Tank overturning design criteria.

[SJEC]

I am working on a tank which is required non-anchorage design due to civil reason.
the tank is equipped with supported cone roof ( roof is supported by structure column from tank floor and beam).

The 12th edition API 650 is giving two criteria for us to choose.

Unanchored tanks shall meet the requirements of 5.11.2.1 or 5.11.2.2.

5.11.2.1 Unanchored tanks shall satisfy all of the following uplift criteria:
1) 0.6Mw + MPi < MDL /1.5 + MDLR
2) Mw + Fp(MPi) < (MDL + MF)/2 + MDLR
3) Mws + Fp (MPi) < MDL /1.5 + MDLR

5.11.2.2 Unanchored tanks with supported cone roofs meeting the requirements of 5.10.4 shall satisfy the following
criteria:
Mws + Fp (MPi ) < MDL /1.5 + MDLR

My question is if I only need consider 5.11.2.2?
why first two criteria in 5.11.2.1 is not applicable?

Since it is none-anchorage design, the required plate thickness is huge different due to overturning check.

Thanks

 

[SJEC]

some background info from API meeting about why and how the change happen in 12th version API 650.

1) Open top tanks are subjected to no roof wind uplift. Under current rules, controlling formulas see 60% of the design horizontal wind in the empty condition. This results in overturning that is un-conservative compared to historical values.

A similar condition occurs on tanks that are much taller than the diameter. The roof uplift due to relative small area is insignificant compared to the horizontal wind on these tanks large H/D tanks. The 60% wind again is un-conservative compared to historical values. It is possible under current rules for an 80 ft tall x 40 ft dia tank subjected to typical wind loading to not require anchorage.

The historical 0.6605 overturning factor limit for unanchored tanks was established for full wind with the tank empty. This value limited uplift of the tank resulting in high compression loads and distortion of the shell. The current limits allow significantly greater tank uplift for these H/D ratio tanks.

2) The API 650 addendum 3 changes failed to eliminate the roof weight from definition of MDL. Correct for addition of term MDLR. This is corrected below.

3) An option has been added to design low cone roof overturning stability using the previous method of applying horizontal wind pressures only (no vertical uplift).

I do not understand why low cone roof is no vertical uplift? and what is criteria for low?

 

[Duwe6]

API, like ASME is experience driven. Apparently, there have been no problems related to roof venturi uplift. Don't overthink the rules - they have been proven to work.

"If it looks stupid and it works well, its not stupid"