A 90mph gust windefinition is last how many seconds?

[StructLin]

I got a customer enquiring about a 90mph (or 100mph) gust definition. His question is how long this 90mph gust last will be defined as 90mph wind. 3s? or 5s?, can anybody help me to answer this question? Thanks

 

[Lion06]

The maps are for 3-second gusts, right?

 

[jnadeau]

ASCE 7 provides a 3-second gust wind speed at an elevation of 33 ft for exposure category C. ...When your talking about wind speed it is important to know the averaging time (which you are inquiring about) as well as the elevation of the wind speed so that you can modify the wind velocity for different averaging periods, elevations, and exposures.

 

[StructLin]

So how do we rating a wind which stays at 90 mph for a long time (>3s)?

 

[StructLin]

I think a wind with 90mph last for several hours will cause much larger damage than a wind of 90mph just last for 3s. Are they still have the same rating?

 

[JStephen]

As I understand it, the 3 second interval is the interval of measurement used, and doesn't imply anything about the duration of the expected wind.

If the wind was sustained at 90 mph, then I suppose due to natural variations in the turbulent flow, you would actually have 3-second gusts that were higher than 90 mph, so it wouldn't be a 3-sec-90 mph wind anymore. And if you had a 90 mph-3 sec wind condition, you might find higher speed gusts that had shorter durations than 3 seconds.

 

[JStephen]

See Fig. C6-4 in the commentary to ASCE 7-05, which shows maximum speed variation based on the length of measurement.

 

[csd72]

Actual wind is not a stable pressure but is turbulent in nature. Thus you will never get a constant pressure at a certain velocity without getting short bursts of higher velocity.

If the 3s gust is 90mph then the 1s gust would be larger possibly 100mph and the 5s gust would be lower say 80mph. As the code is based on the 3s gust this is the speed you should use in your calculations.

A lot of statistical analysis of wind tests goes into the factors that you find in the codes.

 

[transmissiontowers]

Back in the days of Fastest Mile wind that we used for transmission towers, we used to use a 1.3 factor to compute the gust speed so for your 90 mph wind gust the Fastest Mile wind would be 69.2 mph.

Let me then say, there is only steady wind in the lab and it is not found in the real world. The ASCE wind maps present a standard that we can design for. They measured winds at points continuously during storms then chopped the data into 3 second time periods and took the average over those 3 second periods and published the highest average as a map of contour lines. The map is for a 50 year MRI storm. You can choose factors to convert the return period to different times.

These aforementioned measurements were for continental winds away from hurricane zones. I believe the hurricane zones used Monte Carlo simulations to obtain the 50 year MRI wind speeds because we only get 1 hurricane at a point on the coast every few years. This is why Exposure D is not appropriate for coastal areas in hurricane zones which should use C.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[asixth]

The wind speed given in AS/NZS1170.2 is for a 3 second gust. That isn't to say the 1000yr wind event (say 216kph or 135mph) lasts for 3 seconds, the 1000yr wind event may lasts for several hours, but the wind gusts at 216kph.