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Making PLS-CADD and PLS-CADD/Lite Results Match Alcoa SAG10^®

This TechNote will discuss the approach that users should take when trying to make their PLS-CADD and PLS-CADD/Lite (hereinafter referred to as "PLS") reported sags and tensions match those from Alcoa SAG10^® (hereinafter referred to as "SAG10^®") runs.

Before going any further, it should be noted that if all parameters are set the same in both programs and all conductor data is the same, the results of PLS generated sags and tensions will match Alcoa SAG10^® runs. This TechNote will discuss those parameters in both programs and how they need to be set. All references to SAG10^® are made in reference to the demo SAG10^® Version 3.0 Beta N, which can be freely downloaded from the Alcoa website. However, from the experience gathered working with the hundreds of people who have switched from SAG10^® to PLS over the years, these references are valid for all previous Windows versions of SAG10^® as well as the original DOS versions, although the specific details on how to set these parameters may very slightly from the older DOS versions to the newer Windows versions.

First, let's cover some basics.

• There is no such thing as "average" tensions. The only real values known are 1) the horizontal tension, 2) the maximum tension at the end of the approximated ruling span, or 3) the actual maximum tension in a cable element. PLS allows the user to specify tensions in any one of these 3 conditions in one place or another. SAG10^®, by default, uses average tensions and the default must be changed to one of the three tensions above in order to make a comparison.
• The "creep" condition is a simulated condition (tension) that the conductor maintains over a given period of time that causes a given permanent elongation. The NESC states "Final unloaded sag (tension) shall include the effect of inelastic deformation (creep)". PLS allows the user to specify which condition to select for this effective tension. SAG10^®, by default, uses 60º F (this is the "60 degrees - no ice - no wind - no tension - Code "2" row in the loading condition table in SAG10^®), but it can be changed.
• The "load" condition is the maximum load that you want to consider that causes the conductor to experience permanent elongation. PLS allows you to specify which condition to use when determining this effective tension. SAG10^®, by default, uses the heaviest unit load condition (but this can be changed as well by adding a row in the loading condition table in SAG10^®).

With this basic understanding under our belt, let's now discuss how to set the parameters to make the two programs develop the same results.

1. Let's start by making sure you have either developed or loaded your criteria data in PLS. You can do this by entering all of the information under the Criteria menu to meet your loading conditions, or you can simply download and load (Criteria/Load CRI File) the NESC Criteria file appropriate to your area from our website here.
2. Since PLS recognizes only physically possible tensions, you will need to make sure that your SAG10^® run is based on one of these three possible tensions. Since SAG10^® defaults to the average tension mode, which is not recognized by PLS, this is the most common mistake made when trying to compare the results between the two programs. Also, since it is tedious and often quite time consuming in SAG10^® to run Inclined Spans, we'll focus the rest of this TechNote on making the sags and tensions match based on the maximum tension in a ruling span section. To change from the default average tensions method in SAG10^®, select the Options menu item and select the "Vert (At Supports)" option under the Tensions section. If you have an existing SAG10^® run with average tensions used that you are trying to match, please see the last item in the clarifications list below.
3. Now, make sure that your PLS-CADD model is also using the maximum tension in just a ruling span method (and not the more accurate and desirable "inclined spans" method), select the Maximum Tension item under the Criteria menu in PLS and set it to the "for end of ruling span with equal end elevations" option. PLS will now match the level ruling span tension method from SAG10^®.
4. Next, let's insure we are using common creep and loading conditions. In PLS, select the Creep-Stretch menu item under the Criteria menu and select the appropriate weather cases to match the SAG10^® common points, or, set the SAG10^® run to match the same common points you have in PLS.
   • If you didn't select any specific cases in SAG10^® (just used the default values), you will most likely be using a 60 degree, no wind, no ice weather condition from your weather cases (if you don't have one yet, you'll need to add one under the Criteria/Weather table) for the Creep condition and the heaviest cable load condition (probably your NESC condition or a heavy ice condition, possibly the extreme wind condition if you don't have any ice conditions) for the Load condition.
   • To change SAG10^® to match your PLS selected common points, change the temperature on the above described "60 degree - no wind - no ice - Code 2" row to reflect the desired creep condition and then add a line with the desired temperature, wind and ice (leave the tension row blank) with a Code "1" in the far right column to match your desired load condition.
   

5. The final items that you may need to address in Criteria (PLS) and Loadings Table (SAG10^®) are the tension limitation checks and automatic sagging.
   • Under Criteria/Cable Tensions in PLS, all of the tension limitations that you have specified in SAG10^® should be mirrored here by picking the same weather case (you may need to add it to Criteria/Weather first) and then specifying either the limiting percentage of RTS or the maximum tension for that same weather case. One word of clarification here; since PLS distinguishes between Creep and Load, on any "Final" conditions that you may have selected in SAG10^®, you will need to have two lines in PLS for both the Creep and Load conditions to check both "final" conditions. However, for this exercise, since SAG10^® does not recognize this difference, you will need to use the Creep condition only if SAG10^® is reporting "Creep is a factor" and the Load condition only if SAG10^® is reporting "Creep is NOT a factor".
     

