WIND LOAD ON TANKS

[AlexRG]

Hi, we are designing a Storage Tank under API STD 650, but we dont have very clear how to calculate the wind moment as its described in 3.11. Anything would be apreciated.

Thanks

 

[JStephen]

I assume you are using the new Appendix 4. If not, get it, as it changes everything.

Consider an example:
Suppose you have 85 mph 3-second gust, with a tank that is 24' high, 40' diameter.

The lateral wind pressure on the tank shell is 18 PSF x (85/120)^2 = 9.03 PSF. The overturning moment due to that wind pressure is 9.03 PSF x 40' x 24' x 12' to centroid = 104,026 ft-lbs.

The lateral wind pressure on the projected roof area is neglected, per a note there in Addendum 4.

The uplift pressure on the roof is 30 PSF x (85/120)^2 = 15.05 PSF. The uplift force is 15.05 PSF x pi/4 x 40^2 = 18,915 lbs. The moment arm for that force is the tank radius, so the moment is 18,915 ft-lbs x 20' = 378,300 ft-lbs.

Then total overturning moment is 104,026 ft-lbs + 378,300 ft-lbs = 482,326 ft-lbs.

Does that help?

You can compensate for ladders, piping, or stairways in the projected area if you need to.

 

[AlexRG]

it helped a lot. thanks, and by the way, I got the addendum 4, and it changed everithing. thanks again.

 

[LeonEarle]

Gents, In JStephen's response, he is assuming you have a flat roof. Hence, the pressure is being applied to the area of a circle. For API 650, the pressure is to be applied to the horizontal projected area of the roof. See 3.2.1 f of API 650 Addenda 4. To calculate this, you need to know the rise of the roof. The projected area is calculated as Dh/2 where D is diameter,and h is rise to center.

 

[IFRs]

I thought the horizontal projected area of a cone roof of any slope would be the tank area. No?

 

[JStephen]

The horizontally projected area of a roof would be the tank floor area. But see Note 3 in 3.2.1, which says "Windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore they are not included in these pressures." So you don't figure a horizontal load on the roof itself. See Fig. 3-23 for a diagram of the loads.

 

[notnowiambusy]

One thing that I found helps with the uplift is 3.2.1.f.2 which puts an upper limit on the required uplift pressure of 1.6 times the maximum design pressure calculated using Appendix F. For large frangible roofs this reduces the uplift pressure considerably.