If I have to calculate a pipe line under seismic case with SRSS method I have to:
Combine U1: seismic with sustained case: W+P+U1
Or
Combine U1: seismic with operating case: W+P+T+U1.
Because I already tried the two combinations and the first case (sustained) was more critical.
Is this normal?

Depends on the piping code.

A common approach is to add the STRESSES ("scalar" summation in CAESAR II terms) of the W+P1 case and the U1 case. This need not be the same numbers as the stress from W+P1+U1. This sum of sustained and occasional stresses must remain below an allowable limit of k*Sh where k is anywhere from 1.15 to 1.33.

You would be interested in the W+T1+P1+U1 (and W+T1+P1-U1) to determine loads on supports and equipment and pipe position (but not necessarily a code-defined stress).

Thank you for this explanation but can you tell me from where you got the relation k*Sh?

MR. abdelkader,

Search ASME B31.3 CHAPTER 2"design" 302.
302.3.6 Limits of Calculated Stresses due to Occasional
Loads

I got it Thank you