MDL is a moment about the shell-to-bottom joint from the weight of the shell and roof supported by the shell. Is this weight to be considered in corroded condition or new tank weight?
I agree with IFRs. I think this is also something that should be made more clear in the standard. And it's not clear to me if this was actually the intent of the writers, or just the result of poor wording or oversight.
See the definition of Dead Load in 5.2.1. Also note the usage of "nominal" (= "as-built") roof thickness in App. F, sections F.1.2, F.4.1, F.4.2, etc.
Note that when calculating uplift forces on anchor bolts, the corroded thicknesses are used.
May I, Just add slightly from a somewhat different perspective of corrosion.
Since Corrosion is most usually oxidation(i.e.oxygen is added and weight slightly increased per unit area;with the exception if it is peeled-off/ removed.)
Whereas usually it stays-on& should be witnessed clinging and associated on to the surfaces; therefore most probably this weight increase
And consequent possible momentum factor increase on this account is not considered greatly affecting from an overall perspective by the Standard related personnel.
Hope this gives some explanation to your query.I believe!
Thank you all for your input. For anchored tanks it is specifically corroded weight and hence I had a confusion. The same concept is also implied in appendix V. It is the uncorroded thickness unless the corrosion is uniform and all over.
In the wind stability calculations, the as-built thickness is used also, and there again, it is assuming that average thickness is not affected much, just localized thinning.
