Pole Deflection

When a flexible object like a power pole has a load on it, it bends. This is commonly called deflection. The issues for power poles include:

  • a pole that is bending unduly may alarm the public
  • the increased sag could cause a clearance problem

Because a pole is a flexible object the bending itself is not a problem, as long as the stress does not go beyond the pole’s limit of elasticity (viz. the tipload does not exceed the pole’s strength). Even concrete and steel poles are flexible in this way.

Let’s look at a realistic example of a termination pole with an 80m span of Moon.

Pole PropertiesConductor Properties
ClassS2Conductors3 x Moon
Modulus of elasticity18500MPaMass0.34 kg/m
Height of pole tip10.7mDiameter14.3mm
Average diameter250mmSpan80m
Stringing tension (5%)0.94kN

Using typical limit state load factors the everyday load on the pole is 3.1 kN and the maximum wind load is 11.8 kN. These are within the nominal sustained and short-term strengths of the pole, 7.6 kN and 14.1 kN.

Using a linear1 deflection calculation method the everyday load causes the tip to move about 130mm towards the span. When the conductor is initially strung this deflection can be compensated for by raking (leaning) the pole away from the span, leaving the pole vertical once the conductors are tensioned and tied off.

If the pole was subjected to the maximum load of 11.8kN the deflection would increase by about 350mm to 490mm. If the maximum load was a no wind load case (eg caused by ice), the increased deflection would cause a substantial increase in sag—an 80m span with a sag of 2.83m means the conductor is 80.27m in length2. If the span length decreases by 0.35m (due to pole deflection) but the conductor length remains 80.27m the sag increases to 4.29m, which would probably create clearance problems.

However in our example the increased load is caused by 900Pa wind perpendicular to the conductors. Figure 1 shows that for 900Pa wind the conductor blows up to about 75º from vertical and the vertical component of the conductor position midspan decreases to about 1.0m. So the increase in sag due to the increased deflection does not create any clearance issues. As this load condition is by definition a short-term load the pole and conductors should return to their everyday position after the weather has settled.

Figure 1. Blowout and Sag
  1. There are linear and non-linear methods to calculate deflection of a pole due to load. For the loads a distribution pole is normally subjected to the linear method is more appropriate.
  2. Equations for these calculations are available in Appendix R of AS/NZS7000:2016