AS/NZS7000:2016 Theory
This section will outline the process documented in the Australian standards AS/NZS7000:2016 and AS/NZS1170.2.
Region Wind Speed
The first step to in a design is to decide on a regional wind speed. The regional wind speed is a function of three things, the line security level (measure of line reliability), the line design life and the geographic location of a line.
The combination of the line security level and the design life will give a wind return period of the line as per Table 6.1 of AS/NZS7000:2016 as reproduced below. The line security is often a reflection of the importance of the line. The higher the security level the more reliable the line will be. For example a distribution line will likely have a lower security level than a transmission line.
| Minimum design return period - all wind regions | |||
| Design working life | Line security level | ||
| Level I | Level II | Level III | |
| Temporary construction and construction equipment, e.g. hurdles and temporary line diversions with design life of less than 6 months | 5 | 10 | 20 |
| <10 years | 10 | 20 | 40 |
| 25 years | 25 | 50 | 100 |
| 50 years | 50 | 100 | 200 |
| 100 years | 100 | 200 | 400 |
Table 6.1 AS/NZS7000:2016
The return period along with the wind region gives the regional wind speed by using Table 3.1 AS/NZS1170.2:2011. For example the regional wind speed would be 41m/s for a line security of Level II, design life of 50 years in region A.
As a note the regional wind speed might not apply for an entire line. For example there might be a difficult to access section of the line where a higher line security level is selected and hence a higher design wind pressure applied.
Site Specific Wind Speed
Once the regional wind speed is selected then the site specific wind speed is calculated for each structure using the formula:
Note many of these terms are defined in AS/NZS1170.2.
Typically the wind direction multiplier and the shielding multiplier are set to 1. Also the topographic multiplier is often only taken into account in special circumstances. M(z,cat) is only wind speed multiplier implemented in Neara. Click on the link for more details about M(z,cat).
Wind Pressure on Conductors
The last step in the calculation is applying the wind pressure to the conductors and the structure. The longer the span length the more inertia the conductors have and hence the length of conductor used in the calculations can change. This is knows as the Span Reduction Factor (SRF) and Tension Section Reduction Factor (TSRF). Click on the link for more details about SRF/TSRF.