Instability Error

[nali_ameen]

Hi, 

I have modelled a steel structure with user defined sections (as the section I need are not available in Robot). The structure is properly braced in both directions. But I need to model all connections as pin as they can not transfer moment. But I get so much error for instability (1, 2 and 3 type). is it ok to have such error? or is there away to fix it?

Attached is my model.

Thanks in advance

 

[Simau]

Hi @nali_ameen 

Select all bracings and delete releases and set them as Truss.
select bras 16A19 and delete releases
Select all supports and set them as Fixed

Type 3 warning may be ignored

Model attached

[nali_ameen]

Thank you Simau for your support.

In reality my supports will behave as pin support and if I change it to pin, I get type 1 error. does that mean my structure is really unstable?
also, sorry, I didn't understand the 2nd point "select bras 16A19 and delete releases".

Thank you.

[Simau · Accepted solution]

@nali_ameen 

I didn't understand the 2nd point "select bras 16A19 and delete releases".

Sorry for typo, i mean bars or members  16 to 19

In reality my supports will behave as pin support

Setting suppports as fixed will not change results as your columns are released. It avoids instabilities

[nali_ameen]

Thank you Simau. This is very interesting, though the support is fixed but with release it will behave as pin support.

[nali_ameen]

Hi @Simau,

I have made some changes in my model (considering also adjustments you mentioned previously). But now I see an error regarding conversion and type 2 warning:

Is there any workaround for that. Thank you

[Simau · Accepted solution]

@nali_ameen 

Modif made:
Correct a geometric inaccurancy
Supports as fixed with columns released
Pinned-Pinned release set as xxxxff-xxxxff
Load cases 2 3 4 set as auxyliary
Nonlinear analyse parameter modified (see on the model)

Type 3 instability maybe ignored

[nali_ameen]

Thank  you so much @Simau , really interesting. One question please, if I make the columns pin-pin, I get a warning for "Matrix is not positive definite". Does this warning has any relation to stability of my structure?

Thank you.

[Simau]

@nali_ameen 

Take in mind that columns are not pin-pin, but pinned at the bottom and fixed at top.

Look at my previous model

[nali_ameen]

Thank you @Simau, but in reality my connections between the columns and the beam is pin connection. Why would this make an error in Robot?

[Simau · Accepted solution]

@nali_ameen 

You may use this for the columns