Guy cable data
Republished with permission from K7NV, Kurt Andress. See K7NV's web site for further information and other studies. – Editor
Here is some information that resulted from my research into guy cables to support my tower modeling efforts. The object of my research was to arrive at values for the cable diameter and elastic modulus that would provide accurate stretch behavior in the cables for use in the FEA tower models.
Steel Wire Rope
The following information was derived using published information from the Macwhyte Wire Rope Company, Kenosha, WI.
To obtain the equivalent area of a solid wire (to be used in the tower models) to represent any multiple wire construction we use the following formula:
- A = (100 D2) / (F×E)
- A = the equivalent area
- D = the nominal wire rope diameter
- E = the elastic modulus of the steel :
- F = The factor provided by Macwhyte for each cable construction. Different factor for each construction.
The elastic modulus for steel is widely accepted to be around 29 Msi (million lb/in2). Using this value we obtain the following equivalent areas for steel wire constructions:
It is readily apparent that the 1×7 construction has the largest equivalent area and hence, stiffness. EHS guy cables for towers use this construction. It is harder to form around the thimbles, etc.; guy grips eliminate this handling problem. The other constructions are designed to be more flexible for running over sheaves, like in crank-up towers, but we don't want to use them for guying towers. The reductions in the equivalent diameters are caused by the fact that there is less material in the construction than a solid wire at the nominal size and the degree of twist built into the wire construction. It is my understanding that the factors were derived from laboratory testing of the product.
Aramid cables are made from non-conductive synthetic fibers commonly known as Kevlar (TM). There are two common Kevlar compounds used for cable construction. The first and most common is Kevlar 49, the other is Kevlar 149. The information I have indicates the elastic modulus of the Kevlar 49 is around 18 Msi and the Kevlar 149 is around 25 Msi. It is my guess that most of the aramid cable offered to the cost conscious amateur buyer is made from the cheaper Kevlar 49. The Kevlar 149 is stiffer but not as strong as the 49. The 149 is available in certain cable sizes rated at 8,600-32,500 lb breaking strengths. It is not likely an amateur tower builder will get it from the normal outlets, unless specifically ordered. It is a readily available cable in the marine industry for applications that require the increased stiffness.
The following table presents equivalent diameter derivations made from the Philadelphia Resins Corp. Technical bulletin NO. 320-5/80. The bulletin presents stress-strain plots for various cables and does not state which Kevlar compound is used in the cable. I have assumed that it represents the Kevlar 49 @ 18 MSI elastic modulus. This assumption does not introduce any unexpected error, as the plots show elongation vs load, and either choice of modulus would result in the same net elongation reported in the document. They would just arrive at the same elongations via different effective diameters.
The values were determined by finding the average equivalent diameter for 18 Msi modulus material across the range of the linear plots. The technical bulletin did not include the HPTG6700 cable. Its equivalent diameter was calculated using the average reduction from nominal cable size found in the larger cables presented in the plots. The nominal Kevlar fiber bundle diameters were taken from data published by Aramid Rigging Inc., Portsmouth, RI., 10/96, a supplier to the marine industry for these cables. The actual OD measurements of the cable are 0.063 - 0.070 in larger when measuring the jacket.
Fiberglass Reinforced Plastic (FRP) meets our requirement for non-conductive guy material. There is "fiberglass" (used for highly stressed aerospace structures) and there is "fiberglass" (used to make your bathtub), and a lot of stuff in between.
There are different types of glass filament alloys, some have average strength like E-Glass made for electrical and general purpose applications, and there are high strength alloys, like S2 Glass made for high strength applications. There are also a multitude of resin systems applied to these fibers in a multitude of processes to obtain an even wider variety of properties. So, when we talk about fiberglass we need to be exceptionally careful to quantify the constituent materials and their process to get in the right material properties ballpark.
Add to that, some processors and resin formulators are more clever than others and get better properties than others. The important point is that "fiberglass" has the highest diversity of processes and properties of any of the materials we use. So, care should be exercised when ordering these materials. Be absolutely sure to obtain guaranteed minimum material properties.
Current information indicates that the use of FRP rods as tower guys has been confined to rods produced by the pultrusion process. This is similar to the extrusion process for some metals. It is a low cost continuous line process. Most of the fibers are aligned with the axis of the rod providing the highest axial strength and lowest axial elongation. There are numerous manufacturers of FRP rod with this process. It is my opinion that this is the best process for FRP stock for this application. The rods can be terminated with Glass Grips made by the same company that makes the Big Grips.
Guy Cable Comparisons
A comparison of the cables under load will make the information more meaningful. A 3000 lb load is applied to the cables and the elongation is calculated for a 100 ft length of each cable.
Note: The fiberglass rod information is based on a polyester "E" Glass pultruded rod that has a tensile strength of 120 ksi, tensile modulus of 6.0 Msi, and a density of 0.073 lb/in3. Be sure to ask for the properties listed to get something that will perform according to this comparison.
I only offer comments to stimulate thought and hopefully help fellow amateurs. None of the information provided is authoritative in any manner or guaranteed to be correct. The reader is encouraged to research these subjects and make his own determinations about these things before trying to apply them in the real world.
Updated July 7, 2001 Copyright © 2001-2004 Kurt Andress, K7NV All Rights Reserved