Pedestrian bridge with mixed mesh

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HarryvL
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Pedestrian bridge with mixed mesh

Post by HarryvL »

With reference to my post in the topic on mixed meshes (https://forum.freecadweb.org/viewtopic. ... &start=100), here are some early findings and issues I need to get my head around.


PB_2_Def_1.png
PB_2_Def_1.png (261.47 KiB) Viewed 2469 times


1) The mesh has a central beam running the full length of the bridge. This does not show up in the 3D beam result. I presume that this "edge" did not get meshed as a beam because it is also part of a face. The same applies for the part of the pylons that coincide with the transverse plate girder.


PB_2_Mesh_Detail_1.png
PB_2_Mesh_Detail_1.png (233.95 KiB) Viewed 2469 times


The first eigenmode shows that omission of the pylon section creates a hinge that is not there in reality


PB_4_1st_EM.png
PB_4_1st_EM.png (256.5 KiB) Viewed 2469 times

PB_4_1st_EM_Hinge.png
PB_4_1st_EM_Hinge.png (313.67 KiB) Viewed 2469 times


2) I get GMSH errors and warnings that I suspect have to do with the fact that some edges are shared between beam and plate???:

Code: Select all

Error   : Edges from different entities share vertices: create a finer mesh
Warning : Failed to compute equidistant parameters (relax = 0.0078125, value = -nan) for edge 521-529 parametrized with 150 150 on GEdge 1753  
Creating a finer mesh as per the recommendation does make a difference.
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Re: Pedestrian bridge with mixed mesh

Post by bernd »

wow, very very cool !


HarryvL wrote: Mon Apr 09, 2018 8:59 pm 1) The mesh has a central beam running the full length of the bridge. This does not show up in the 3D beam result. I presume that this "edge" did not get meshed as a beam because it is also part of a face. The same applies for the part of the pylons that coincide with the transverse plate girder.
No it is not the mesh IMHO it is the inp exporter. See https://github.com/berndhahnebach/FreeC ... .cpp#L1184 This was done by me and could be totally wrong. There seam to be many examples where an needed edge could belong to a face and a needed face could belong to a volume. I need to think about this problem.


HarryvL wrote: Mon Apr 09, 2018 8:59 pm The first eigenmode shows that omission of the pylon section creates a hinge that is not there in reality
This gained my interest because. If you would make a simple example of this I would try to trigger this problem.



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Re: Pedestrian bridge with mixed mesh

Post by HarryvL »

Thanks Bernd. I will construct a simple example. I suspect it has to do with the "drilling" degree of freedom of the shell element, because only the bottom connection hinges. This is where the stiffness would have to come from a "drilling" rotation of the shell. I don't think shell nodes have stiffness for that type of deformation. In fact, I am pretty sure. So this can only be solved by generating a beam on the face (to make the pylon continuous) or by putting an extra shell element perpendicular to the plate girder.
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Re: Pedestrian bridge with mixed mesh

Post by bernd »

Ahh, is it similar to the problem described here https://forum.freecadweb.org/viewtopic.php?f=18&t=23067 ?
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Re: Pedestrian bridge with mixed mesh

Post by bernd »

how about make the pylone a continous beam even through the faces? For this again the mesh export problem that edges which belong to faces are not exported needs to be fixed.
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Re: Pedestrian bridge with mixed mesh

Post by HarryvL »

bernd wrote: Tue Apr 10, 2018 6:23 am how about make the pylone a continous beam even through the faces? For this again the mesh export problem that edges which belong to faces are not exported needs to be fixed.
Yes !!! That would be fantastic.
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Re: Pedestrian bridge with mixed mesh

Post by HarryvL »

bernd wrote: Tue Apr 10, 2018 6:07 am Ahh, is it similar to the problem described here https://forum.freecadweb.org/viewtopic.php?f=18&t=23067 ?
Yes indeed - similar issue. A shell node has no stiffness for rotation in its own plane.
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Re: Pedestrian bridge with mixed mesh

Post by HarryvL »

That's called a drilling degree of freedom.
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Re: Pedestrian bridge with mixed mesh

Post by bernd »

Another good example for edges which belong to faces and are needed to be exported would be a bridge with edge beams at each side. The modell would have shell elements for the bridge decke and two beams at each side for the edge beams (ideally with axes moved). Attached an example of a section from a channal bridge. I'm gone give it a try to fix this mesh export problem.

bernd

bridge-section.jpg
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Re: Pedestrian bridge with mixed mesh

Post by HarryvL »

Yes Bernd. There are indeed many practical examples of beams integrated in a plate/shell. Also for shells integrated with solids (for example: foundation plate on soil, surfboard, sandwich panels - as in my buckling post).
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