Load combination for Seismic

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,

Dear Seok,

I think A is more conservative and common practice. Note that the max. seismic loads are in X+0.75Y and Z+0.75Y directions. You may analyze both A and B cases and compare results.

Thanks for reply,

You are right, Farhad. I fully understand that is of course common practice but, the problem is that it's really difficult to find out load combination basis for piping in Caesar II program or Code. One more, the other problem is that seismic location is in Chile, one of place where seismic is most strong.

To verify it, I need someone's help who has experience for Chile project or who has enough knowledge for seismic theory and code or Coade staff.

I would really appreciated if you give me an advise.
thank you.

Best regards,

Dear Seok,

If load case B has been mentioned in project spec, go ahead with case B.