Ring Wall (Mrw) vs Slab Overturning Moment (Ms)

[Soln]

I'm having a difficult time understanding the distinction between ring wall overturning moment (Mrw) and slab overturning moment (Ms). Mrw is less, and defined as the portion off the total moment that acts at the base of the shell perimeter, Ms is defined as the total overturning moment acting across the entire tank base cross section.

Is Ms is too conservative when designing for a ring wall?

What would be the effect of using the Mrw when designing a slab, or pile foundation?

In addition to slab/pile foundations, would Ms be applicable moment to use when designing circumferential/radially spaced columns of an elevated tank with multiple column/rafter support?

Would a ring wall foundation with concrete pavement inside the ring wall (as opposed to sand/structural fill) be considered a slab?

I appreciate comments, I'm trying to get a better understanding of how Mrw/Ms are related, and what is the distinctive action behind the different magnitudes.

 

[JStephen]

On a normal flat-bottom tank, the hydrostatic pressure applied to the tank bottom is assumed to be transferred directly to the soil below.

For seismic action on a flat-bottom tank, there is some variation in the pressure across the bottom. Where the tank is supported on a soil-type foundation, with or without a ringwall, that variation just gets transferred directly to the soil as well. So you might get somewhat higher soil pressures during an earthquake, but that doesn't really affect the tank itself.

If the whole tank is supported on a slab, and you are considering overturn or bearing of the entire slab, then that variation of forces across the bottom results in an additional moment.

(Is Ms is too conservative when designing for a ring wall?)
For a lot of tanks, there won't be that much difference, or the seismic won't matter that much. For larger-diameter tanks, the slab moment may be much higher and would be overly conservative. It you just want a more conservative design, it'd make more sense to increase the importance factor instead.

(What would be the effect of using the Mrw when designing a slab, or pile foundation?)
Soil bearing or pile loading would be higher than what you calculated.

(In addition to slab/pile foundations, would Ms be applicable moment to use when designing circumferential/radially spaced columns of an elevated tank with multiple column/rafter support?)
Maybe, if it's a flat-bottom tank on grillage. It would be an uncommon situation, though. Note that the normal elevated-tank seismic design doesn't consider sloshing, and that the normal flat-bottom tank design doesn't consider the tank being supported on structure.

(Would a ring wall foundation with concrete pavement inside the ring wall (as opposed to sand/structural fill) be considered a slab?)
No, not unless the pavement was made strong enough to support the liquid weight on the ringwall.

 

[mariog123]

API 650 model considers two seismic moments.
Mrw is the "ringwall moment", a result of the seismic moving content exerted to the tank shell.
Ms is the "slab moment", which is the result of seismic action of the content to the tank (shell and bottom).
Ms-Mrw appears to be the seismic action of the content to the tank bottom.
The calculation has been developed by George Housner in several scientific works.
In fact:
- Mrw is the seismic moment that overturns the tank;
- Ms-Mrw changes the hydrostatic pressure on that tank bottom and is transmitted to the bottom support (to the slab, in case there is one) ;