Include Liquid in Overturning?

[meca]

I have a question regarding tank design per API-650. I have a small (30 ft OAH x 12.5 ft OD) tank that is shop fabricated. The tank is in a very high wind zone, and I am verifying that the tank is adequate for overturning. My question is whether you must consider the tank to be empty (no liquid) when calculting overturning? If there is a minimum liquid level in the tank, is it acceptable to consider that this liquid is present when determining the overturning?

 

[MJCronin]

meca....

Yes, consider an empty tank coincident with a wind gust.

The calculation that you are making, of course, determines the need for anchor bolts

As a general rule of thumb, IMHO, I would require anchor bolts on a tank whenever the shell height exceeds the diameter.

-MJC

 

[IFRs]

API 650 section 5.11 is pretty clear how to analyze for wind, when to include the liquid inside and how much liquid to assume.

 

[meca]

IFRs,

I'm looking at API-650 (11th Edition - June 2007) Para 5.11, and I don't see anything about how much liquid to assume for an anchored tank?

 

[IFRs]

Section 5.11.2 specifies the liquid height in the MF calculation where you check the stability of an unanchored tank ( 1/2 height ).

 

[meca]

I saw that, but I have an anchored tank. Is there any direciton in the code for an anchored tank?

 

[IFRs]

I use 5.11 to determine if a tank needs to be anchored and then if the answer is yes, use the loads thus calculated to determine the load in the anchor bolts. I don't see why one would recalculate the loads in the anchors once you determine that anchors are required (and therefore know the loads).

 

[JStephen]

If it's already given that the tank is anchored, you don't need to check overturning.

Typically, overturning stability of a foundation would not include any allowance for product- it would be based on empty weight.

API added the uplift on the tank roof, which tremendously increases the overturning of many tanks. At the same time, they added the allowance for product, which counteracts much of the increase.

Note that if you calculate the overturning moment about the edge of the tank per API-650, this is not the same as the overturning about the centerline of the tank that would be used in Mc/I-type calculations for bearing.

 

[IFRs]

Personally, I would still check the anchors to make sure they are strong and numerous enough and calculate the force per anchor using un-anchored tank analysis.