The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Corrective factors for uncertainties in wind action calculation

Written By: Lexatus - Sep• 14•13

In the previous article we have talked about the Regional wind speed and we have seen as this value is function of an additional corrective factor for tropical cyclones regions C and D. This factor is included to allow for uncertainties in the prediction of ultimate design wind speeds for those regions.

Structural Engineering often deals with probabilities of occurrence and it is for this reason that we use such modified values. It has been noted that the frequency of Category 5 cyclone crossing in Region D in the years 1998 to 2002 is much greater than the predicted from the historic data on which the wind speeds in the Australian Standard are based. A good example for this matter is the Cyclone ‘Vance’ occurred in 1999.

Future studies on probabilities of occurrence could lead to more accurate predictions and thus to the cancellation of these factors. For example, the American Standards are based on simulations of several thousand years of cyclones for a given coastal site. In this way, reasonable predictions of future wind gust speeds for a given return period are obtained.

So, at the moment, we use to increase the Regional wind speed by 10% in region D and 5% in region C at Ultimate limit states. For all the remaining regions and for Serviceability limit states, the corrective factor is set to 1.0.

Note: in the 2011 version of the Australian Standard 1170.2, this factor is only applicable to structures with R ≥ 50 years.

For any issues or questions, you can contact the author at:

onorio@strutturista.com

Eng. Onorio Francesco Salvatore

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