Skirt - Seismic Response Modification 3 or 2 per ASCE 7?

[ConstantEffort]

What is your recommendation for the "R" Response Modification Factor for a skirt supported, welded steel vertical vessel such as a process column?

ASCE7-05 table 15.4-2 is the applicable reference.
Link: Table 15.4-2 (see page 2)

Under the non-building structures I am torn between
- "elevated tanks, vessels, bins, or hoppers" entry for "single pedestal or skirt supported - welded steel" which has a R of 2
- "All other steel ... distributed mass cantilever structures ... including ... skirt supported vertical vessels" which has an R of 3

Would not any skirt elevate a vessel?

 

[JStephen]

My two cents-
The "elevated tanks...bins, or hoppers" are generally going to be squattier type structures, although possibly on a tall pedestal.
If this is a tall thin vessel, I'd go with the second option. Something squatty, first option.

 

[Trestala]

I suggest getting a copy of "ASCE Guidelines for Seismic Evaluation and Design of Petrochemical Facilities."

"The response modification factor, R, incorporates the combined effects of the inherent ductility and the overstrength capacity of the structure. ... Using higher R-values in design tend to result in a flexible structure that will tend to increase calculated displacements."

As far as on-ground skirt supported pressure vessel, my understanding is that to use R=2.

And, "R=3 may be used if the vessel and skirt are checked for seismic loads determined using I/R=1 with FS=1.