Deduct corrosion allowance when comparing seismic OTM to overturning ratio? 1

[Soln]

I don't beleive API 650/AWWA D100 requires the deduction of corrosion allowance when calculating the overturning moment and overturning ratio of ground supported tanks. However, I've seen seen designs in which the CA is deducted from the tank material weight, resulting in the requirement of anchorage. As a tank fabricator I would prefer to use the "as purchased" material weights (corrosion allowance included) to avoid adding cost to the project with anchorage if the code allows. What does the code intend?

 

[jtseng123]

We always ask fabricator to check both new w/o CA and hot w/ CA for seismic. Guess what ? You shall now know the answer.
No engineer is going to run any risk. Cost is not a concern.

 

[IFRs]

For wind loads, API uses the "nominal" shell weight which would not have the corrosion allowance deducted. Presumably this is because there is not expected to be uniform corrosion over the entire shell that would reduce the total tank weight. Corrosion is expected to be local, not global. For seismic where API does not add the word nominal and simply uses the shell weight, and I would use the nominal shell weight here also, for the same reason. If the owner feels that this is not appropriate and expects the entire tank or a major portion of it to be subject to uniform corrosion then the are free to require that the corroded shell weight be used in the seismic and/or wind stability calculations.

 

[JStephen]

"No engineer is going to run any risk. Cost is not a concern."- I've not seen any tank owners that felt that way- otherwise, everything would be built out of 1.50" stainless steel.

But see the definition of "Dead Load" in 5.2.1 of API-650, which includes corrosion allowance unless noted otherwise.

 

[jtseng123]

JStephen, funny that you twist on citing my comments.