Wind Loads on Roof 9

[reslife]

Hello all,

I am mounting an antenna on a roof of a building, and I am trying to figure out the wind loads it will experience. Once I have these loads I can calculate the stresses on the bolts it is mounted on. Here is my question:

How will the angle of the roof (14 degrees) affect the wind load? I had planned to use 130 mph for the wind load, but I'd like to know how much the wind speed coming over the top of the roof will change.

Any help is appreciated!

 

[MiketheEngineer]

Unless you are in a hurricane or tornado - I think 130 mph would be adequate.

 

[Unotec]

Are you in Kansas? 130mph seems a little high to me.
The roof slope should not affect your wind load negatively. At best it will block wind from one side, at worst, the wind will hit the antenna head on.


<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[reslife]

The antenna will be in Tampa Bay, Fl. I'm not really coming up with a new design, but rather telling the customer whether it will hold or not.

I wasn't given too much information about wind loads in the area, so I figured I'd assume a hurricane rolled in and they didn't take the antenna down.

Im using this equation to find the force on the antenna dish: Drag = 0.5*rho*wind speed^2*Area of object*Drag Coefficient. Since this force is greater than the weight, I will use the drag force when finding my stresses in the mounting bolts.

I hope I'm doing this correctly. This is my first assignment out of college, so I'd like to get it right.

Thanks for the help!

 

[IRstuff]

Wouldn't there be compression of the air flowing upslope, hence, an amplification of the windspeed?

TTFN

FAQ731-376

 

[JoeTank]

I perform a lot of project reviews for antenna installations on water tank roofs. Normally the wind forces on antenna components are tabulated on the antenna manufacturer's equipment data sheet. I suggest that you obtain their data sheet for the unit that is installed. That eliminates the guess work on loads.

Joe Tank

 

[MiketheEngineer]

Check with your local code enforcement. I have seen 150 mph specified in the coastal areas. And as mentioned - check w/ the mfg.

 

[zcp]

ASCE 7 "Minimum design loads for buildings and other structures". Chapter 6 is wind loads. It has everything you need. For Tampa, you do appear to be in the 130 mph zone (Figure 6-1), but your force from the wind may have more to do with the surroundings and the building it is attached to than the wind speed.

ZCP

http://www.phoenix-engineer.com

 

[JStephen]

See 6.5.15.1 in ASCE 7, which specifies an increase for roof-mounted equipment and structures. (Pointed out to me by some other helpful person here on the forums).

 

[reslife]

I'm thankful for all the responses I've gotten to this post.

I looked up 6.5.15 in ASCE 7 and used the equation: F=qz*G*Cf*Af. This is giving me a value of about 435 lb, as opposed to me using the force of drag equation: 0.5*rho*wind speed^2*Area of object*Drag Coefficient, which gave me about 630 lb.

Which should I use in my stress calculations?

 

[IRstuff]

Doesn't that depend on how much margin you intend to have? Depending on the customer and the severity of the consequence of failure, design margins can range from 25% to 100% to 200%

TTFN

FAQ731-376

 

[JStephen]

Did you include the increase in 6.5.15.1? This can almost double the loading.

 

[reslife]

I took a look at 6.5.15.1, but it looks like it only applies to buildings under 60ft, while mine is about 75.

 

[zerosum]

Isn't the wind velocity given in ASCE 7 referenced to 10 meters and your antenna is up at around 25 M? Perhaps the equation F=qz*G*Cf*Af corrects for the increase in wind velocity with altitude; I don't currently have access to the ASCE code.

 

[swearingen]

The qz that you calculate must include the factor Kz which depends on the height - that's where the correction shows up.


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[zerosum]

Makes sense. Thanks, Swearingen.