Sags & Tensions of Span Attachments

Another scenario you could run across is a wire or cable that is obviously strung to far less tension than normal. There can be specific design requirements for the pole line that require this, such as poles with no room for proper anchoring. In other cases you may visually notice some obviously slack wires or cables that should be re-tensioned. They could have been subjected to loads that exceeded what they were designed to expect (weather events, traffic accidents…). The pole line may also have some corner and deadend anchors that have pulled somewhat and may in fact be under-sized for the type of soil. In any event, under-tensioned wires and cables that could impact clearances must be addressed through re-tensioning and consideration of the existing anchors.

Slack Span Construction is a technique that can prove useful in dealing with difficult situations, as a last resort. It is prudent for the designer to consider all other options first as pole lines under full tension provide more reliability and are easier to maintain and build.

The good thing about Slack Spans is that you can always assume that the wire or cable is in a "new material" state, regardless of when it was initially constructed. This is because the everyday tension is very low on the Initial Curve and even heavy loads will not create very high stresses on this curve. If everyday tensions are low, long term creep will not be significant. If the wire or cable does not see high stresses, it will fundamentally only operate within the purely elastic area of the Initial Curve. This means that so little permanent stretch from either creep or plastic stretch will have occurred, that the material can be observed in its current state (sag/tension) and assumed to be "new material" for calculation purposes.

To capture the "starting condition" for slack spans, the best way is to estimate the existing sag of the wire or cable. Knowing this value, the span length, the temperature and the relative attachment heights will accurately define the tension (which defines the stress). It can also be advantageous if you have pole survey crews capturing similar data in the area as no other special tools are necessary. You can also directly measure the tension in the wire or cable, however this is usually more time consuming to coordinate and costly. You can also directly measure the tension on existing head-guys, in some situations, and iteratively adjust the tension in a whole-pole loading analysis until you reach values that appear reasonable for the same temperature. Some organizations also having been using some "rules of thumb" which could be very close, or close enough, to the appropriate values to use. For instance, for spans up to 20 meters, an everyday tension value of 100 Newtons (20 lbs.) might be used.