Visualisation of internal forces and moments in fiber sections

Hello! In this section we'll talk about the post processing module and the data visualization.
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dudchenkoav
Posts: 24
Joined: Mon Feb 17, 2020 3:27 pm

Visualisation of internal forces and moments in fiber sections

Post by dudchenkoav » Wed Oct 21, 2020 5:47 pm

Dear STKO Team,

I have a question regarding representation of internal forces in beams. Could you clarify how we can plot internal shear forces and moments in forcebeamcoulomn and displacementbeamcoulomn elements? Can we do it without section aggregators? Could you please also explain or give me a link where I can read about how various internal force are calculated?
I ask this because for simple cantilevar beam I get the following moment distribution.

Best Regards,
Aleksandr
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STKO Team
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Re: Visualisation of internal forces and moments in fiber sections

Post by STKO Team » Thu Oct 22, 2020 9:30 am

Dear Aleksandr,

This is an important topic that often creates confusion.

Introduction:
  • Results like force (global system) or localForce(element's local-system) are element-nodal forces, i.e. the internal forces at the beam-ends.
    I ask this because for simple cantilevar beam I get the following moment distribution.
    Note that the forces at the second node have opposite sign (that's why you see those strange colors, but they are perfectly correct in their meaning). In fact the external forces coming from 2 connected elements and converging to the same node should be in equilibrium, thus the opposite sign.
  • Results like section.force are instead sectional results, thus always in the element's local-system. They have the correct sign, because they are not yet post-processed by the element for the computation of internal forces
.
Could you clarify how we can plot internal shear forces and moments in forcebeamcoulomn and displacementbeamcoulomn elements? Can we do it without section aggregators?
If you plot force or localForce, they always have the transverse shear (always pay attantion to the opposite sign at the second node), because of equilibrium.
If instead you want to use a gauss result (section.force), because you are interested in what's going on along the beam, and not only at the beam ends, then it depends on whether you assigned a shear-deformability to the cross seciton (section aggregator).

This leads to another important point: If you assign a shear behavior to the cross section via a section aggregator, you are making your beam SHEAR-DEFORMABLE (Timoshenko), otherwise it will be a standard Euler-Bernoulli beam.

And finally this leads to another very important NOTE:
The displacement beam column element DOES NOT check whether or not the cross section has a shear-behavior, so it will always pass a shear deformation = 0 to the cross section, meaning that if you use a dispBeamColumn, it will always be an Euler-Bernoulli beam!

An anticipation for the next version:
We are currently implementing the beam diagram plot. It will be very flexible: if you select gauss results, it will keep the original sign and it will do a plot that changes along the length of the beam, thanks to the internal values provided by the interior gauss points. Otherwise, if you select results at the beam-ends, it will take care of changing the sign of the second nodes.

Finally, here is an example that compares 2 very thick beams (very thick just to show you that the dispBeamCOlumn will be stiffer because it neglects shear deformations). You will see the results in terms of element-nodal-forces are the same because of equilibrium with external forces. However the tip displacement will be different. And you can see that in the gauss plot, only the forceBeamColumns shows a non-zero shear stress!
beam_output.zip
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dudchenkoav
Posts: 24
Joined: Mon Feb 17, 2020 3:27 pm

Re: Visualisation of internal forces and moments in fiber sections

Post by dudchenkoav » Thu Oct 22, 2020 1:08 pm

Dear STKO Team,

thank you for your answer. It is more clear now.

Best Regards,
Aleksandr

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