Indu
I agree with what Seth posted. One should analyze the foundation with strength level loads to find out the true strength level demand. Talking in terms of STAAD.Pro, one needs to create REPEAT LOADs to combine the gravity and lateral loads with appropriate load factors and then analyze the foundation to find out the strength level moments/shears. Multiplying the results from service load cases by strength level load factors should be avoided because of the nonlinear nature of the problem as you rightly pointed out in your post.
I agree with what Seth posted. One should analyze the foundation with strength level loads to find out the true strength level demand. Talking in terms of STAAD.Pro, one needs to create REPEAT LOADs to combine the gravity and lateral loads with appropriate load factors and then analyze the foundation to find out the strength level moments/shears. Multiplying the results from service load cases by strength level load factors should be avoided because of the nonlinear nature of the problem as you rightly pointed out in your post.