   • If you will be using the Automatic Sagging function in PLS (strongly recommended), you will need to also add these values in the Sections/Automatic Sagging table. You can easily do this by copying the entire Cable Tensions table (click in the upper left hand corner box and then select "copy") and the paste (again, click in the upper left hand corner box and then select "paste") in the Automatic Sagging table.
6. Finally, you will need to insure that the conductor properties being used by each program are the same.
   • In SAG10^®, under the Conductor Selection button, you can select the conductor type and then the applicable conductor and then click on the "Lookup Wire Data" button. You will now see the Area, Diameter, Weight, RTS and Chart number for this conductor - jot these down or print the screen so that you can reference them later. Now, check on the "New S-S Chart" checkbox at the bottom of the dialog box and a Add / View Stress-Strain Charts dialog box will pop up. Select the drop down arrow for the Chart Number and find the referenced chart from your desired conductor and then click on the View Chart button. Again, jot all of these numbers down or print the screen so that we can refer to these numbers later.
   • In PLS, select Sections/Edit Cable Data menu item and either find your desired conductor, or just type in a name if you are creating one from scratch. You will now be presented with all of the data in a single cable data dialog box. This data should all be the same as the SAG10^® conductor data that we just referenced. Most of it is self explanatory, but it helps to know that the information in Column 6 of the SAG10^® Stress-Strain chart is the Final Modulus of Elasticity and Thermal Expansion Coefficient of the respective Outer and Core strands. Homogenous conductors will only have information for the Outer strands and even some composite conductors use only the outer core strand information. You can refer to our TechNote on Correlating Alcoa SAG10^® Cable Data with PLS-CADD Cable Data if you need any additional help on this. The thermal rating properties are only used for thermal rating functions in PLS and thus are not required to perform sag and tension calculations. Should you want this information, it can be obtained from several sources, but the most common and easily accessible source for thermal properties is Southwire's Overhead Conductor Manual, which can be obtained by request from http://www.southwire.com.

You should now be ready to make a run in SAG10^® and compare that to the Sections/Sag-Tension Report in PLS. If you have set everything up the same between the two programs as described above, you should now obtain the same results. A few words of clarification:

• If SAG10^® reports that "Creep is a factor", then you will need to compare the Final results in SAG10^® to the Creep results in PLS and of course, if "Creep is NOT a factor", then you need to compare the Final results of SAG10^® to the Load results in PLS. By observing the PLS sags and tensions under Creep and Load, you will quickly see why PLS distinguishes between Creep and Load and reports them both; creep is ALWAYS a factor - it either controls the final sag or the final tension and thus should always be considered by the prudent engineer for damper designs and other final condition calculations. For a full explanation of this, please see our TechNote on Why Creep is ALWAYS a Factor.
• The ruling span distance is highly sensitive - even a span difference of only five feet can make a fair difference in the resulting sags. If you are trying to match the results to the hundredth of a foot, you will need to ensure that you are comparing the same ruling spans between the two programs. PLS automatically calculates the ruling span (as described in Appendix I of the PLS-CADD Users Manual), or you can select the Override Calculated Ruling Span checkbox and fill in your ruling span in the Sections/Modify dialog box (Line/Setup table in Lite) for your conductor. (This exercise can also demonstrate the need to calculate the ruling spans as accurately as possible, as the traditional practice of rounding off to the nearest 50', 100' and in some rare cases, even 150' can cause significant errors in the resulting sags.)
• Make sure that all of your weather cases between the two programs are identical: comparing load case by load case, including the "k" factor for your NESC conditions. A common mistake is to intend to use a "12 psf wind" condition, use 12 psf in SAG10^® and then use the PLS function of putting in the wind speed of 69 or even 70 mph, to which PLS calculates the wind at something slightly different than 12 psf. Obviously you may have additional load cases defined in PLS that you may not have in SAG10^®, since PLS is a significantly more advanced program that allows you to do much more than just check sags and tensions.
• If specifying an NESC 2002, ASCE wind, or other wind condition that uses adjustable gust response factors, insure that your SAG10^® wind pressure reflects the proper height and span adjustment factors on the wind pressure. PLS does this automatically and thus the resulting wind pressure in PLS may be different than just using the unfactored wind pressure in SAG10^®.
• If you have an existing SAG10^® run that you are trying to match, odds are that it will be in average tensions. You will need to convert these average tensions to either horizontal tenisions or maximum tensions first. A simple way to do this is to use the following equation:
  • HTEN = AVGTEN - (UR^2*RS^2)/(16*AVGTEN)

    Where

  • HTEN = Horizontal Tension (to plug into PLS-CADD) - lbs
  • AVGTEN = Average Tension (from a "default run" of SAG10) - lbs
  • UR = Resultant load per unit length of the wire for this load case - lbs/ft (includes wind and ice loads and NESC constant as well as weight of the cable)
  • RS = Ruling Span - ft

  Once you obtain the equivalent horizontal tension, you can use this value in the Sections/Modify dialog box (Line/Setup table in Lite) for your sagging condition. Keep in mind that you will need to use the "controlling condition" from your SAG10^® run to make this work. Note that you will most likely see violations of our conductor tension limits as PLS-CADD checks these limits with the Maximum Tension value, not the Average Tension value like the SAG10^® run you have most likely does.

Setting all of the parameters the same between the two programs will allow you to compare "apples and apples" should you have that need or desire. Once you are satisfied with the result, don't forget to "smarten up" your PLS criteria to take advantage of the full power of PLS. Add the other condition - Load or Creep - to your Cable Tensions and Automatic Sagging tables to insure that you are not violating any code or other criteria under either condition. Set your Criteria/Maximum Tension to "using actual geometry of tension section" (note this input is not available in PLS-CADD/LITE). This is the equivalent of running SAG10^®'s Inclined Spans option on every single span of every single cable - a task that could take days in SAG10^® but only a fraction of a second in PLS. There are also some other options such as Aluminum Compression and Weight Span Models that you may or may not want to take advantage of in your PLS models, but those are topics well covered in the User Manual.

To see how to use PLS-CADD/Lite to make a simple ruling span based sag-tension run, see our TechNote on Generating Ruling Span Sag-Tension Runs in PLS-CADD/Lite.

SAG10^® is a registered trademark of Alcoa Fujikura Ltd.