I am currently working on a handful of piping systems which also have to be taken care of for seismic conditions. I have defined 8 spectrum load cases; each case comprises of a vertical spectrum and a horizontal one which are combined in SRSS method (I put plus and minus signs for the factors of the directions)
I check the results and as odd as it may sound I see no difference between cases in opposite directions for instance (+X and +Y) and (-X and +Y)
can I have a second thought on what I may have missed here?

thanks

Modal summation used in CAESAR II seismic analysis by the response spectrum method washes away all signs (e.g. SRSS, ABS, GROUP); deflection, load and stress will be the same in your example.
If using a static equivalent method, for linear models, stresses will be identical but deflection and load will show opposite signs.