I read Interpretation 2-17 differently.
This is what I see:
Interpretation 2-17
Subject: Seismic Loads
Date: June 8, 1983
Question: How shall stresses in a piping system due to restraint deflection from seismic motions, and those due to seismic inertial loadings, be applied in accordance with ANSI/ASME B31.3?
Reply: It is the intent of 302.3.6 that the inertial loads from earthquake be included in the calculation of stresses due to occasional loads. The effects of displacement of restraints due to earthquake may be treated as an externally imposed displacement in accordance with 319,2,1(c).
It says seismic displacements MAY be included as part of the expansion stress range.
As I said earlier, (the more recent document) B31E includes the effect of those displacements in the sustained plus occasional stress evaluation.
Remember, too, that interpretations do not necessarily extend to Code updates. There have been many changes since 1983.
One final point, the Code committees now avoid addressing "intent" questions. Published interpretations must draw on what is written rather than propose why it says what it says.
_________________________
Dave Diehl