Releases

[Anonymous]

Hi all,

I am attempting to model a steel frame building, I am applying the default pinned-pinned releases to each beam to make them simply supported but I am getting instability in the model. I have pinned supports on my column and have no releases on each column. Can you tell my why I would be getting this instability as I think I have modelled each element correctly? 

Thanks,

Sarah 

[Artur.Kosakowski]

First of all check the shape of deformation as well as values of displacements of nodes. The large values indicate having excessive releases. In such case you may restrain RX rotations at both ends of elements and see if you still have the same situation. Make sure that you have bracings as your beams don't resist bending at the locations of columns.

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.

[Anonymous]

Hi Artur,

Thanks for your prompt reply. 

I have restrained the RX rotation at both ends for all of the beams and I am still getting the instability... I checked all of the displacement shapes and although they are large values they appear to be working as I would expect them to appear. 

V bracings are modelled according to our drawings (and I have made these members pinned-pinned also).

There is a rigid diaphragm modelled on the beams. I have turned on and off the DSC Algorithm function and also the RLink function to see if this helps. 

Would there be any other reason why I am still getting this instability? 

Thanks,

Sarah 

[Artur.Kosakowski]

The type 3 may also be caused by large difference in stiffness of elements of the model (short and rigid vs. long and flexible). You may also try to set the solver to Skyline and switch off RLinks (provided the model isn\t too big for this solver). If there is no instability reported then the reason was RLinks (very stiff elements) and you can ignore them (when you use solver to speed up calculation time). Mind that type 1 and 2 instabilities should not be ignored.

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.

[Anonymous]

Hi Artur,

I had already tried setting the solver to Skyline and switching off RLink, this also gives the type 3 instability. Is it possible to email on the model privately. I will be using this model for seismic analysis - equivalent lateral force method and a modal analysis. I would like to know why this instability is appearing, possible my realeases are not set up properly could be a possible reason? 

Thanks for your help,

Sarah 

[teixeiranh]

Try to delete all the real realeases. Then apply one realease at a time and always calculate the structure (not the fastest way but it works).