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Re: Seismic load combinations and ASME Section VIII pressure vessels

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The 1/3 stress increase for load combinations that include wind or seismic
is a structural concept that has been around a long time.  It's
justification has always been questionable.  That is why it is gone from
all recently issued structural codes and specifications.  The 1997 UBC is
one of the few remaining vestiges of the 1/3 stress increase.  (Even AISC
has issued a supplement to remove the 1/3 stress increase from the ASD
specification.)

That issue aside, the ASME pressure vessel codes are on an allowable stress
basis.  As such the strength based E defined in the 1997 UBC should be
reduced by 1.4 to "convert" it to an allowable stress basis.  This reduced
value for E should be used in accordance with the ASME requirements and
load combinations, not the UBC load combinations.

Remember the pressure vessel people are concerned about containment, and
they have very different objectives than the life safety objective of the
UBC.  The ASME code should specify what loads should be combined with E,
not the UBC.

Rick Drake, SE
Fluor, Aliso Viejo, CA
rick.drake(--nospam--at)fluor.com
949-349-5791

*********



                                                                                                                                       
                      "Tom Barsh @                                                                                                     
                      Codeware" <tom           To: <seaint(--nospam--at)seaint.org>                                                                 
                      04/29/03 09:41 AM                                                                                                
                      Please respond to        cc:                                                                                     
                      seaint                                                                                                           
                                               Subject:  Seismic load combinations and ASME Section VIII pressure vessels              
                                                                                                                                       
                                                                                                                                 ..... 
                                                                                                                                       




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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