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Re: Seismic load combinations and ASME Section VIII pressure ve

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>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.

'Don't mix Codes and don't imagine that the design margins mean the same

'thing for pressure vessels and buildings. Any ASME Code stamped
pressure 
'vessel has to meet the ASME Code first. That means you need to meet all

'the applicable provisions for the pressure loading alone before
anything 
'else. If you have additional loading besides pressure you may need to 
'increase the thickness to address these loadings according to UG-22 
'subject to an upper limit for calculated stress address these using the

'provisions of UG-23. If those provisions don't satisfy a building 
'official (I've never had such a thing happen) you may have to address 
'that separately yet again. But don't fiddle with pressure vessel code 
'allowables if you want the vessel to be stamped. 

You raise some very good points: ASME Code must be met first. I think
that pressure vessels full of hazardous/flammable fluids should be
considered as part of life safety issues.

In a cylinder the longitudinal stress due to internal pressure is about
1/2 that of circumferential stress due to pressure. This means that for
a vertical vessel (axis of cylinder is vertical) there is a lot of
reserve capacity left for carrying stresses due to weight of the vessel
and overturning moments due to wind, seismic, etc. 

For most structures the circ stresses govern. But for some vessels,
those that  are particularly tall, skinny, flexible, etc. the
longitudinal stresses may govern the design thickness. It is this class
of vessel that concerns me. Ironically, this is the class of process
(not storage) vessel that is probably the most costly and possibly most
dangerous when it comes to life safety issues.

I see that ASCE 7-02 and IBC 2000 continue to reduce the E load when
combined with other effects. It's hard to judge how this washes out
though since the combinations also include a reduced dead load D. IBC
2000 allows increase in allowable stress, ASCE 7-02 does not unless "it
can be demonstrated...".

What is the 'grunt' engineer to do? Look at this both way? allow the
stress increase and then check again without it? (hmm, does that make
sense?)

How do you folks in California handle refinery vessels regarding this
issue? 

thanks,

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



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