Dear All,
I hope you can help me to understand something more about my problem.
We are analyzing an Existing Tank,which Diameter is 10.43 meter and it has 6 Anchor Bolts.
The design pressure of my Tank is 2KPa.
My Tank has a fixed rafter supported cone roof.
Sismic analysis has not been performed because data are missing.
The results of my analysis say that we need 11 Anchor bolts.
But there is something that I'm missing.
I'll try to explain.
According to API 650 (my software edition is Tank 2012) par. 5.11 above 1.55 Kpa the Tank shall be anchored.
This means that above this pressure,I shall consider the Uplift loads in table 5.21 in par 5.12 because the Tank will be Anchored.
But,the output of the software says that, all the Uplift loads in this table are 0 except of the Frangibility pressure.
The problem is that our Tank has a fixed roof and not Frangible!
The software says so that,above 1.55 KPa we need Anchor bolts, and it says that we need 11 Anchor Bolts to satisfy the requirement of the maximum spacing of 3 meter between them.
Considering Appendix F for roof,the software says then that Max Design Pressure, limited by uplift.(KPa)is 2.0059.
My Questions are :

1)How can Understand if,above 1.55 Kpa,my existing number of Anchor Bolts(6),eventhought the distance between them is more than 3 meter,is enough to resist the loads?
(it says that I need 11 for the condition of spacing,but till which pressure 6 anchors could be enough?how can I get this information from the software?)

2) How can I say to the software that it shall not consider the frangibility pressure?

3)If in the table 5.21,all the uplift loads are 0 (due to the W1,W2,W3) and the only positive load is the frangibility pressure,that we shall not consider, which one is the load that gives me the necessity of anchor bolts?

4) which is the relationship between the pressure calculated in appendix F (maximum design pressure limited by uplift) and the uplift loads in table 5.21?

Please try to answer clearly to the questions,
Thanks all for your support!!!!

You referred to 5.11 where checking the overturning stability due to wind and internal pressure requires satisfying two conditions.

I don’t have your details, but I can speculated that you are in case when the internal pressure is controlling the stability, I guess the first check fails (i.e. 0.6Mw + MPi < MDL /1.5 + MDLR) and consequently anchorage is required.

However, the criterion for Design Pressure + Wind in Table 5-21 is [(0.4P + PWR – 0.08th) × D2 × 785] + [4 MWH/D] – W1 which, with the same input data, may be negative, so anchorage appears to be not required.

IMO there is a contradiction in API: by one hand they have one criterion where full design pressure has been considered, by the other hand they have a table with a criterion where only 0.4P or Fp*P has been used.

If this is your case, I haven’t an "API solution"; it is my opinion that you have to consider the most conservative condition (5.11) maintaining anchorage; in the same time you cannot calculate the anchorage based on uplift so my suggestion is to apply the minimum anchor size and minimum spacing required by API 650/ 5.12.

Also IMO, you try to force N=6 for your case but you cannot say you remain under API 650 provisions. Complying with API 650 means to assure the minimum spacing required.

Best regards.

I should correct "minimum spacing" in the previous post; actually it is "maximum bolt spacing".

I would add that in Addendum 3 of API 650, the expression for "the maximum design pressure limited by uplift" given in F.4.2 is mathematically equivalent with 5.11.2 condition (1)- i.e 0.6Mw + MPi < MDL /1.5 + MDLR.

I'm not sure this is true for previous versions of API 650 (I do not intend to check this) and I don't know which version of API650 implements your Tank 2012. I would suggest you a manual checking vs. Addendum 3 formulas to confirm the values of 1.55 kPa and 2.0059 kPa.

You must also understand that the level of details you've given cannot assure a "diagnostic" of what it is in your calculation. In fact I tried to guess a possible explanation.

Best regards

Dear Mariog,

you really show a deep knowledge of the API and that's really helpful for me!
Your answer was clear, the contradiction inside Api was a possibility that we were thinking about.

So,as you say,We won't be able "to force " the software to verify the tank with 6 Anchor Bolts,but that was something that I was pretty sure,you have a software based on Api,and Api must be respected!

Thank you for all,
I will write again for other doubts!

Best Regards