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#235 - 11/15/00 04:06 PM Sustained vs. Dead Load
MF Erne Offline

Registered: 11/15/00
Posts: 1
Loc: Ann Arbor, MI, USA
On a recent analysis, structural requested a breakdown of load components. I provided the operating case and the sustained case. I then tried a weight only case and a pressure only case. The weight only (and also the WNC case) yielded loads which varied from the sustained loads. No values showed up for the pressure only case. At some supports, the weight only case loads were greater, as some less. Sometimes the magnitude of difference was large, sometimes small. I told structural to use the sustained case loads. Is there a logical explanation of why these difference occurred?

#236 - 11/15/00 05:36 PM Re: Sustained vs. Dead Load
John Breen Offline

Registered: 03/09/00
Posts: 482
Loc: Pittsburgh, PA (& Texas)
Interesting question.

Doubtless Rich Ay will give you the real CAESAR II answers that you are looking for. I would like to offer some food for thought. Perhaps it will stimulate some more discussion.

A piping system is an irregular space frame. If it is in equillibrium, it will be connected to some supporting structure(s). The total load must be transferred to the supporting structure(s) to keep it in equillibrium. The total load includes live weight (including e.g., contents, ice, large birds that land on it, etc.), dead weight (pipe, in-line components (e.g., valves, strainers, flange sets, etc.)) insulation (and cladding), hanger hardware, accumulated debris, external loads (e.g., wind), thermal expansion/contraction thrusts, and other stuff that doesn't readily come to mind. The summation of all these loads and the supporting reactions equals zero. If anything changes, a redistribution of forces (e.g., weights) is compelled to occur. The distribution of forces is a classic "beam 3 moment" distribution problem. The obvious change is the one that occurs in thermal expansion/contraction when the length of the piping changes and thermal thrusts are imposed on (at least) the terminal points (i.e., "anchors"). It is clear how this forces a change in the force (load) distribution. Of course the operating case has a thermal component and will include terminal equipment movements. Hanger angularity associated with thermal (and wind) loadings will also transfer thrusts to the terminal points ("anchors"). Perhaps less obvious is the various effects of pressure.

Pressure stiffening will cause slight changes in the stiffness matrix if it is turned on. Bourdon effect only comes into effect in pipe with a large D/t ratio and significant pressure. Small diferences in pressure can affect the weight distribution though.

Be careful of those "structural guys". They say "dead load" when they really mean total load. Include the loading of the "sprung" and "unsprung" hanger hardware weight. Hope that when "Bubba" walks along your pipe, he treads lightly. And just when you think you got it right the electricians will hang a bank of conduit from you pipe. There is a special place in hell for "pipers" who hang their new piping from somebody's existing (well designed) piping system.

Consider the rather significant change in load distribution that occurs when the cold pipe heats up (and the opposite)due to the reduced modulus of elasticity. Piping will sag noticeably if the hangers are too widely spaced (then fluid may collect in the "low points"). If you do a sustained load run with cold Young's modulus then do it again with hot Young's modulus you will see diferences,

What else should be said, my estemed colleagues?

Best regards, John.
John Breen

#237 - 11/15/00 08:56 PM Re: Sustained vs. Dead Load
Richard Ay Offline

Registered: 12/13/99
Posts: 6226
Loc: Houston, Texas, USA
To answer your question accurately, I really need to get a copy of your job - please feel free to e-mail it to me at

In the mean time, assuming the load cases you are questioning are:

1) W + P1 (SUS)
2) W (SUS)

I would expect to see pretty much identical results for displacements and restraint loads. There are a couple of things that could cause (large) differences though:

  • If you have an untied expansion joint, the pressure thrust could cause significant displacements and resulting loads when pressure is included, but not in the weight only case.
  • As mentioned in the previous post, if Bourdon pressure effects are turned on, the pipe will experience displacements when pressure is included, but not in the weight case.
  • Depending on the magnitude of the pressure, the "pressure stiffening effect" on the stiffness matrix could affect the results.

There may be something else I just haven't thought of, which is why I'd really like to look at your job.

Richard Ay (COADE, Inc.)
Richard Ay - Consultant

#238 - 12/06/00 02:45 PM Re: Sustained vs. Dead Load

Well I agree with all of you guys! Marty for being astute and supervising the solution, John for bringing the world of engineering science to the table, and Rich because thats how the program is supposed to work!

As a trial take a single element, anchor x Y restraint and run it with W + P, W, and W(NC) watching the loads on the y restraint. The diffeences shoul=d be easily reconciled. Your job must have something "special" going on!

Good to see you three guys together!

Best Regards,

John C. Luf

[This message has been edited by John C. Luf (edited December 06, 2000).]


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