Discussion Groups

Robot Structural Analysis

Reply
Valued Contributor
eusmor
Posts: 71
Registered: ‎12-30-2012

Isolated joints and instabilities

275 Views, 7 Replies
02-06-2013 05:55 AM

Hello, I modeled roof trusses with axial loads only. The analysis run but I have two types of warnings:

 

The first one refers to isolated joints in the model, I can’t see them and then I can’t erase. At the model there are rigid links because the joist are at the same place of trusses joints just above of the top chord of trusses (the top chord of trusses is slope).

 

The instabilities type 3 appears at nodes of trusses without members in other plan than the truss plan (i.e. node 2073 in direction Ry). Is it necessary making supports in only perpendicular address for these nodes? Or how I can eliminate the instabilities?

 

The file was send a private message to Artur Kosakowski

 

Thanks a lot for your help

 

 

David Moreno

Please use plain text.
Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: Isolated joints and instabilities

02-06-2013 07:31 AM in reply to: eusmor

 

The first one refers to isolated joints in the model, I can’t see them and then I can’t erase. 

 

This is caused by lack of accuracy in defining the geometry of the top part of the roof. The nodes are the calculation ones. In addition you defined overlapped claddings e.g. 2598 and 2599.

 

The instabilities type 3 appears at nodes of trusses without members in other plan than the truss plan (i.e. node 2073 in direction Ry). Is it necessary making supports in only perpendicular address for these nodes? Or how I can eliminate the instabilities?

 

This seem to be caused by large difference in stiffness of elements (bars with the same section having 8cm and 671 cm). Mind that you should carefully check the obtained results (if as expected) especially displacements and rotations of nodes.

 

The file was send a private message to Artur Kosakowski

 

I will send you the corrected model (top part of roof). Mind that you should redefine loads for this part of the model.

 


If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.



Artur Kosakowski
Please use plain text.
Valued Contributor
eusmor
Posts: 71
Registered: ‎12-30-2012

Re: Isolated joints and instabilities

02-11-2013 01:53 PM in reply to: Artur.Kosakowski

Hi,

I was eliminate all little beams and I was revise carefully the releases, but instabilities type 3 still appears, maybe it’s due to rigid links and or additional supports a perpendicular plan of trusses.  

Thanks for see my model again

The new corrected file was send a private message

 

Thanks,

David Moreno

Please use plain text.
Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: Isolated joints and instabilities

02-12-2013 12:24 AM in reply to: eusmor

David,

 

I would consider re-defining releases so that one of the bars that 'meet' in the same node is 'fully' attached to it (with no release applied).  E.g. for two bars that meet in node 8078 - if you define release on one of them only it is enough to prevent any bending moment generation in this location. Defining releases for two bars you sort of 'detach' the node from them allowing for its 'free' rotation which results in the instability warning.

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

 

 



Artur Kosakowski
Please use plain text.
Valued Contributor
eusmor
Posts: 71
Registered: ‎12-30-2012

Re: Isolated joints and instabilities

02-12-2013 07:08 AM in reply to: Artur.Kosakowski

If I have more the two bars, is it necessary that “only” one releases bending? Or only one holds? For example at node 638 I have a warning instability type 3 in direction RY

Thanks.

Please use plain text.
Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: Isolated joints and instabilities

02-12-2013 07:16 AM in reply to: eusmor

In general for bars that 'meet' in one node you should define releases for all but one bar.

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.



Artur Kosakowski
Please use plain text.
Valued Contributor
eusmor
Posts: 71
Registered: ‎12-30-2012

Re: Isolated joints and instabilities

02-12-2013 07:27 AM in reply to: Artur.Kosakowski

In this case the bar 848 is fixed a node 638 in RY local (same global) address, but I have the warning.

 

Thanks

Please use plain text.
Product Support
Artur.Kosakowski
Posts: 4,750
Registered: ‎12-17-2010

Re: Isolated joints and instabilities

02-12-2013 07:33 AM in reply to: eusmor

The instability of type 3 warning may be triggered by other factors too. Please look at the message 2 in this topic.



Artur Kosakowski
Please use plain text.