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Graphical Sag Options: Making the Wire System Match "As-Built" Survey Points

The PLS-CADD "Sections/Graphical Sag" command has a number of options to help you fit wire through points. It is most frequently used to make a PLS-CADD wire match sags surveyed in an as-built field condition. It can also be used as a design tool to determine what must be done to a line to meet certain clearance requirements. The various graphical sag options are discussed below.

1) Ruling Span

This mode operates under the ruling span assumption that for any given temperature, all spans within a tension section have the same horizontal component of tension. This assumption is generally considered acceptable provided the spans are fairly level, span lengths are fairly similar, the line was correctly strung and nothing unusual has happened to the line that would cause unbalanced tensions (no structure/foundation movement, no changes to structures after construction, no cutting/splicing wire...)

To sag in this mode you click on a single point and specify the wire temperature and wire condition (initial/creep/load) under which the wire should pass through the point. PLS-CADD will determine the horizontal tension that is required to fit through the point and will sag all of the spans within the tension section accordingly. This mode can be used to match a surveyed sag or to determine the tension that would be required to have the wire clear the selected point. When using this mode to as-built a wire it is a good idea to check the fit in any other spans you have surveyed wire points in to make sure you got a good fit. If you did not get a good fit you may have to consider one of the more advanced modes below.

2) Finite Element with Insulators Plumb at Sagging Condition (requires FE sag-tension plug-in)

In this mode the program assumes that all spans within the tension section had the same horizontal component of tension when the line was sagged. The program does not rely on ruling span approximations other than for determining the initial length of wire in each span. Once these lengths are known they are fed through a finite element sag-tension calculation to predict where the wire should be under other conditions. This method is capable of dealing with large variations in span length and severely inclined spans where the ruling span approximation may not work well. Like the ruling span method, this method is unable to handle changes in wire length or structure position that occurred after the line was sagged.

Sagging a wire using this mode is the same as for the ruling span mode except that you should first use "Section/Modify" to set the sagging temperature and wire condition to values where the insulators were plumb (constant horizontal component of tension).

3) Finite Element with Wire Length Adjustment in Selected Span (requires FE sag-tension plug-in)

In this mode you click on a point and the program adjusts the length of the wire in the span crossing that point to fit. This mode is most useful when you have a clearance problem in one span and want the computer to tell you how much wire to cut out to meet clearance. Please note that length changes in one span will impact sags and longitudinal loads throughout the tension section. Care must be taken when doing this to ensure that the structures can handle the induced loads.

4) Finite Element with Wire Length Adjustments in All Spans (requires FE sag-tension plug-in)

This is the mode to use if you don't get a good fit after using one of the previous modes. Best results are obtained if you first use one of the previous methods to get a crude fit before using this one. In this mode the program will adjust the length of wire in every span of the tension section to simultaneously fit through a selected point in each span.

When starting a graphical sag in this mode the program will automatically pick a default point in each span to fit through. The point it selects is at the current mid span wire elevation (this is why first getting a crude fit with one of the earlier modes is a good idea). Unless you change these automatically selected points the program will in effect lock the spans at their current mid span elevations. You can change the selected point in each span by simply clicking on a different point. There is also an option to have the program automatically select the wire survey point that is closest to mid span in each span (great for fitting to lidar data).

The main advantage of this mode over the earlier ones is that it is completely immune to ruling span approximation errors, sloppy stringing technique or anything that may have happened after construction (structure movement, wire length changes, structure changes…). If you know where your structure to insulator attachment points are and know points along the wire this mode will allow you to obtain a fit.

5) Finite Element with Manual Length Adjustment (requires FE sag-tension plug-in)

In this mode you click on a span and then key in how much wire you want to add or subtract from a series of spans. Through trial and error you can determine how much change in wire length is needed to obtain a desired clearance. It can be used to experiment with cutting out wire length or shifting slack from one span to an adjacent one (reclipping insulators) as solutions to clearance problems. As is the case with any adjustment in wire length,, sags and tensions throughout the section are affected so you need to make sure the structures can take it.

6) Finite Element Multiple Point Fit (requires FE sag-tension plug-in)

This is the mode to use when the locations of your insulator attachment points (insulator to structure connection) are questionable. The other graphical sag modes assume the insulator attachment points are correct and will not produce good results if they are in error. This mode is different in that it can calculate where the insulator attachment points should be and can even move your structures to match these points.

The program searches for points of a designated feature code close to the currently displayed wire position. It will automatically pick three points in each span, one near the left end, one in the center and one near the right. It will then fit a catenary curve through these three points. There’s an option to refine this catenary by doing a least squares curve fit through all points within a certain distance of this catenary. Once the program has the catenary curve in each span it can calculate the wire attachment points from the intersection of these curves and then from these it can determine the insulator attachment points. Options are provided for adjusting wire length to match the arc length along the catenary between wire attachment points and to adjust structure positions to get the attachment points in the right place. There are also options to create survey points at the calculated wire and insulator attachment points.

Back to finite element sag-tension technical notes.

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