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bjornhgr
Posts: 69
Registered: ‎09-28-2011
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Phantom reaction forces?

481 Views, 7 Replies
01-02-2012 12:15 AM

Hi.

 

I've got this steel structure with wind braces and simply supported column bases all around. This configuration mostly gives the expected distribution of reaction forces: with even vertical reactions and horixpntal reactions concentrated in the bracing. However, after moving some of the braces, their former base supports still get significant horixontal forces. Does anyone have an explanation to this? All beam rotational DOFs are released from the columns, and all supports are pinned.

 

See attached picture for comparison of two similar supports, but with very different horiszontal reactions.

 

Thanks!

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Artur.Kosakowski
Posts: 5,057
Registered: ‎12-17-2010

Re: Phantom reaction forces?

01-02-2012 12:35 AM in reply to: bjornhgr

Could you attach the missing pictures as well as the corresponding rtd files please? I assume that you did not ignore any instability warnings? In case you did address them first.



Artur Kosakowski
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Valued Contributor
bjornhgr
Posts: 69
Registered: ‎09-28-2011

Re: Phantom reaction forces?

01-02-2012 12:57 AM in reply to: Artur.Kosakowski

Certainly, image attached. The .rtd however, is 56 MB, so it's perhaps a bit on the heavy side? Is there a way to "purge" anything but the essential geometry and load information?

 

No instability warnings.

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Artur.Kosakowski
Posts: 5,057
Registered: ‎12-17-2010

Re: Phantom reaction forces?

01-02-2012 01:05 AM in reply to: bjornhgr

Select the rtd without results file type in the Save as dialog and save it with a new name. Ten compress it.



Artur Kosakowski
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Valued Contributor
bjornhgr
Posts: 69
Registered: ‎09-28-2011

Re: Phantom reaction forces?

01-02-2012 01:08 AM in reply to: Artur.Kosakowski

Nifty! Much smaller model file attached.:smileyhappy:

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Artur.Kosakowski
Posts: 5,057
Registered: ‎12-17-2010

Re: Phantom reaction forces?

01-02-2012 02:09 AM in reply to: bjornhgr

Please open the file in the repair mode, save it under a new name and then calculate again. Could you describe me step by step the way you created and edited this model? I assume that you created it in Revit and then exported to Robot. What kind of modifications were done afterwards? In particular what was the way you moved the bracings (I assume that prior to this operation the results were as you expected them to be).



Artur Kosakowski
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Valued Contributor
bjornhgr
Posts: 69
Registered: ‎09-28-2011

Re: Phantom reaction forces?

01-02-2012 04:07 AM in reply to: Artur.Kosakowski

Great! Opening in repair mode seems to have done the trick, and all reactions are as expected. Thanks!

 

In the interest of finding the cause: The model was indeed created in Revit, and I added loads and supportsin Revit as well. After exporting to Robot, I created load combinations and modal/seismic cases before running the analysis. I don't remember the initial result any longer, but I am pretty sure all was well until I moved the braces.

 

The move was done partly by deleting and creating new ones in Revit, partly by using the move/copy function in Robot. It seems phantom reactions occurred in both cases.

 

If you have an explanation or tips on how to avoid this in the future, I'd like to hear about it. Anyways, thanks a lot!

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Artur.Kosakowski
Posts: 5,057
Registered: ‎12-17-2010

Re: Phantom reaction forces?

01-02-2012 06:17 AM in reply to: bjornhgr

The only dangerous situation I can think of is when you make the changes in both files (rvt and rtd) at the same time rather than in sequences. What I mean is:

 

Model in Revit > export to Robot > modification in Robot > update (export to) in Revit > modification in Revit and in the same time modification in Robot > update from Revit to Robot

 

rather than

 

Model in Revit > export to Robot > modification in Robot > update (export to) in Revit > modification in Revit  > update from Revit to Robot >modification in Robot



Artur Kosakowski
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