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Pipe Stress - response spectrum analysis question

Pipe Stress - response spectrum analysis question

Pipe Stress - response spectrum analysis question

(OP)
Hi, I have two general questions regarding response spectrum analysis (RSA), questions which would also apply to any frequency domain loading such as harmonic loads from pressure pulsations or unbalanced vibrating equipment

1) Since results of RSA and other frequency domain loading are positive values only, how is this "limitation" of RSA results typically accounted for in piping design code compliance and for calculation of equipment loads? The sign of the RSA results could make a difference when combining with other loads

2) Is the pipe/structure interaction typically ignored in pipe stress analysis and design? In a seismic event in particular, it would seem that the coupled effect of piping (and equipment) with the structure could have a significant impact on results. The sway of a structure loaded with equipment and piping during an earthquake could whip the piping around in unexpected ways that could potentially damage equipment connected to piping and/or overstress the piping. Of course, the structural eng. should give notice to piping on large deflections from lateral loads

In structural analysis and design, we typically model pipes, equipment, cable trays, etc. as loads, and we include these loads as part of our mass model when there is dynamic analysis. This is not a perfect solution for dynamic analysis, since gaps and friction are ignored, many of the loads are approximated with 'fudge factors' for safety margin, and equipment loads that are offset from the supporting frames are often assumed to be acting on the centerline of the frame.. but still, even that approach seems to be miles ahead of the simplified "rigid support" assumptions that I see being commonly made in pipe stress analysis.. although I don't see any other practical alternative short of spending a ton of time creating a nonlinear structural model with everything

RE: Pipe Stress - response spectrum analysis question

ZippyDDoodah,
             To answer 1 - Althought the results from a RSA are shown as absolute values they must be considered as positive and negative.
             To answer 2 - Normally Secondary response spectra are calculated using FEA model of the structure (if structure is significant and cannot be considered as being "rigid")The piping is then analysed using multiple response spectra applied to the piping system. The Seismic anchor motions of each applicable raft are input to the analysis.

RE: Pipe Stress - response spectrum analysis question

Zippy....

My gosh......these questions bring me back thirty years to the dawn of pipe stress analysis for nuclear plants and systems !!!

Q1: DSB is correct....the need for piping response spectra analysis mandates that an organized method for calculating pipe support loads is developed. It is correct to assume that both plus and minus RSA loads must be considered. Extra exitement is generated when harmonic and/or time-history loads must also be considered !!!

Q2: It is my recollection that the piping/support interaction issue was addressed by mandating a certain minimum stiffness in the pipe support design (this, by the way was a contributing factor to the decline of the nuclear power industry IMHO).

There were other "fun" issues for some of us nuclear geezers ...they include:

-  Seismic anchor motion..!!
-  Response Spectra spreading and Enveloping !!
-  Percent damping to be used for Various piping diameters !
 
and.... best of all..(drum roll...please )

guess the allowable equipment nozzle loads !!!

Sigh...memories of Boston & the night life ..Circa 1974

Good Luck !!

-MJC
 

   

RE: Pipe Stress - response spectrum analysis question

(OP)
Thanks for the replies MJ and DSB. Regarding consideration of - sign to RSA loads in addition to absolute valued, that was what I was thinking.. it's what we typically do in structural design also. Obviously time history dynamic anlaysis is different, and preferable, because it gives you + and - signed output results.

I've seen factors applied for piping static analysis as an alternative to rigorous coupled pipe/structure dynamic analysis, but I've yet to see a reasonable technical justification for their use. There's just too many different considerations for those factors to have validity in my opinion. But given practical limitations, it appears that that's pretty much how piping is typically designed.. uncoupled from the structure

Boston nightlife in the mid-70's sounds like a hoot. I attached a pic which may be a younger MJCronin from the 70's :)  

RE: Pipe Stress - response spectrum analysis question

OMG.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stress - response spectrum analysis question

Zip...

OMG...OMG....OMG....!!!!!

Its Me  !!!!! I had just learned "the Hustle" !!!!!

Thanks for the chuckle !!!!!

   

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