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Re: SEISMIC: Pipe Supports and Thermal Stresses

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>I have a client asking for help in bracing some large pipes carrying both
>superheated and chilled water in an existing building and want to make sure
>I do things right.  Can someone steer me to a good reference that might help
>with the following questions:
If you mean water which remains liquid because the pressure is high 
enough to prevent boiling, you'd better get familiar with the piping 
codes. As a general caveat, limit analysis dosn't apply to piping and its 
supports because of the nature of service loading--highly cyclic, a very 
high percentage of live loading without much structural redundancy to 
rely on. The pipe has to remain undamaged or you could have a serious 
problem on your hands. 

>How much inter story drift can a steel pipe riser of given diameter,
>supported at floor levels in a high rise subjected to an earthquake, be
>expected to tolerate without failure?
Normally this is calculated by assuming the pipe is a member of a rigid 
frame system and limiting the stress to the Code allowable, but there are 
end conditions and internal pressure to complicate matters. The 
applicable piping codes and you local pressure vessel law will set stress 
standards. If you have long runs, you'll want to consider dynamic 
analysis. And don't forget loads imposed on equipment like receivers. I'm 
not trying to be vague, but there's no simple answer to your question 
before you do the numbers.

>To what extent can transverse bracing on intersecting lines or elbows be
>relied upon to brace a pipe against longitudinal movement without worrying
>about breaking the intersecting pipe and/or elbow joint?
The only way to tell is to do some simple calculations. When there's 
thermal stress involved, the bracing problem gets tricky because you 
can't always make restraints that resist mechanical loading and allow 
thermal expansion. 

>How much do I have to worry about locking in thermal stresses?
The term 'locking in' isn't clear. If you mean do you have to worry about 
restraining the pipe and generating thermal loads, then yes, you have to 
worry enough to do the numbers and figure what thermal expansion is doing 
to you and making provision for it. 

>Can I safely ignore the effects of hydraulic forces at bends assuming normal
>flow rates?
'Normal' isn't a very good word here. I investigated a fiberglass pipe 
failure where the normal flow rate was thousands of gpm of chilled water. 
When the mitered elbow let go as the result of momentum loads the 
occupants had quite a lively afternoon. The allowable pressure for the 
pipe was well in excess of the service pressure, but the elbow 
construction was garbage. Don't ignore anything you don't understand.


Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw