Problem: I seem to have missed where the depth, width and density of the foundation ringwall is put in so that a "counterbalancing weight" (F7.5) can be calculated, and I am not getting Appendix F7 calculations. The following comments are based on the assumption there is no input, but if I am mistaken please re-direct me. Appendix F does not exclude anchored tanks, they are covered in F7, though the rules seem to match table 5-21 calculations for windload on anchored tanks (overlapping scope in API-650).

When I enter my anchored tank under 50 ft in diameter into tank, I get a negative Pmax per F4.2 even though F4.2 calculations only apply to self anchored tanks and should not even be calculated. Seems the only practical way to use TANK is to recursively try increasing design pressures, check the wind load calc per Table 5-21, and check the area of reinforcement for that pressure per appendix F, ignoring the Pmax calc per Appendix F and using the input design pressure as Pmax. The TANK report output looks wrong but the useful numbers can be picked out. I find I can not use the "attached structures" input in basic tank data as it is apparently applied to the tank, giving the appearance that the bolts are not needed.

Strictly speaking, it appears Appendix F7 uses the Design Pressure, Test Pressure and Pf failure pressure (F5) to calculate the required counterbalancing weight as if in designing a tank from scratch, but when working with an existing tank, it would be nice to calculate the Pf from the given counterbalancing weight. The program does not seem to report the REQUIRED counterbalancing weight nor give a place for inputting the dimensions of the ringwall to calculate said weight.

Bottom line, the F7 calculation instructions seem to agree with Table 5-21 calculations for anchored tanks. The Appendix F output section of tank should suppress F4.2 calculation and instead pick those values up, instead of reporting an incorrect negative Pmax from a paragraph that does not even apply.

One side question: API-650 put a limit on the anchor bolt pitch of 10 ft. Many older tanks do not satisfy this, making TANK hard to use in evaluation of existing tanks. It should give a warning rather than refuse to run at all. Can you tell me the specific concern that prompted the change? I assume it was the span of concrete ringwall that could be self supporting between two anchors, but I am curious about the official reasoning. Also, should "cover" applied to the ringwall (civil engineering speak for extra concrete thickness corrosion allowance for carbonation and acid soil) be subtracted from the counterbalancing weight where corroded shell thicknesses are also used? In some of our tower foundations, as much as 6 inches of "cover" has been specified by the Civil PE after soil sample evaluation.

TANK doesn't account for the ringwall weight. Appendix F is applied if you specify an input pressure. If any of the checks in this appendix fail, F.7 is attempted - by increasing the pressure with the fluid head and performing a re-calculation starting with the shell thicknesses.

Not sure I understand what you are saying, I will call separately. Can't do F7 without the counterbalancing weight.

Simply, the required bolt load reported in the wind load calcs multiplied by the number of bolts gets the required counter weight. The bolt load reported includes the safety factor of 3 required by F7 but does not include some of the overburdens on the inward protection of the ringwall as credits. F7 actually allows liquid inventory existing over that portion of the ringwall projecting inside the tank wall to count as hold down weight and that would be left on the table v.s. the Table 5-21b calcs for UPLIFT but it would be conservative to do these based on an empty tank.

The "frangibility pressure" is calculated considering the specified design pressure as part of 5-21b (last line in the table). The F4.2 Pmax counts the wall weight entirely for balancing pressure against uplift, which double dips the weight needed to resist wind moment

When evaluating an existing tank
1. Research the existing ringwall size and estimate the weight
2. Input the current or desired design pressure as part of Basic Data
3. Is the ringwall weight greater than design bolt load times number of bolts reported from wind load calc?
4. Is the minimum required Area of reinforcement less than the actual area of reinforcement? (recalc for different design pressure if no.)
5. Stop if yes

Increment the design pressure up and recalculate until maximum allowable design pressure is found if an uprate is being sought.