Modifying Lines Without Changing Existing Wire Lengths

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Modifying Lines Without Changing Existing Wire Lengths

This page shows common situations that require going beyond ruling span sag-tension to a more sophisticated approach based on the length of wire in each span.

Most sag-tension programs assume that the horizontal tension is the same in all spans between dead ends. While a line may be strung with equal horizontal tension in adjacent spans there are situations where changes made after construction invalidate this assumption. Examples of several such after construction situations will be shown below including:

Moving structures
Raising structures
Inserting structures
"Shift & Cut" thermal upgrade (shifting slack between spans or removing slack)

Each of these situations results in a horizontal tension imbalance that defies analysis using ruling span techniques. These situations must be analyzed using an approach that is based on the length of wire in each span. Customers with both PLS-CADD and SAPS can switch from the more conventional constant horizontal tension approach to a wire length based approach by going to Section/Modify and checking the "clip insulators (lock unstressed length)" check box. Once in "clipped insulator" mode you are free to use the normal calculation and editing commands and PLS-CADD will transparently invoke SAPS as needed for wire length based finite element sag-tension calculations.

The examples below show a line with four 500' spans between dead ends and a drake conductor initially strung to 1500 lbs. The initial line configuration is shown in blue and the altered configuration is shown in red. The vertical scale is exaggerated by a factor of 10. While PLS-CADD is able to account for structure flexibility (see "getting more sophisticated: structure deflections and coupling between wires", soon to be available on our web site) we have elected to ignore structure deflections here in the interest of simplicity.

Structure #3 moved 5' to the left (Structure/Move command)

Note how moving structure #3 significantly impacts the sags and insulator swings in every span.

Structure	Original Longitudinal Load (lbs)	New Longitudinal Load (lbs)	Ahead Span Sag Increase (ft)
1	-1500	-1328	2.9
2	0	148	5.3
3	0	-930	-7.3
4	0	211	-4.8
5	1500	1898

Structure #3 raised 5' (Structure/Modify Height Adjust command)

Note how raising structure #3 has minimal impact on the first and last span.

Structure	Original Longitudinal Load (lbs)	New Longitudinal Load (lbs)	Ahead Span Sag Increase (ft)
1	-1500	-1502	0
2	0	-2	-2.6
3	0	0	-2.6
4	0	2	0
5	1500	1502

Structure inserted between #2 and #3 (Structure/Add command)

Note how the structure insertion significantly increases sags in the first span and the last two spans.

Structure	Original Longitudinal Load (lbs)	New Longitudinal Load (lbs)	Ahead Span Sag Increase (ft)
1	-1500	-1329	2.8
2	0	112
2.5		0
3	0	-142	2.2
4	0	-61	1.3
5	1500	1420

"Shift & Cut" thermal upgrade: 5' of wire shifted from right side of #3 to left side (Section/Graphical Sag command)

Note the significant sag increases to the left of structure #3 and decreases to the right.

Structure	Original Longitudinal Load (lbs)	New Longitudinal Load (lbs)	Ahead Span Sag Increase (ft)
1	-1500	-1333	2.8
2	0	143	5.7
3	0	-912	-7.6
4	0	208	-4.8
5	1500	1894

Back to finite element sag-tension technical notes.