321 PIPING SUPPORT
321.1 General
The design of support structures (not covered by this
Code) and of supporting elements (see definitions of
piping and pipe supporting elements in para. 300.2)
shall be based on all concurrently acting loads transmitted
into such supports.
Well, there are pipe supports (shoes, guides, etc), and support structures which support those pipe supports. Seems that unless you have a coupled pipe/structure model (nice in theory), there's not much beyond "educated guess" based on experience when designing pipe supports and piping for seismic loads unless you involve the structural engineer. You can have a rack structure with 100 ton aircoolers on top with other heavy equipment and piping with pipe racks deflecting 4" or 5" under seismic loads while still meeting drift requirements. Obviously that affects piping and pipe support loads in a big way, yet most pipestress models that I've seen ignore this.
In a seismic analysis, unless that heavy equipment, piping, cable trays, etc are part of your mass model and structural flexibility for these loads is considered, then you're dealing with a too-limited model for realistic seismic design of pipe supports(my opinion). This is where structural input might be required. Structural engineers won't trust piping loads given to them by pipe stress engineers for seismic cases for the very reason that piping models are too simplistic to capture these coupled effects, and in seismic design, those coupled effects are often too great to safely ignore
