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Author Topic: Discrete modelled stiffener - Web Beam concept offsets  (Read 2369 times)

Gawain

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Discrete modelled stiffener - Web Beam concept offsets
« on: January 16, 2020, 09:11:55 AM »
Hi

I’m currently trying to size a single sided Orthogrid panel which has been discretely modelled with BAR and QUADS.  I’m using a ‘One stack unstiffened’ concept for the plate and a ‘Web Beam’ concept for the stiffener.  At this moment in time I’m not using Stringer Segments, as I believe the preferred process is to size a panel using Plates and Bars first, and then use Stringer Segments as a final check as this is a quicker way of doing things.

The issues that I’m having relates to defining the BAR ‘reference plane’ and how this calculates the Offset within the BAR element when it updates the FEM.  At this moment I’m trying to size a single sided machined stiffened panel, hence as the elements are modelled on the plate centroid then I’m using either the ‘Top’ or ‘Bottom’ reference plane, depending on which side I’m offsetting the stiffeners.  The first issue is that the ‘Top’ reference plane does not seem to change the BAR offset during the HyperFEA loops, whereas the ‘Bottom’ reference plane does.  Is there a reason why I can only Offset the BAR in one direction? 

The next issue is regarding to the BAR Offsets during the HyperFEA process, as it looks like the full stiffener height is being used as the BAR offset instead of using ½ the stiffener height during the FEM update.  This would produce an incorrect panel stiffness/frequency and therefore an incorrect load redistribution.  Is there a reason why the BAR offsets are set to the full height of the stiffener and is there a setting to correct this?  This does not seem to happen when using stringer segment.

Regards
Gawain

Stephen

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Re: Discrete modelled stiffener - Web Beam concept offsets
« Reply #1 on: January 16, 2020, 10:43:26 AM »
Gawain,

This issue was actually previously identified and fixed in a more recent version of HyperSizer. It was indeed a bug and not the intended functionality.

If updating to the latest version is going to be a challenge, I think the remaining options would be to use DSM from the beginning (which will have implications on the speed of sizing and analysis), or to have a script that post-processes the updated FEM and corrects the bar offsets. The latter is definitely possible but would be somewhat involved (as you would need to map between property IDs and CBAR elements to determine the appropriate element offset).

Apologies that I can't really think of a better solution right now..

-Stephen

Gawain

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Re: Discrete modelled stiffener - Web Beam concept offsets
« Reply #2 on: January 16, 2020, 12:56:54 PM »
Thanks Stephen

Post processing the updated FEM before the Nastran run seems a possible solution.  I'll try that.

Thanks

Gawain