Used TANK 2015, API 653, tank dia 7.5',height 18', self supt dome roof, Mat'l SB-127, .1875" thk shell & roof, .5" btm plate, 4 anchor bolts. TANK calls for bolt dia 3.5". This seems too large for such a small tank. Any help would be much appreciated.

Hello Sam Tank

Yes, we know about the problem. The uplift force is defined in Table 5.2a/b of API 650. The frangibility pressure is a problem which forces the programme to require large bolts. In the TANK software that is going to be released in 2017, the frangibility requirement has been removed, so the bolting check will give more reasonable bolt sizes.

Hi SamTank,

If possible please submit a Support Request with the input file.

Mr.Delaforce,

I don't think API has a problem only with "frangibility pressure" but also with "1.5*pf" criterion when the tank is enough internal pressurized.

By note a. in table Table 5.21, this case is valid for special cases: "Failure pressure applies to tanks falling under F.1.3 only. The failure pressure shall be calculated using nominal thicknesses."

F.1.3 stipulates that "F.1.3 Internal pressures that exceed the nominal weight of the shell, roof, and framing but do not exceed 18 kPa (21/2 lbf/in.2) gauge when the shell is anchored to a counterbalancing weight, such as a concrete ringwall, are covered in F.2 and F.7."

In my understanding, F.1.3 refers to tanks having uplift in operational and/or design pressure. By API approach of frangibility, a frangible tank has the have no uplift in case of roof failure. It appears that a tank which has uplift in operational/design pressure (before roof failure) cannot be frangible, because the tank shall be certainly subject to uplift at roof failure pressure.
I would conclude that F.1.3 refers to non-frangible tanks, however the current interpretation is that the uplift case with 1.5×Pf refers to frangible tanks, isn't it?

And second issue: it is hard to explain which is the meaning of the uplift case with 1.5 × Pf.
I expect to have roof failure at pf, but still I can apply 1.5×Pf as internal pressure? That means to apply a 50% overdesign to a failure event...

Hello mariog

The original question was why the bolts had to be so big. I showed the reason, and provided the solution. Those other issues you mentioned were not really part of the original question.

However, thank you for your input.

Thank you for the reply.
If I understand well, the solution was to eliminate the frangibility requirement for tanks other than those designed to 5.10.2.6 d
i.e. removing
Uplift criteria Frangibility Pressure [(3 × Pf – 0.08th) × D2 × 785] – W3
in SI units.

Do you remove as well:
Uplift criteria Failure Pressure [(1.5 × Pf – 0.08th) × D2 × 785] – W3 ?
or rather do you interpret this criterion as in package with "frangibility"?

Best regards.

Hello mariog

No, we retain the requirement for the 1.5xPf condition.

OK, just be informed that the "failure pressure" criterion for uplift is present in the Addendum 1 of API 650 12th edition but disappeared in the Addendum 2.

Please see the files attached reproducing Table 5.21 Uplift loads in the two referred addenda.

Hello mariog

I feel satisfied that I have addressed the original enquiry from Samtank. On the forum it is best to stick to the issues raised. However, If you wish to raise ancillary issues, you are of course free to do so by opening a new thread.

I guess I was not able to explain properly my point, that's way it appears as ancillary.
What Sam remarked is that "This [bolt size] seems too large for such a small tank.".
For long time, Table 5.21 imposed considerable uplift giving bolts' and foundation oversizing vs. common sense. In case the tank has a frangible roof, that is- the criterion "3*pf" is what you need to consider.
In case the tank is not designed by API 650/5.10.2.6 d., of course the uplift associated to frangibility must be removed- as you said you decided to proceed in 2017 edn. I would remark that your competitor- former e-tank- made this criterion as "non-applicable" for non-frangible long time ago. In fact, today it is the same note as in the edition 11th of API 650 "Frangibility pressure applies only to tanks designed to 5.10.2.6 d", that's why an ancillary discussion would be which is the factor that made you to remove "3*pf" frangibility criteria just now.
What is new is that also criterion "1.5*pf" is withdrawn. This criterion was also a source of oversizing bolts and foundation. Since you said you plan to keep it, I just hope shall be removed as soon as possible.