Steel cable supporting Wind Load

[butch81385]

I have a steel cable supporting a net. The entire system is about 60' high, but the application will have a steel cable supporting 50' vertical of netting. I have to treat the net as if it is seeing all of the wind load, which comes out to be about 23psf after all factors. The span is going to be 20'-30'.

How do I go about transferring the 1150plf line load on the cable into the tension on the supports. I am familiar with the catenary equations, which relate the horizontal load, weight per foot, and sag, but how do I eliminate one of the variables? How do I know how much sag will occur and therefore will be able to calculate the horizontal force?

I assume that there is a way to relate the amount of sag to the properties of the steel cable, but any help is appreciated.

Thanks

 

[MiketheEngineer]

First of all - I design a certain amount of sag in the line and design for that. If it sags more - the better. But my design is more conservative.

1. I suppose you could go through all the calcs and figure how much it lenghthens - but why bother.

2. Put the cable up and pull at the center with all your might - there is your sag??

3. Archoneng.com has neat little software for about $40 that does a great job.

 

[BAretired]

If the force is horizontal, the shape will not be a catenary. It will be a parabola. Knowing the equation of a parabola, you should be able to calculate the sag for a given length of cable. Or alternatively, you can specify the sag and let the contractor calculate the length required to produce it.

If the offset is h, the tension in the cable is wL^2/8h.

BA

 

[butch81385]

Thanks for the quick reply guys. I guess what I was getting caught up on was dealing with the amount of sag. I was thinking of just using a value for the sag and seeing what the force would be, but then I was worried that in application it might sag more or less depending on the material and therefore change the tension.

Is it common to call out a specific sag under load?

Also, good catch on being a porabola under horizontal load.

 

[ishvaaag]

You may find useful to check the

Structural Engineering Handbook
Section 27, Lew, Scarangello
Suspension Roofs
Design of Suspension Systems
4. Single Cable under Uniform Distributed Load

Gaylord, Gaylord, Stallmeyer
4th Ed. (that I have)
McGraw-Hill NY 1997

Respect the question and using the equations there given, one may assume a guess answer for the standing sagitta (hence an elongated longer length of the cable than the span), and then iteratively test for compliance of the set of equations for the assumed section. I think this can be done with Mathcad set to find the sagitta (varying it) that meets all the conditions, but have not done. May do it, next weeks.

Also pay attention to the dynamic effects etc named in the text.

 

[BAretired]

I believe it is more common to specify sag in the unstressed condition. The tension produced will elongate the cable slightly and that will increase the sag and reduce the tension.

The amount of elongation is variable depending on the type of cable used and whether or not it has been pre-stretched.

BA

 

[csd72]

according to roarks
y = L(3wL/64EA)^.3333

for a cable with zero initial tension.

 

[ishvaaag]

I have in the end made the worksheet in Mathcad and the numerical result perfectly matches Roark's solution as quoted by csd72. Hence you know how much your cable will sag once its (valid in strength and remainint in the elastic realm) properties are known. For a particular case where the safety factor respect fpy ending the elastic behaviour is 2.43, the maximum stress along any point in the cable gets 1.17% bigger (a factor of 1.0117 hence) than the value at midspan (or horizontal reaction at end) as stated per BAretired above.

 

[butch81385]

Thanks everyone for all of your help. I have found the information I was looking for, and have come to a much better understanding of this.