Dear Colleagues,
As you have seen, the last addenda of the API -650, third addenda, has changed the procedure to calculate the bolt size.
In the previous edition, the table 5-21b defined for seismic load, and for design pressure + seismic the slab moment for the bolt design (Ms).
In the last edition they changed the Ms for Mrw (ring wall moment).
Can someone explaining it to me? why do they change something that has been for several years.

You should really direct this question to the API-650 Code Committee.

I can give you an explanation.

Housner work in 60’s initiated studies regarding seismic action on tank's shell and bottom.

The impulsive and convective action of liquid on wall and bottom were considered as supplementary pressures and the math integration leads to Mrw (wall action) and Ms (wall and bottom action).

This is not transparent in API650, but still exists in some contemporaneous works. Please see the figure in http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ08.pdf

So you can see that the bottom transfers a part of seismic load to slab, when there is a slab. A slab will receive Mrw by shell- annular plates- anchors and the rest by bottom (Ms- Mrw)

When there is no slab, (Ms- Mrw) is transferred to soil which is inside the ring wall, not to the ring wall.

But anchors are always (slab or ring wall cases) loaded by Mrw.
This was correctly taken in consideration when API committee written Appendix E.

You may note that E.6.2.1.2-1 calculates "the minimum anchorage resistance" based on Mrw (fact which is- and was- correct) but this approach was in contradiction with "old" 5-21 table.

I try to explain bellow by math.

Wab = Calculated design uplift load on anchors per unit circumferential length, N/m (lbf/ft)
Wab= 1.273Mrw/D^2-wt*(1-0.4*Av) formula E.6.2.1.2-1
1.273 = 4/PI and wt = W/PI*D
So, Wab = (4Mrw/(D^2*PI)-W/(PI*D)*(1-0.4*Av)=
= (4Mrw/D)/(PI*D)-W/(PI*D)*(1-0.4*Av)

Wab is the uplift per unit circumference so the net uplift is

Uplift = (PI*D)*Wab =
=(PI*D)*[(4Mrw/D)/(PI*D)-W/(PI*D)*(1-0.4*Av)] =
= 4Mrw/D – W*(1-0.4*Av)

Compare with the formula in "old" Table 5-21 that was [4 × Ms/D] – W2 (1 - 0.4AV)
The same formula is now: [4 × Mrw/D] – W2 (1 - 0.4AV)

Best regards.