Bottom Plate Thickness in Overturning Stability 5.11.2

[fastphantom]

In 5.11.2 does the thickness of the bottom plate effect the overturning.
I am replacing a 40 year old tank. Due to process conditions the client has requested the tank be built form SAF2205, resulting in 5mm walls and a 6mm floor.
The tank is 4.5m diam, overflow Height 6.6m with a cone roof. In its original life the tank did not have hold down bolts - it satisfied the overturning criteria - and the client (due to a short plant shut down window) does not want to modify the footing to including hold down bolts. The new tank has to be unanchored.
Ultimate Wind speed 50m/s (41m/s permissable).
I must be reading 5.11.2 incorrectly - I can not seem to satisfy equation 1 and 2.
Moment about the shell to bottom joint (Mdl) includes only the shell and roof (But not the floor).
I believe I should be able to increase the floor thickness so the tank will be stable, but where is this wieght used in the stability calculations.
Can someone help please.
Thanks
Tony

 

[JStephen]

The wind overturning criteria has been revised considerably in the last 40 years.

The only place that bottom thickness enters into the wind uplift is in the calculation of MF, the weight of liquid used to resist uplift.

Note that API-650 specifies wind uplift on the roof, which will be much of the overturning moment, but other wind codes (such as ASCE 7) do not, and may give much-reduced overturning moments. And API-650 does allow the use of other wind codes.

 

[IFRs]

You can possibly claim improvee stability if the owner can maintain a calculated minimum amount of liquid heel in the tank.

 

[fastphantom]

Thanks for the replies
JStephen - it makes sense to use the mass of the floor plate in the MF calculation but it is not clear in API650. It states "MF = moment about the shell to bottom joint from the liquid weight".
I will very happily use the floor plate mass here.
Is this what you are saying??
Thanks again

 

[fastphantom]

And to calculate wl - I read the standard (for SI units) to be:
the lesser of 14.8HD or 59tb*sqrt(Fby*H).
Is this right???

 

[JStephen]

MF doesn't include floor weight, but is affected by the thickness of the floor.