Hi all,

I don't understand why in the occasional load combination (in my specific case OCC=SUS+EARTHQUAKE) even if I modelled a Z stop with 65 mm GAP at node 700, when I look at the displacements in the Z direction in this load case, the real displacement at the node where the restraints is applied is about 130 mm (node 700 has a z restraint with 65 mm of GAP and X restraint with no GAP).

See attached image.

There something specific in B31.3 rules? or am I missing something?

TY.

http://s10.postimg.org/tmfye5nsp/Immagine.png

When you combine load cases you are combine the results of those load cases. So each of your two cases you combine may close the gap of 65 mm giving a total of 130 mm.
If you wish a structural evaluation of "SUS+EARTHQUAKE" then run a load case combining all those basic components (e.g., W+P1+U1).
Remember that the piping codes want to see the sum of stresses (sustained and occasional) and that may not make sense in terms of structural response.

OK.

Thank you for your clarification.