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Seismic load combinations and ASME Section VIII pressure vessels

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My concern relates to seismic criteria as per UBC 97 (or similar codes
that restrict using increased allowable stress with a seismic load
reduction factor in load combinations) and how this relates to the
allowable stress to be used for design of ASME Section VIII pressure
vessels.

Specifically, UBC 97 1612.3.1 combines E/1.4 with D but generally does
not permit an increase in allowable stress. 1612.3.2 combines E/1.4 with
D and L and does permit an increase in allowable stress.

ASME Section VIII Div 1, UG-23(d) allows a 20% increase in allowable
stress for pressure components.

On one hand, if internal pressure is considered a "live load" then it is
justified to apply the reduction of 1.4 as well as increasing allowable
stress. People tell me that of course "pressure is a live load, it's not
weight..."

On the other hand, when subjected to pressure there is little or none of
the structural redundancy that exists in most structures (buildings,
bridges, etc). When pressurized every part of the vessel is subjected to
essentially the same level of stress. It seems that UBC and other codes
recognize that specified live loads may not exist on the entire loading
surface area at once: there are reductions permitted for live load
applied over wide areas. ...the reality is that this certainly does not
happen in a pressure vessel!

It seems to me that because of the distribution of stresses in the
pressure vessel that allowable stress should not be increased if the
seismic reduction 1.4 is applied.

I appreciate any comments and insights...


   Tom Barsh, P.E.
   Technical Support Engineer
   Codeware Inc
   www.codeware.com
   Phone: 281-497-5705
   FAX: 281-497-5839



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