Duplicate element IDs in the same deck runs without error. Why?

sudhish.kumar over 5 years ago

I have a Sol101 analysis deck that has a set of element Ids that are duplicated.

A set of CTRIA3 /CQUAD4 elements have been given the same ID as that of another set of CBEAM element.

I am not seeing any fatal error or warning in the F06 file.

Why is this happening?

0 Don.Graff over 5 years ago

That is just how it works... it is 'ok' in Nastran if different element types have the same id... it will fatal error if the same element type has the same id. But, this is generally discouraged.
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0 sudhish.kumar over 5 years ago

Thanks Don,

follow-on query: if I apply PLOAD4 loads on a model which has duplicate element IDs will it correctly identify that and apply it on the 2D elements?
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0 Don.Graff over 5 years ago

The PLOAD4 entry only applies loads to 2d and 3d elements... so there will be no issue for 2d or 3d elements with the same id as 1-d elements. But, if there are 2d elements with the same id as 3d elements, then the PLOAD4 will apply the loading to the 2d elements if G1 and G3 (fields 8 and 9) are NOT defined on the PLOAD4... if G1 and G3 are defined, then the PLOAD4 will apply the load to the 3d elements.

I suggest you make a sample input file with the types of duplications you have and test out how the PLOAD4 operates. You can add OLOAD=ALL to the case control and see the resulting applied loads in the f06, and check that they are going to the correct grids.

For your specific question, as long as G1 and G3 are not defined, it should work as you are intending and only apply load to the 2d elements.
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0 Dave Popejoy over 5 years ago

As Don mentioned, Nastran allows you to do that, though it's a very poor practice. Plus, I don't know of any pre-processors that would support this...I assume most (if not all) would insist on renumbering elements if you import an input deck set up this way.

This oddity of Nastran reminds me of the "stupid pet tricks" that David Letterman used to include on his late night show.
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0 Martin MSC Nastran over 5 years ago

Back when memory was measured in KB, disk in MB, the GUI was paper-pencil, and usage was charged by the CPU minute speed - these limitations drove code writing logic. Storing element information first by type then by the ID was the efficient choice. Plus this method made it easier to add additional element types and it still is.

I do not miss those days nor do I have any nostalgic feelings for them.
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0 anthony.carlson over 5 years ago

Not sure if this adds anything to this topic or not but with grids I have noticed that one can have redundant grid id's defined and not get a fatal error if the grids positions are defined exactly the same. If the grids with the same id have a positional difference one does get a fatal error that looks like this.

*** USER FATAL MESSAGE 9994 (BULKPM)

GRID with ID=1001 near line 3

was entered more than once, with differences starting at field X1.

I'm not sure if this behavior is true for other entities (element, property, etc) as I haven't tested it.

Is this topic covered in any existing Nastran Docs?
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0 Don.Graff over 5 years ago

Yes, this is controlled via Nastran System Cell 402, or DPBLKTOL. Here is a description from the QRG:

DPBLKTOL (402) Specifies Bulk Data tolerance value for GRID, CORD2C, CORD2R, and CORD2S entries.

I've attached a full screen capture.
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0 anthony.carlson over 5 years ago

Don, good stuff. Thanks for that info. TC
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