The Span Reduction Factor (SRF) is a concept outlined in AS/NZ7000:2016 appendix B. The SRF effectively reduces the span length that the wind pressure is applied to when calculating the loading on a specific structure. The concept behind the SRF is the wind gusts will not be uniform across the length of the span and the conductors also have inertia. In PLS-CADD this is referred to as the Gust Response Factor. The SRF will effectively reduce the wind span of a structure

See Wind and Weight Spans for more detail.

Likewise the Tension Section Reduction Factor (TSRF) reduces the wind pressure applied to the conductors over an entire strain section thereby reducing the conductor tension.

In summary the SRF is used when calculating the loads on a particular structure (due to wind on the conductors) and the TSRF is used when calculating the tension within the strain section (which will also impact on the structure loading if it is an angle structure or termination structure).

There are two different types of wind events, synoptic and downdraft. Synoptic can be thought of as a regular wind event with the wind parallel to the ground. A downdraft, sometimes referred to as a microburst is where the wind is directed straight at the ground. Downdrafts can often be more destructive that synoptic winds. The SRF and TSRF is defined differently for synoptic and downdraft wind events.

The first step in Neara is to set up the environments (load cases) with the Gust Factor as either Synoptic or Downdraft. Note if None is entered for the wind type then the SRF/TSRF will not be applied for that environment.

Note the column to the right, the Height Factor controls M(z,cat).

The second step is to click on the tension section and in the properties pane select the option Use SRF and TSRF.

Note in the Project -> Design Settings there is a setting where you can control if new conductors have SRF and TSRF enabled or not.

To view the SRF in the model build a custom report with Assemblies -> SectionAttachments (For help creating custom reports see How To Create Custom Reports). Note this is only calculated if the Use SRF and TSRF check box is ticked for the tension section. Note this is only calculated if the Use SRF and TSRF check box is ticked for the tension section.

Then add the fields synoptic_srf and downdraft_srf.

To view the calculated TSRF set up a custom report with StrainSections (For help creating custom reports see How To Create Custom Reports). Note this is only calculated if the Use SRF and TSRF check box is ticked for the tension section.

Next add a field using the + button on any column to insert a new column.

And select the fields synoptic_tsrf and add another field with downdraft_tsrf.

These fields will now be in the report and the values will change as the model is updated.

AS/NZ7000:2016 Appendix B defines the SRF and TSRF for synoptic and downdraft wind as following:

Below is a plot of of the SRF and TSRF for both synoptic and downdraft wind conditions.