Cable Design for Fabric Screen Wall - Sag & Tension Questions

[Kstructuralguy]

I'm designing a cable for a fabric screen wall on the edge of a roof penthouse (and its connections to a steel post and CMU wall). Since there is very little dead load, I'm not concerned with vertical deflection. I'm designing it primarily for the horizontal wind force.

The issue I'm having is that in order to calculate the tension in the cable, I need to know the sag of the cable. To know the sag, I need to know the elongation of the cable. And to know the elongation, I need to know... The tension. So somewhere I need to specify one of these things (sag or tension) or make an assumption and solve in iterations and hope to converge on a solution.

The horizontal cable reaction is qL^2/(8h) where q = uniform load, L = span, and h = sag. So lower sag means requires much higher tension. When putting the screen wall up, they are going to pull the cables tight so there is little sag under the light dead load. I can design for this tension force and calculate the sag. But does this mean under the (much higher) wind loads the cable will 'sag' laterally much more without an increase in the cable tension? Or would tension AND sag increase? When I use the sag induced by dead load only for the lateral wind load case the calculated tension force is incredibly and unrealistically high. I have a higher tolerance for lateral deflection than vertical but at the same time I don't want to sag significantly under the dead load only case.

 

[KootK]

Or would tension AND sag increase?

That one.

You run into similar issues with cable vehicular barriers. Googling that might turn up some useful and relevant information.

when I use the sag induced by dead load only for the lateral wind load case the calculated tension force is incredibly and unrealistically high.

Yeah, that'll happen. Could you factor in the flexibility of your posts bending in the plane of the screen to achieve a higher, and more realistic, sag under wind?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Denial]

The cable will sag more when the wind load is applied because its stretch will increase, and any iterative solution scheme you develop will have to accommodate this effect.[ ] To calculate the cable's elastic extension correctly you need to know the "average tension" in the cable.[ ] A hard-to-find formula you will find useful here is
[ ] [ ] [ ]T_av = T_H * L_s / H
where
» T_H is the horizontal component of the tension in the cable
» L_s is the stretched length of the cable (measured around the catenary curve)
» H is the HORIZONTAL* distance between the cable's end points.
* The formula applies whether or not the cable is inclined.

The axial stiffness of a cable is best obtained from its manufacturer, and the cable should have been prestretched before installation so that its individual fibres have nestled into their final positions. If you have doubts about the value of axial stiffness, in this context it is conservative to go with a high value.

It will probably be overkill for this situation, but my website contains a spreadsheet that analyses cables. It allows for non-horizontal cables, cable stretch, and a point load anywhere along the cable. See URL

http://rmniall.com/downloads/software-downloads/#LoadedCable