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Are Weight of Floor Plates not Considered?

Are Weight of Floor Plates not Considered?

Are Weight of Floor Plates not Considered?

Based on API 650 calculations from various fabricators, It would seem that the weight of the floor plates are not considered when calculating overturning moments (API 650, sect. 5.11) and Uplift loads (API 650 sect. 5.12). This is also the case for calculations performed in TANK (a program for API 650 tanks from the makers of PV Elite). Section 5.11 verbiage clearly does not require the inclusion of the weight of the floor plates, but section 5.12 is a little more vague, as the phrase "any dead load” could potentially be interpreted as including the floor plates. I don’t have near the experience of the for-mentioned sources, but it seems logical that the weight of the floor plates should be included in both cases.

That being said...
  • Is not including the weight of the floor plates the correct procedure?
  • If so, what would be the potential reasoning for excluding it?
A little guidance is needed.

RE: Are Weight of Floor Plates not Considered?

Weight of the floor plates is not generally included.
Reasoning (my opinion)- the code equations are set up for a large tank, where the tank would have to uplift a good distance before most of the floor plate was lifting up. It would make more sense to include the floor weight on a 10' diameter tank than on a 300' diameter tank for that reason.

In the wind and seismic equations for the product near the shell resisting uplift, that doesn't actually include the floor weight, but the uncertainty in that number is probably a good bit larger than the floor weight. So you may calculate 1,500 lbs per foot of contents resisting uplift, if you added in 3' of bottom, that'd be another 30 lb/ft, then there's +/- 40% uncertainty on that whole number, so not much point figuring the extra 30.

RE: Are Weight of Floor Plates not Considered?

Considering tank size makes sense. I would think that some percentage of the weight could be considered based on the diameter of the tank. Is there a trusted publication that gives general rules/guidelines on when to consider the floor plates based on size and thickness?

I follow your reasoning in your example regarding wind and seismic, but I am not clear on how one would determine uncertainty. Can you elaborate? When you say "uncertainty" do you mean one's confidence in the value or are you referring to a calculated value based on a written standards? Maybe I missed it, but I do not recall coming across any discussion of uncertainty in API 650.

RE: Are Weight of Floor Plates not Considered?

I pulled the 40% out of the air, the point is, that contents-resisting-uplift is a very approximate relationship.

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