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# Static Seismic Analysis Load Cases

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## Static Seismic Analysis Load Cases

(OP)
Previously I've dealt with piping systems in relatively low seismic zones so horizontal acceleration by itself was considered. In a new project with much higher seismic values we'll be looking to combine the horizontal and vertical components. Based off the Caesar II user guide, the load cases (shortened/condensed here) are intended to be:
L1 = Operating
L2 = Operating + U1 (horizontal)
L3 = Operating + U2 (vertical)
L4 = Sustained
L5 = L2 - L1 (algebraic)
L6 = L3 - L1
L7 = L5 + L6 (SRSS)
L8 = L4 + L7 (abs or scalar)

My questions:
1) The Caesar II user guide has another example where the operating case as "Operating + U1 + 0.67U2". Is there any difference in stresses/restraint loads between this load case and the method shown above? If so, what drives the decision to use either method?
2) I assume the L7 SRSS case (combining U1 & U2 before adding to the sustained) is another intermediate case that can be suppressed in the output (no restraint forces, displacement, or stress values to review). Is this accurate?

Thanks,

### RE: Static Seismic Analysis Load Cases

The way EQ loads are combined does depend on the local EQ loading code and the piping code. B31E has some guidance.

EQ Load values and combinations for piping stress check and the structural design of supports and foundations maybe quite different.

### RE: Static Seismic Analysis Load Cases

(OP)
So with regards to pipe stress, is there a difference between using an SRSS combination like my example above or using "Operating + U1 + U2"?

### RE: Static Seismic Analysis Load Cases

Why are you not using the 100/30/30 rule for combining horizontal and vertical seismic effects?

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