Hello everyone,

I feel concern about load combonation method for seismic.
What I know and have been doing for seismic load combination method is as follows;

A. OCC Case only for seismic
1. U1(X)
2. U2(Y)= 2/3 Fh
3. U3(Z)
4. U1+U2
5. U2+U3
6. SUS+U1+U2
7. SUS+U2+U3

Is above not enough for OCC stress check?
Now our client which is responsible company like PMC for seismic review want that all three orthogonal directions should be considered (x,y,z) simultaneously like this;

B. Client's requirement
1. 1.0X+0.3Y+0.3Z
2. 0.3X+1.0Y+0.3Z
3. 0.3X+0.3Y+1.0Z

For your guidance, the project what I'm doing is power plant(CCPP) in Chile where seismic is very strong.

My questions are following 3 cases. please let me know if anybody know...... especially, I am concerned for No.2 and 3.

1. Which is more conservative between A and B?
2. Is there any code requirement for load combination to use A or B method?
3. How can I verify basis if A is correct?

Best regards,