Type 3 Instability - Tension only bracing

[ByrnePM]

Hi I would really appreciate if someone from support team could take a look at my model attached,

I have transferred this model from Revit and have tried to bring in line with ROBOT (adjusting bracing etc...)

I have done sanity checks on the model and all members seem to be supported and released correctly,

I am analyzing the structure for only its self weight and some additional slab concrete weight applied to floor cladding,

I am getting a good few Type 3 instabilities,  (I'm using tension only truss bars for vertical bracing members)

I don't see why these nodes are giving me instabilities ?

https://we.tl/t1LddiGeTr

Thanks

[Artur.Kosakowski]

Is there any reason for having short (calculation) bars like the one on the picture below?

[ByrnePM]

Hi Artur,

No, I would prefer if I could get rid of these nodes that have no members connected to them ? (if there is an easy way to identify them within the model?)

Such nodes are only present as a result of the initial model being generated in Revit (bracing connection was not properly connected to top of column (node 1027)

[Artur.Kosakowski · Accepted solution]

Yes, they can be easily deleted. 

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.

[ByrnePM]

Thanks Artur, so what you have done here is to provide 10% relative stiffness to all of the members that have pinned connection defined ?

This means that end moments will be transferred between pinned members ?

[ByrnePM]

Could you explain to me why is this a necessary step?

Ideally I do not want to design connections for such moment transfer

[Artur.Kosakowski]

You don't have to but as you switched on use of DSC elements (recommended for nonlinear analysis) they influence values of elements of the stiffness matrix. If you switch off DSC there will be no instability for the original definition of the releases reported. I'd consider comparing the results for both situations for original releases and see if you can could reduce the elastic stiffness of the releases for 'my' model and get no instability for DSC.

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.

[ByrnePM]

Thanks Very much Artur,

I will investigate both

[ByrnePM]

Artur,

Using your model an elastic stiffness of 0.01 is possible ...which is good (less moment transfer the better)

I now want to insert diaphragms on the panels shown in Image 1 

The definition of the diaphragms are as shown in Image 2

This is now throwing up instabilities in random locations ...I cant figure out why 

Any help would be appreciated 

[Rafal.Gaweda]

If you do not have this instability in model without diaphragms - I think you can ignore it after checking nodal displacements and rotation results.

If you see large displacements or rotations - you have to correct model.

[ByrnePM]

Hi Rafal,

I have checked model by applying unit loads in relevant translation / rotational directions to specific nodes showing instabilities,

The resulting displacement / rotations look logical for these small loads....

Therefore I think I will ignore the Type 3 instabilities at this time,

Thanks for your help