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Most transmission engineers have been trained over the years to think that creep is either a factor, or is not a factor, one or the other. Other programs even go so far as to print this in their output (e.g. "creep is a factor" or "creep is NOT a factor") and then just report the "Final" sags and tensions. However, contrary to what the uninformed engineer might think and what other programs might tell you, creep is always a factor and should be considered by the prudent engineer in all designs. This TechNote explains why creep always matters and why PLS-CADD and PLS-CADD/Lite sag-tension reports both the "Creep" and "Load" conditions.
First, let's explain the differences between "Creep" and "Load" conditions of a cable:
These two conditions are discussed in depth in Section 2.3 of Southwire's Overhead Conductor Manual, which can be obtained by request from http://www.southwire.com.
Now that we understand the basics of Creep and Load, let's discuss why these concepts are important and why both should always be considered. When other engineers and software programs say that "Creep is a Factor", they are just referring to the fact that the resulting sag (permanent elongation) after the "Creep" condition is higher than the resulting sag (permanent elongation) after the "Load" condition. Conversely, if they say that "Creep is NOT a factor", they just mean that the sag after the "Load" condition is greater than that after the "Creep" condition. In these other programs, normally whichever condition results in the most sag is what is reported in the "Final" column and it is left for the engineer to think that these values are what he is to use for his maximum sag and tension values.
The major flaw in this assumption of only one or the other condition occurring is that "Creep" is ALWAYS a factor. Basic physics tells us that tension and sag are directly related; when the cable sags more, the tension goes down and when the conductor sags less, the tension goes up. So, while "Creep is NOT a factor" for the sag (e.g. the "Load" condition results in more sag), "Creep" is a factor for the controlling final tension. Load or Creep will control the final sag and the other will control the final tension.
This becomes a significant engineering concern when considering Aeolian vibration. Let's take the case where another sag-tension program says "Creep is NOT a factor". It will properly report the maximum sag under the "Final" condition as being the maximum sag that is associated with "Load" condition. However, it also reports the corresponding tensions with those controlling sags. When your damper manufacturer now asks you what your "Final" tensions are, you will be giving him the lower tensions that are associated with the "Load" conditions. These are fine, assuming that your cable sees the controlling load case in the first few years. However, most maximum loading conditions are ultimate conditions and are designed with 25, 50, or even 100 year return periods. Your cable may never experience that loading condition or may have to wait for 25, 50 or even 100 years for it. This leaves you with a cable that you THOUGHT would be at 18 percent of the RTS average everyday tension in its final tensions, when in reality, it could be at 20 percent or more. Regrettably, engineers can find this out the hard way when they have fatigue failures due to Aeolian vibration because their cables were at a significantly higher tension for a longer period of time than they were led to believe. This situation where the line never experiences the maximum load condition can result in "under dampening", or even worse, not dampening cables that should have been.
We conclude with a couple of case studies that you can duplicate on your own. Let's assume that we are going to have a Linnet ACSR cable on a 1000' ruling span, using NESC Heavy Loading criteria (1/2" ice, 4 psf wind, 0º F) with a utility specified maximum loading of 1½" ice. Using PLS-CADD/Lite, downloading the Linnet conductor and the NESC Heavy Criteria file from our website (here) and then modifying the default NESC Heavy criteria file to add the 1½" ice condition and using that condition as the controlling condition for the after "Load" tension, we can generate a sag-tension report. (To see how to do this for yourself, see our Generating Ruling Span Sag-Tension Reports in PLS-CADD/Lite TechNote). The resulting sag-tension run can be downloaded here for your inspection. You can easily see that while the Load condition controls the maximum final sag, creep will control the final tension if this cable never actually sees a 1½" ice condition. The major and in some cases catastrophic problem occurs when engineers use the "industry standard" of preventing the everyday tension from exceeding 18% which traditionally means dampers are not required. Examining the sag-tension run, the tension under the everyday 60 degree condition (Load Case #16) is 2538 lbs, or 18% of the RTS under the Load condition. However, under the Creep condition, the tension is 3292 lbs - an increase of 754 lbs or nearly 30% and results in a final tension of 23% of the RTS - well above the normal requirements for installing dampers and a recipe for a catastrophic fatigue failure some years in the future.
Now, let's take a Drake ACSR conductor and string it on the same 1000 foot ruling span and pull it up to the maximum NESC limiting tensions (Rule 261H1, page 179/180 of the 2002 NESC), understanding that we will most likely be adding dampers. Using other software, the engineer would have miscalculated the controlling tension as the 25% "Final", or 7871 lbs in this case, as being after Load (e.g. it would have reported that "Creep is NOT a factor"). In reality, as can be seen in the sag-tension run from PLS-CADD here, this results in an actual maximum "Final" tension of 8571 lbs under the Creep condition, a 700 lb difference and a violation of the NESC by nearly 10 percent. Additionally, the damper selection would be made on a tension 700 lbs less than what the final tension most likely will be, unless the cable sees the 1½" ice condition in the next 10 years. This is something that we simply cannot accurately predict and certainly cannot rely upon occurring.
In summary, it is absolutely critical for an engineer to consider both Load and Creep final conditions and design his cables to meet the code and select the proper dampening designs based on both cases. You should seriously question anyone or any design that says, "Creep is NOT a factor".