of columns that will support steel cables which string lights will hang from to illuminate a large 'garden' area. The top of columns will be ~20' and the width of the garden is ~150'. I'm assuming there will be a lot of initial sag in the cables in an 'at-rest' state and will require post-tensioning to limit the maximum sag to ~5'-0" or less. Are there any references for how to calculate the final tension in the cable and then the reactions at the columns?

 

[Tomfh]

https://www.engineeringtoolbox.com/cable-loads-d_1816.html

When you tighten cable up approaching flat the tension loads increase dramatically. Hence most street cables and decorative lighting having appreciable sag.

 

[Denial]

It sounds like your "string lights" are continuous with the supporting cable(s) and so contribute only an end-to-end uniformly distributed load to the problem.[ ] If so, and if the bending stiffness of your columns is sufficient to prevent significant lateral movement at their tops, then Tomfh's solution should do the job for you.

However if the column tops are likely to move significantly under the lateral load applied to them by the cable(s), or if the lights apply a concentrated load to the cable(s), then my website (

http://rmniall.com)

offers a spreadsheet that might be more directly applicable.

_{[ ]—————————————————————————————————}
^{Engineering mathematician/analyst.[ ] See my profile for more details.}

 

[BridgeSmith]

You say a "series" of columns. How many columns?

Btw, with our designs for span-wire structures, the wind load effects dwarf those of the gravity loads.

 

[Charred]

I have attached a site plan showing the intended column and light layout and have included a few notes/dimensions. Where would I find references for wind load effects on cables?

 

[Celt83]

The AASHTO Signs and Luminaires spec provides good information that can be applied to string lights: Link

Make sure you consider ice and wind on ice loading. The loads on top of the columns can get out of hand fast so it's best to temper expectations about acceptable sag early.

 

[TLHS]

A helpful tool for this can be PLS-CADD Ultralite. It's the free catenary calculator that will do wind and temperature effects. It's intended for overhead line design but you can use it for other stuff to a degree.

I think it might be a little awful from a user experience perspective and I'm likely blind to it because I've been in the PLS-CADD ecosystem for a long time now, but it might be worth playing with.

You should be able to download a .wir file for various steel ropes from the pls-cadd website if it's not in the included database. Ultralight may not have all the tools for adding extra wind area and weight above the cable weight, but worst case you can use a weight multiplier or mess with ice radius and density to accommodate probably.

 

[BridgeSmith]

The AASHTO sign spec also includes an Appendix (don't remember which one off the top of my head) that gives an analysis method for span wires and poles. I've used that as the basis for my span wire design in Excel.

 

[dauwerda]

I too was going to suggest PLS-CADD Ultralite, but TLHS beet me to it!
The download link is here:

https://www.powerlinesystems.com/plscaddultralite