Wind loading limit state factors in new Australian codes

[RiBeneke]

Hi. It has been so many months since I last participated here that I feel almost like a newcomer. I have been very overloaded with work lately.

I am designing small (maximum 5 metre high) steel mast structures for Australian conditions. The new Australian loading codes AS 1170 part 0 (general) and part 2 (wind) contain limit state factors that seem to give low wind forces. Can anyone tell me if I am interpreting them incorrectly, or do these codes, published June 2002, really give lower wind forces ?

The numbers are (AS 1170.2 section 2 etc)
Region A1 (non-cyclonic) with 3 sec gust wind speeds (50 year return period) Vr = 39 m/s.
Wind from any direction and structure with any orientation : Md = 1.00
Terrain category 2 and no nearby change of terrain category : Mz,cat = 0.91
No shielding : Ms = 1.00
No special topography (hill shapes or lee areas) Mt = 1.00

Then basic wind pressure p = 0.5 * 1.2 * (Vr * 0.91)^2 = 756 Pascal
to be multiplied by the appropriate pressure coefficients for the type of structure to give the wind force W.

For the ultimate limit state the overturning forces on the structure are calculated using factors
1.2 * G + Wu + 1.5 * Q (AS 1170.0 section 4 etc)

The puzzling part is that Wu is elsewhere confirmed = 1.00 * W (AS 1170.0 Commentary C4.2.2)
and therefore the ultimate limit state basic wind pressure is only 756 Pa.
This seems low by South African standards (which use Wu = 1.3 * W), but more importantly, it is also low in comparison with other calculations based on the older Australian codes of practice that were previously done for similar structures in similar locations.

Any ideas ? Thanks

 

[austim]

Ribeneke,

I am not in the best situation to offer help, since I don't have immediate access to the latest standards. What follows is based on my reading of draft standards 99309 and 99419 (precursors to the revised AS 1170.1 and 1170.2). It could all now be totally irrelevant.

First, the matter of 1.3*W or 1.0*W for ultimate wind. Even the previous standards told us to use 1.0*Wu. The point being that Wu was selected as an ultimate speed in the first place, and therefore needed no extra factor.

Second, are you sure that in using R=50 you have the correct return period wind ? That seems to boil down to a choice of structural importance level.

Table C1 (in the draft of 1170.1) seems to offer a good scope for differing judgements here. Either level IV ('normal structures') or V ('structures presenting a low degree of hazard to life and other properties') seem possible. These look to be directly comparable with values of Mi= 0.9 or 1.0 in the 1989 standard.

If I consider your poles to be 'normal structures - importance level IV', then Table 2.1 points me to level C design actions, for which Table 2.2 specifies a 500 year (R=500) ultimate wind, giving me 45 m/s in Region A, which is pretty comparable with the 1989 standard for Perth, Melbourne, Adelaide.

If I treat your poles as importance level V, then Table 2.1 says level E design actions for a 25 year design life, or level D for 50 years. That leads to R=100 or 200 years respectively.

I do hope that this helps.

 

[RiBeneke]

Thanks Austim. Your reply helps a great deal.

Thank you for confirming that Wu = 1.0 * W is correct.

I did not properly differentiate between design life, annual probability and what now appears to be the superseded concept of return period.

The new code AS 1170.0 refers to the Building Code of Australia (which I do not yet have) for importance levels in Australia, and only provides a table 3.1 of importance levels for New Zealand. After re-reading the commentary to this table, it is clear that for the South African concept of 50-year return period, one has to choose a 1/500 annual probability in order to achieve the same statistical reliability. Thus I had chosen a wind speed that was too low by some 13% .

Thanks again.