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Why is Creep ALWAYS a Factor?

We are often asked why PLS-CADD and PLS-CADD/Lite sag-tension reports both the "Creep" and "Load" conditions instead of just a single "Final" condition. This is done becasuse creep is always a factor regardless of whether or not the maximum weather load causes additional sag or not. 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:

PLS-CADD Creep Stretch Dialog Box

These two conditions are discussed in depth in CIGRE Brochure 324, Task Force B2.12.3 "Sag-Tension Calculation Methods for Overhead Lines", which can be purchased at the e-cigre online bookstore. This document is an excellent resource if you have any questions about the behavior of sag and tension or want to understand the underlying theories and calculations.

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 by the engineer. 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. If Load results in the maximum sag, the tension associated with this Load sag will be lower than the tension associated with the smaller Creep sag. Creep will always happen; it is a function of time. Load may never happen; it is a function of weather. If Creep results in more sag than Load, then it will result in the Maximum Sag and the Maximum "Final" Tension. If Load results in more sag, then it will result in the Maximum Sag, but it will NOT result in the Maximum "Final" tension as the maximum weather event used that resulted in the Maximum Sag may never occur (or it may occor many years later after the full Creep state has already been reached.) Thus, the tension associated with Creep will always be the "Final" tension.

This becomes a significant engineering concern when considering Aeolian vibration. Let's take the case where the "Final" maximum sag of a conductor occurs after a "Load" event. If only considering the "Load" condition as "Final", then the "Final" tension will also be after "Load". 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 after the line is up. However, most maximum loading conditions used in line design 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 dampened.

We conclude with a couple of case studies that you can duplicate on your own. Let's assume that a Linnet ACSR cable on a 1000' ruling span, using NESC Heavy Loading criteria (1/2" ice, 4 psf wind, 0 F) with a utility specified maximum loading of 1" 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 1" 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, review 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 1" ice condition. The major and in some cases catastrophic problem occurs when engineers use the "industry standard" rule-of-thumb 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.

PLS-CADD Sag-Tension Report

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. Let's consider Load as the 'Final' tension at the 25% "Final", or 7871 lbs in this case. 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 1" 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 applicable code and select the proper dampening designs based on both cases. Both Creep and Load conditions must be reported by the sag-tension analysis in order to do this. "Creep is ALWAYS a factor".


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