Elastic-perfectly plastic steel-beam

[Anonymous]

Hi Autodesk!

I've started my master thesis and am about to do some modelling involving plasticity and redistribution of forces.

I've made a model of a continuous beam with beam-elements, see picture below and/or attached .rtd.

 

An IPE100 cross section is chosen and the "Elasto-plastic analysis" box is ticked where the "Elastic - perfectly plastic" stress-strain curve is chosen.

Both web-, flange- and beam discretization have been chosen to 30, 30 and 0.025, which is deemed more than enough.

No matter if my stop criterion is "Structure collapse" or not under "Criterions to stop analysis", full yielding (reaching the Re = 235 MPa) is not happening.

1) Why is the yielding plateau not reached at the middle support accompanied by increased moments in the spans as the load is increased?

2) What exactly defines the collapse? A rotational limit that somehow can be played with?

Thanks in advance and best regards,

Jasper Cotto Hansen

[Krzysztof_Wasik]

Hi @Anonymous 

I have made similar example and results seems to be correct, except of stress irregularity near central support which is general issue in Robot (calculation element length was set to 0.1 m). Top structure is fully elastic bottom elasto plastic. Both sections are IPE 100

For elasto-plastic model stress does not exceed 235, span moments are bigger as well as deflection

Try to activate Matrix update after each iteration. It will improve convergence.

Collapse condition means calculation divergence (usually due to too big displacement). Refer to Robot help with dialog description.

Keep in mind that Robot has limited capability for elasto-plastic structures analysis

[Anonymous]

Hi Krzysztof,

Thanks for your reply!

By ticking the "Update matrix after each iteration" I do not get convergence for some reason; I do get various types of instabilities instead:

I did adjust the calculation beam element length to 0.1 m as you did. Hmm.

Did you tick anything in the "Additional criterions to stop analysis"?

I notice that your model may not be in the "2D frame" setting as my model is. It looks like you have specified your support conditions to the "3D frame" or "Shell design" setting? Not that this should matter at all...

Can you attach your model to this thread?

Thanks for your time,

Kind regards

Jasper

[Krzysztof_Wasik]

Hi @Anonymous 

Refer to attached model

[Anonymous]

Thanks!

Well, running your file I still get tons of instabilities:

Still though, the final nodal deflection picture is alright:

Also, the stresses are plastic as you showed. Thanks, well done!

How would you know if you can trust such analysis, when all types of instabilities (1, 2 and 3) occur?

Best regards,

Jasper

[Krzysztof_Wasik · Accepted solution]

Hi @Anonymous 

Instabilities are caused by maximal load obtained for collapse criteria. Program increases loads up to structure collapse and the due to big displacements instabilities occurs (then program stops analysis presenting last converged increment). If you switch off collapse criteria and define load close to 6.75 model is calculated with no instabilities.

In case I do not trust in calculation I usually double check using manual calculation or other software. Keep in mind that Robot approach to elasto- plastic analysis is simplified. More precise results will be obtained for solid model (Robot actually do not support elasto-plastic for solid)

Simple solid model created in Nastran (loaded with 6.6 kN/m) shows similar plastic zone distribution 

and displacement bigger than in Robot (46 mm Nastran  vs 39 mm in Robot)

[Anonymous]

Thanks a lot for your time. I will consider Nastran for my project!

Best regards,

Jasper

[Krzysztof_Wasik]

Hi @Anonymous 

Another program you can consider is Fusion 360