topic Re: Instability Error in Robot Structural Analysis Forum

Instability Error

nali_ameen — Fri, 01 Dec 2023 13:28:38 GMT

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

Re: Instability Error

Simau — Fri, 01 Dec 2023 15:33:46 GMT

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

Re: Instability Error

nali_ameen — Fri, 01 Dec 2023 15:51:29 GMT

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.

Re: Instability Error

Simau — Fri, 01 Dec 2023 18:23:18 GMT

@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

Re: Instability Error

nali_ameen — Mon, 04 Dec 2023 09:36:36 GMT

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

Re: Instability Error

nali_ameen — Tue, 05 Dec 2023 13:37:34 GMT

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

Re: Instability Error

Simau — Tue, 05 Dec 2023 15:46:27 GMT

@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

Re: Instability Error

nali_ameen — Tue, 05 Dec 2023 17:23:41 GMT

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.

Re: Instability Error

Simau — Wed, 06 Dec 2023 09:18:55 GMT

@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

Re: Instability Error

nali_ameen — Wed, 06 Dec 2023 09:45:25 GMT

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?

Re: Instability Error

Simau — Wed, 06 Dec 2023 12:58:21 GMT

@nali_ameen

You may use this for the columns