Addendum 4 Wind Uplift on the Roof

[TankDude]

I know this has been visited before in this forum, but has anyone learned anything new regarding the method for adding wind uplift to the overturning moment per Add. 4? It has appeared to me from the beginning that this value needed to be reduced (along the lines used for geodomes), however it seems that API is sticking to this method. This greatly increases the overturning moment in most practical situations. Another issue of confusion is whether or not this uplift value is to be factored into the roof design. I would certainly hope not. Any feedback would be appreciated.

 

[JStephen]

I don't know anything about committee activities on the subject. In non-coastal areas, the uplift hasn't been a big problem. I assume that a tank roof does not have to be designed as a pressurized roof simply because of the wind uplift, as the standard also limits wind uplift based on failure pressure.

There are a number of inconsistencies still in the uplift design- especially when looking at wind and seismic on an App. F tank.

 

[notnowiambusy]

I have a general concern with the new wind load and overturning criteria. The first equation given in section 3.11.2 can be rearranged to show that

0.9 Mw < MDL

is acceptable (assuming there's no internal pressure).

That looks to me like a factor of safety less than 1.0

 

[TankDude]

notnowiambusy,

I am in agreement with you that the way it is written in the standard would actually serve to reduce the safety factor. I am wondering if the 0.6 reduction factor wasn't added in error. Without it, the reduction of the resisting moment would serve as a sufficient safety factor. It's funny because my colleague and I feel that there should be a 0.6 reduction factor placed on the calculated wind uplift area. It just seems like overkill to use the entire horizontal roof area.

 

[JStephen]

My take on it is that they felt that the loads as stated were reasonable (or necessary for some outside reason) but at the same time recognized that there weren't that many failures, either. So they increased the moment such that it is now several times what it was previously, but also adjusted the way the design check is handled to try to keep things reasonable.

One consequence is that the equations show that a tall tank can be more stable than a low tank of the same diameter, which just seems wrong.

If I remember correctly, the anchor bolt sizing equations are written so that they don't correspond with the new way the moment is calculated. For sizing anchor bolts, the M should be only the lateral force moment, with the vertical force deducted from the weight.