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





RE: Skirt - Seismic Response Modification 3 or 2 per ASCE 7?
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.
RE: Skirt - Seismic Response Modification 3 or 2 per ASCE 7?
"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.