Wind Loading Question

[Lion06]

I have a question on wind loading. I have a case where I need to determine the actual wind speed that will cause a ballasted sign to topple. The sign is a small sign (about 1x2) with a weighted ballast base.

I'm approaching this from two fronts. First, to determine the actual ballast weight required to keep it from overturning in accordance with ASCE 7. So I'm calc'ing out the weight needed for the wind speeds and coefficients defined in ASCE 7, including Kz.

The second approach is to determine the max wind speed the sign can handle based on the maximum ballast weight.

In both cases, the pressure is being reduced significantly by Kz. So when I determine the force associated with a given wind speed, it's based on that wind speed 50' above the ground. Would you describe the wind speed as the wind speed at the height of the sign (about 15' off the ground) or the design wind speed (50' above the ground)?

 

[bones206]

Maybe both? You can state the wind velocity at 50' and the wind velocity (reduced by Kz) @ 15'.

 

[JStephen]

Define "actual wind speed".
If that is the highest gust measured at the sign, use a Kz of 1.0. There may be some adjustment factor for shorter gusts than 3-seconds.
If that is the 3-second gust measured at the nearest weather station at an elevation of 10 meters/33' (I thought that was the standard), then use whatever Kz you calculate from the code.
Note that building codes are designed to give conservative wind loads, but not necessarily to give accuracy in the reverse direction.
For example, look at Cf for rounded members, and there's a limitation for D*sqrt(qz) <=2.5, where the wind loads jump 50% at that point. Obviously, actual wind loading is going to vary through that range, not jump 50% due to an extra micron on the diameter of the post.

 

[phamENG]

Also: why are you trying to find this? The end goal is key in determining the steps taken to get there.

 

[lexpatrie]

On what planet are you getting a wind speed 50 feet off the ground?

 

[Aesur]

You have your sign dimensions, you can easily calculate the pressure required to topple it based on ballast, overturning and sliding forces. Is sliding a concern, do you know friction coefficient? Usually I combine siding and overturning, but I have seen a lot of people refuse to do so resulting in less ballast. I could maybe see a time and place for there argument, ie doesn't matter if it slides for some reason and will be put back into place.

Once you have your max pressure, find your Cf for signs, find your basic wind pressure and then setup a spreadsheet with various heights and back calc your design wind speed at each height that gives you that wind pressure.

 

[Alistair_Heaton]

Lexpatrie most airports will give a local height profile time of day dependant from the runway threshold readings.

Some also have windshear masts to get true readings up to 200ft. But that's only at airports with special issues.

 

[ThomasH]

I would start with the wind velocity and ultimatly the wind pressure profile, wind pessure as function of height. I don't remember the details in US code but in the Eurocode this is fairly straightforward. If you need to include the dynamic properties for the sign it will complicate things but not to much.

But when you have the pressure profile you can calculate the overturning moment by integration, pressure(z) * z, pressure at height z multiplied by height.

What I described is just the basic principle. I skipped shape factors and things like that.

In Europe wind speed is usually measured 10 m above ground. But if you know the terraing category it can be calculated at other levels.

 

[Lion06]

Bones - I like the idea of giving both. That will also allow for a little commentary.

JStephen - Agreed on the 33'. My bad on the reference to 50'. I will just stick with the 3 second gust for now. Do you know of any good literature that discusses factors for gusts shorter than 3 seconds?

Pham - I'm checking a ballasted base for a small sign support. I wanted to give an actual wind speed (assumed to act at the sign, not 33' in the air).

Lexpatrie - My bad on stating 50' instead of 33', but in any event it's well above ground level.