Addendum 4 3.11.2 Confusion 1

[FTank]

Has anybody figured out why the 2nd criteria formula has to be satified? Once you have satified 1st criteria (formula) what's the point in calculating the liquid moment.
Example: 20' Dia. x 40' Shell height Tank. Design Pressure: 6" H20; Wind Velocity: 81 mph; Shell Weight: 20,506#; Roof Weight: 3421#; Total Dead Load Weight: 23,926#; Following are the results of my calcs.:
Mw= 85812 (roof horiz.)+ 131111 (shell vert)= 216,923ftlb
MPi= 97930 ftlb; MDL = 239,265 ftlb.
0.6Mw+MPi = 228,084 ftlb which is > MDL/1.5 = 159,510 ftlb
Therefore anchorage required.
Now if I add 10,287# of shell & roof weight, Total Dead Load Weight becomes 34,212# & MDL becomes 342,135 ftlb
MDL/1.5 = 228,090 which is > 228,084 ftlb. Therefore anchorage is not required by Criteria 1 Formula.
Question: why should we have to satisfy Criteria 2 which involves liquid moment? We've already satified 1st Criteria by adding extra tank weight.
Api 650's nomenclature for Criteria 2 is ambiguous & full of errors as a previous contributor got API 650 person to admit (ref. e-mail John Walters to Gordon Robertson Aug. 16/06 in which Robertson admits 4 errors).
The closest I could come to satisfying Criteria 2 (after reverse calculating & solving for MF) is that an additional 15,163# of shell and/or roof weight will be required.
Question: what purpose does this aditional weight serve? I'm obviously missing something here.
Suggestion: we should ignore Addendum 4 and continue using Addendum 3 until API 650 gets their house in order. Why should we penalize the oil industry by using a standard that increases the cost of the storage tanks either through additional steel cost or costly anchor chairs that were not required before given the same design perameters?
Observation: Addendum 3 allows "shell weight available to resist uplift, less any corrosion allowance, plus dead weight supported by shell.....". We always took this to mean corroded shell weight + full roof weight + rafter weight + nozzles, manways & attached appurtenances to shell & roof.
Now addendum 4 only allows "weight of shell and roof supported by shell". Are we all interpreting this in the same way?

Sorry for so many questions, but could somebody please -

Help!!

 

[JoeTank]

API has now chosen to account for the wind uplift on the roof, which can be a huge contributer to uplift at the base of the shell. In addition, they have clarified that the only tank dead load that can be used to counteract overturning is the load that is on the shell. Therefore, only the PORTION of the roof that is supported by the shell becomes effective in reducing overturning effects. I suggest that you wait for the errata to come out. Hopefully that will clarify things. Regardless of how API corrects the editorial errors, there will now be many more anchored tanks due to the inclusion of the wind uplift effects of the tank roof.

Joe Tank

 

[IFRs]

Note - there is an upper limit of the uplift pressure so for many tanks the need for anchors will be reduced.

 

[JStephen]

If you'll notice, Equation 1 only includes 6/10 of the wind moment, not the full design moment. The intent is that under normal operating conditions, the dead load itself is adequate for all uplift resistance with a safety factor of 1.5. But when full wind load is applied, it is recognized that most tanks are not maintained completely empty and pressurized, and allows for a part of the average liquid contents to be included for that check.

 

[JoeTank]

IFRs,
Although there is a limit to the equivalent moment due to internal pressure, there overall overturning will still be high per the latest-greatest criteria. Tank owners are reporting that more new tank are requiring anchors than in previous editions.
JStephens,
All API agenda items have pretty good tracking systems that maintain copies of support papers and basis for the changes. You may be able to get a copy from API that explains in detail the new load combination philosophy used for develoment of these changes.

Joe Tank

 

[FTank]

Thank you Joe Tank and all of the other for your replies.

Question for Joe Tank - you mentioned that they (API)"clarified that the only tank dead load that can be used to counteract overturning is the load that is on the shell. Therefore, only the PORTION of the roof that is supported by the shell becomes effective in reducing overturning effects. I suggest that you wait for the errata to come out. Hopefully that will clarify things."

Question - how would I get a copy of API's clarification on the tank dead load interpretation?

Thanks

FTank

 

[JoeTank]

FTank
When API revamped the load combinations much of the load section was revised. While in the agenda item stage, the ballots always include a technical justification/explanation of the agenda item. This creates a traceable path for the changes. Since the documentation is retained for future reference it may be possible to sweet talk them out of a copy. I'm not sure if they would do that, but you could ask for the background papers for the the load combinations.
Regarding the clarification about roof dead load resisting the shell uplift, I think its always been somewhat unclear, so the issue was tweaked a bit to make it more clear. The logic is sound in that the roof plate and rafter loads that are supported by the interior columns can not be used to counteract the wind overturning effects.

Joe Tank