This post first appeared in the February 2005 issue of Tower Times, published by NATE (National Association of Tower Erectors).
Guy Cable Myth #1
All 19 strand guy cables are bridge strand on guyed towers.
Both bridge strand cable and extra high strength (EHS) cable are commonly used in the design of guyed tower structures.
Tower professionals should never attempt to adjust tensions or re-guy a guyed tower without knowing the construction of the cables used on the tower. Why? Because the breaking strength difference between cable constructions can be significant.
For example, consider a tower guyed with 19 wire, 3/4″. As the table on page 37 illustrates, the difference between bridge strand cable and extra high strength (EHS) is 970 pounds – a difference of almost 15%! Unfortunately, without tower drawings or metallurgical testing, there’s really no way to distinguish between 19-strand EHS and bridge strand cable – they look identical in the field.
Guy Cable Myth #2
Tower professionals don’t need to know which type of cable was used if they are using an “in-line” measuring device or a digital readout unit because they can see the actual weight on the dial or readout.
Without knowing the cable construction, there’s no way to know a cable’s initial tension. The current measurement may not be correct; it may have been adjusted by someone who did not recognize the distinction between cables used in our industry.
A calibration for an EHS cable will not be the same as a calibration for a bridge strand cable since each type of cable has different properies. Measuring instruments should be calibrated for both types of cable.
|Cable||Breaking Strength||10% at 60 degrees|
|19 wire, 3/4″ Bridge Strand Cable||68,000 pounds||6,800 pounds|
|19 wire, 3/4″ EHS Cable||58,300 pounds||5,830 pounds|
|Difference||9,700 pounds||970 pounds|
The Long-Term Solution
Marking towers or guy cables should beecome an industry standard. Meanwhile, I strongly encourage tower companies to:
- Educate their tower hands about this critical difference prior to adjusting or replacing guy cables; and
- Find out the construction of the cables on the towers they are responsible for. This information should appear on your tower drawings. If you do not have drawings, you can usually obtain copies from the manufacturer (likely with a cost to obtain a copy).
Please feel free to contact us with questions concerning this issue.