Overturning Stability API-650 12th Ed

[sdz]

Background: I'm actually designing water tanks in Australia but due to lack of relevant information I have turned to API-650 for some design criteria. API-650 uses Allowable strength design (ASD) while Australia used Limit state design (LSD). I'm trying to understand the API-650 provisions to formulate comparable LSD criteria for unanchored tanks.

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

Fp is the pressure combination factor, see 5.2.2; the ratio of normal operating pressure to design pressure, with a minimum value of 0.4. MPi = 0 in my case for a normally vented water tank.
Mf calculated for tank 1/2 full (5.11.2.3)
​

Now I think I can more or less understand the first two. These assume a low probability that maximum overturning effects will occur concurrent with minimum restraining effects; i.e.a completely empty tank.
1) corresponds to an empty tank with 60% of maximum design wind load plus maximum internal pressure; with a 1.5 safety factor applied to the shell dead load (but why not to the roof dead load?)
2) corresponds to full wind load with a partially empty tank plus plus normal operating pressure, 50% full; with a 2.0 safety factor applied to the shell and contents (but why not to the roof dead load?)
But what does 3) correspond to? Why is there a roof dead load but no roof wind load contributing to overturning?

 

[HTURKAK]

The tank will not fly due to internal pressure like balon but the risk is ovalisation of the base plate and uplift from the base . The uplift criteria stated at item 5.11.2.1 similar to Appendix F F.4.2 For self-anchored tanks, the maximum design pressure, limited by uplift at the base of the shell, shall not exceed the value calculated from the following equations as applicable unless further limited by F.4.3 ) The previous
ELEVENTH EDITION it was ( 0.6Mw + MPi < MDL /1.5 ) and MDL was = moment about the shell-to-bottom joint from the weight of the shell and roof supported by the shell.


It was ( Mw + 0.4MPi < (MDL + MF)/2 ) at 11th edition. I will suggest you to compare appendix F of both editions.
Pls notice that ( Figure 5.27—Overturning Check for Self-anchored Tanks ) The wind overturning moment Mw includes the effects of horizontal plus vertical wind pressure.


Because this combination does not include wind uplift. Refer to ASCE 7 wind load concept changes.


I would like to remind that the limit states of static equilibrium are independent of the strength of the material. ( e.g. the OT of retaining wall limit state of static equilibrium ( and provide FS against OT ) , but, the reinf. design of stem wall and ftg ULS governs.
Just for curious ; if you are looking for water tank , the internal pressure will be zero. what is the reason to look API 650 ?

My opinion only..


Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[JStephen]

The overturning criteria in the AWWA and API tank standards have been tweaked in each new addition, and can be reasonably be expected to change again.
ASCE 7 also has updated wind loading, and is not yet coordinated with the tank standards (or vice versa).
In the olden days, wind overturn consisted of a conservative estimate of horizontal wind forces (more conservative in AWWA) applied to a completely empty tank. The problem is that in reality, there is apparently wind uplift on the roof, which increases overturning, but there is also nearly always some amount of product in the tank, which makes the peak wind with completely empty condition rather more unlikely than would be expected. The second equation in API is trying to work with that effect. The third equation is the "old" overturning criteria.
I don't know the logic behind the 1.5 vs 1.0 safety factors on the roof and shell weights. A lot of this may just be committee politics more than actual engineering application.
Most tanks have ladders, stairs, handrails, platforms, vents, pipe risers, etc. and neither the tank standards nor ASCE 7 are at all clear how, or even if, those items should be factored into the wind loading.
API has some discrepancies on the amount of roof plate that should be figured to resist uplift. Specifically, if the internal pressure plus wind uplift exceeds the weight of the roof plate, then logically, the entire roof plate would be considered to resist uplift, whereas if internal pressure plus wind uplift is less than roof plate weight, then only the roof plate weight supported by the shell would be included. Another issue is whether corroded weights or as-built weights should be used in the criteria.

 

[sdz]

I see there is a 13th Ed which has the same equations but with the following change

MDL is the moment about the shell-to-bottom joint from the nominal weight of the shell and roof structure supported by the shell that is not attached to the roof plate

 

[HTURKAK]

Yes ..you are right . IMO, API 650 getting more and more stringent . I am still with the description 12th Ed. and neglecting the positive effect of roof plate should be reasonable for the fixed roof tanks with intermediate columns .

I would like to hear others' opinion .



Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM