API 650 Foundation Design Moment 2

[BearcatEngineer]

Appendix E provides an equation to calculation the "overturning moment due to seismic forces applied to the bottom of the shell" as the follows
M=ZI(C1WsXs+C1WrHt+C1W1X1+C2W2X2)

My question is when determining what the bearing under the ringwall should be, should the I in Mc/I be that of the entire shell bottom(1/4*pi*r^4), or only I of the ringwall itself ((1/4*pi*(rout^4-rin^4)? Obviously you get two greatly different answers. I typically run my analysis until I obtain the required ringwall width. Just looking for some guidance, have been in debate for a while as to which way is correct.

 

[venkicsa]

I could not find this equation in API 650 Appendix E. I am referring to the 10th edition, year 1998. I could only find one equation for Ring wall moment. I calculated the ring wall moment using the equation, Mrw=Sq.rt.(Ai(WIXi * WsXs * WrXr)Power 2 + Ac (WcXc)power 2). Question is how do I proceed further using this value. I mean to check for overturning and bearing pressure. Appreciate your views. Thanks.