Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations MintJulep on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Wind Speed Comparisons 2

Status
Not open for further replies.

Trenno

Structural
Feb 5, 2014
831
Something that was bugging me at 5pm this afternoon - getting an idea of the differences in wind speeds and wind loads when comparing Australia Vs Europe.

The problem arises in how basic wind velocities are calculated... so attempting to compare basic service winds at 10m height in open terrain...

V50 (1:50) 3 second gust wind in Melbourne is 39 m/s.

V50 (1:50) 10 minute average wind in London is 21.5 m/s.

I understand there are a few different methods to convert/compare these numbers. Is there a basic or conservative factor I can apply to compare them?

When I have time, I'll do a side by side calculation to see if differences come out in the region/shape/area coefficients.

 
Replies continue below

Recommended for you

apologies in advance, but 39 > 21.5 so critical ... unless you have different allowables for sudden (gust) loads compared to "steady state" winds.

another day in paradise, or is paradise one day closer ?
 
I think you're comparing apples and oranges.

For example, I'd like to know how to convert a 3 second gust value into a 10 minute average value.

 
Can't help directly, but NBC uses an hourly wind and a gust effect factor. The research referenced is "Gust Loading Factors" by Davenport, Journal of Structural Division, so that might be a start.
 
I'm assuming you need to design to airloading. A higher wind speed (from a gust) would create the critical load, unless you have different allowables for sudden loads. If you need a gust load and only have mean data, then there'd be some adjustment (as per Canwesteng).

another day in paradise, or is paradise one day closer ?
 
I might be misunderstanding, but it sounds like you're looking for the Durst curve -- a statistical correlation between wind speeds over different time frames.

It can be found in the commentary of ASCE 7, among other places.

----
The name is a long story -- just call me Lo.
 
From memory, the Coastal Engineering Manual has a conversion equation (probably in the section on wave forecasting/hindcasting).

Edit: Remembered I have a spreadsheet with this conversion formula in it. 3sec:39m/s is 10min:27m/s using the CEM relationship.
 
Thanks Lomarandil & Steve,

I can roughly back up Steve's numbers with V3 = (1.52 / 1.06) * 21.5 = 30.8 m/s.

Therefore London basic pressures are roughly 38% less that of Melbourne.

Example calculation using the Durst Curve below, if anyone is interested.

d1_yfy4sn.png


d2_gic9u9.png


d3_keg5uk.png
 
It's probably not strictly relevant to a comparison between Melbourne and London wind loads, but AS1170.2 appears to say its wind speeds are 0.2 second gusts (see notes to Table 3.1), not 3 seconds as it used to. The references to 3 seconds were removed in 2012 and replaced with 'peak' gusts. If that's the case, then 0.2s:39m/s corresponds to 10min:26.2m/s according to the Coastal Engineerig Manual formula.

I've also attached the CEM formula: more convenient than reading from a graph in a lot of cases; and goes beyond 1 hour durations with a supplementary formula. Fairly good agreement with Durst's curve.

Link to image below. I can't figure out how to display it within this post from engineering.com. Any hints appreciated.
Durst_vs_CEM_zaamv1.png


Link
 
steveh49 said:
Link to image below. I can't figure out how to display it within this post from engineering.com. Any hints appreciated.

From Eng-Tips.com (NOT engineering.com):

CapturePNGPLUS_dwsdvu.png


Resulting in:

Durst_vs_CEM_aivb0a.png
 
Thanks for the clarification, Steve. Didn't pick up on that amendment...

Revised numbers --- LDN pressure = 0.67*MLB or 33% less.

I couldn't edit the post above for some reason.

 
Status
Not open for further replies.

Part and Inventory Search

Sponsor