Sags & Tensions Calculation Verification

The maximum tension and maximum sag of a wire attachment is required to feed the structural analysis and clearance processes, plus to address some code requirements directly. The maximum tension condition will be one of many weather conditions that a tool is expected to evaluate. It should not be assumed to be the worst storm condition as sometimes low temperature conditions can generate even more tension.

As a guide, the following conditions would be expected to generate calculation results:

  • Maximum storm conditions.
  • Storm conditions at the freezing point and with no wind. This is an ice loading only scenario.
  • Various temperatures from the lowest temperature expected up to the thermal limit required in the code.
  • Other weather conditions dictated by the structure owner.
  • Non-linear load and creep material characteristics (extremely rare not to be present).

Level Spans

The software tool should be evaluated to see how it can address a wide range of wire attachment scenarios to provide clarity to its abilities. For Joint Use Distribution Pole Lines, the following variations are suggested:

  • Wire attachments of different types (single material type and double material type), such as:
    • Aluminum wires
    • Copper wires
    • Steel wires
    • Combined Steel & Aluminum wires
  • Various sizes of wires:
    • small,
    • medium,
    • large
  • Various span lengths
    • 40 meters
    • 80 meters
    • 120 meters

The following wires were chosen for this evaluation:

sag wire types

All of the above variations should be tested under level span conditions with wind (if applicable) applied at 90 degrees to the span. Both the maximum tension and the maximum sag should be extracted from these results and compared to the baseline. It should be obvious in the results how to determine:

  • The Horizontal guy-able load
  • The Vertical load
  • The Lateral load due to wind (if any).
  • The Tension at each pole

 

Un-Level Spans

unlevel spans

Many situations exist where the Groundline elevation of the structures or the wire attachments height of attachment are different. This creates a situation where the maximum sag point is no longer exactly mid-span and the tensions at both ends become different. Since many code clauses have requirements for mid-span clearances and separations, the possibility that the attachment of a wire is higher at one end than the other should be taken into account for the least error. If a software tool cannot address un-level spans, the supplier should note this in the compliance matrix.


The ability of a software tool to handle un-level spans can be demonstrated with fewer tests than required for basic Sags & Tension calculations. The following tests are suggested:

  1. One medium sized wire from each of the Level Span categories and using the same stringing condition.
  2. 60 meter span, CSA Heavy Loading condition
  3. Attachment Point elevation differences:
    1. One meter
    2. Three meters
  4. Reporting of the following results:
    1. Maximum Horizontal Tension under CSA Heavy Loading
    2. Maximum Vertical Load on both supporting poles; under any condition
    3. Maximum Vertical Sag value and the distance from the lowest pole where this occurs; under 100 °C Thermal conditions