ExpGapInd option in PipePlus

[psobral2]

The program (Pipe Plus) recommends setting the -ExpGapInd option when dealing with non-linear supports. I have a pipe supported by several vertical one-way supports (non-linear), and anchors in both ends. When the pipe is hot, some of these one-ways (the ones near the anchors) are lifted up from the support structure, this means that the pipe has a positive vertical displacement, leaving the supports "floating". Running the analysis first without the -ExpGapInd option, and then with it, gives very different results (looking at the support summary).
I am not sure which results should I trust.
Any help appreciated, sorry for my poor English...

 

[GBor]

How different are the results? And which one looks correct?

Someone with experience with Pipe Plus would probably have a better idea what this specific option is, and I can check when I get back to the office later today, but let me take a guess and say that the -ExpGapInd is probably necessary. I'm guessing that this converts the supports to compression only elements and allows the pipe to "move away" from the base of the support, but still be supported when the pipe tries to push against the support.

Without the -ExpGapInd option, I'm assuming you have a linear element, like a truss, holding the pipe in place no matter which way the pipe expands. This could be adding artificial stiffness if you do not actually "capture" the pipe with the support.

By the way, Unless you are from Germany or Russia, your english is better than my ability to speak your language...and it's probably better than my ability to speak either of these as well.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community

 

[psobral2]

Thanks GBor, for the fast and exhaustive response!
Apparently this option tells PipePlus to use a different approach to calculate thermal displacements.
From the PipePlus reference manual:

PipePlus supports two approaches to expansion load cases:
1. In the first approach (the default), displacements, forces and moments are obtained by subtracting the sustained case from the operating case:
Thermal = (dead weight+pressure+thermal) - (dead weight+pressure)
2. In the second approach the iteration process is carried out independently.
To select this approach include the following line in the pipeplus.cnf configuration file:

-ExpGapInd

If -ExpGapInd is used PipePlus will run a separate nonlinear iteration for each thermal case to determine the status of supports, and the expansion displacements, forces and stresses. If there are nonlinear supports, i.e. directional stops, one-ways, or friction that interacts with the gap status and the system weight, this method is not recommended. If the gaps in the system are only activated by the thermal expansion of the pipework and are not influenced by the system weight then -ExpGapInd will produce satisfactory results.

Every time I try to run an analysis which includes non-linear elements (one-ways, limit stops, or friction) and the -expgapind option is disabled, the program warns me, and recommends enabling it. But, according to this reference manual, this method is not always recommended. This if why I'm quite confused...

B.T.W.: I'm from Argentina and we speak Spanish (sort of...) here.