Square hollow section chassis

@xyont,

Ok. shells and beams expands in similar ways.
The offset aproach descrived in the manual to construct beams of nearly arbitrary cross section should be used carefully.

@dichtstoff

Not sure what do you mean sorry. I hope the example is useful to you.
As Xyont has confirmed, this assemblies requires a bunch of nodes to get reliable results but, not recommended doesn’t mean it is restricted.

i known before when reading an articles from STRUCTURE Magazine by Arturo Montalva, P.E., Jeff Baylor and Klaus Wittig (Oct, 2010)

right, i did several test with many section type in the past and found a questionable results in condition near the support, member intersection, stress distribution etc.

i have checked the example,
very interesting. i have checked the buckling modes.
I prefere to work with beam cross-section created with solid elements.
how you fix the boundary, if you use a U-cross-section?
in connection with gravity and shear center!?

right, as CalculiX taken advantages in uniformity of the solver. even though still required many test to validate and improvement the codes.

A bit of caution. Beams have a variety of bugs so you have to tread carefully. For instance:

  • A rotational DOF constraint on a transformed node of a beam can act in the wrong direction.
  • A rotational DOF constraint on a beam can cause incorrect external force.
  • Some components of external force can be missing.
  • Stress at constraints can be unrealistic. Refine the elements.
  • Section forces on elements connected to non-beam elements can be (are always?) wrong.
  • Concentrated loads create spurious local stress and deformation of the cross-section. Again, use refinement.
  • A truss connected to a beam can disconnect itself in some configurations.
  • A truss connected to a shell can constrain the shell’s rotation in some configurations.
  • Inhomogeneous boundary conditions can cause incorrect stress.
  • Velocity appears as 0 on some nodes for dynamics.
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There can be also surprisingly large inaccuracy when fully integrated beam elements are used with plasticity and when the mesh of beam elements is not significantly refined:

https://forum.freecadweb.org/viewtopic.php?f=18&t=27903
https://forum.freecadweb.org/viewtopic.php?f=18&t=23318&p=521544#p521544

hi all,

this what i found in 2016 for preliminary review. i did not refined the model and investigate further, it seems beam element in CalculiX mostly dependent on how user modeling.

please, don’t judge quickly about the reliability and accuracy or even a bugs cause your model may inappropriate to set (i.e mesh, element type & additional restraint) or limitation itself.

i seen many user direct compared to 1D/2D classical element were is not comparable due to expanding element in CalculiX and generates knot or MPC existence. user need different model approach to refined and set up properly.

for example, curved shell with symmetry boundary conditions. i’m reading the documentation again, it’s required to transform nodal support to local axes.

another example is beam in elasto-plastic bending analysis in which using B32 element not B32R as recommended.

or an example from FreeCAD forums, it’s clear for me when someone notify about shear-locking and hourglassing.

some notify: beam element in CalculiX is unique, advances and challenging. no matter how it, keep imaging as solid element and MPC existences not as simple as classical element.

best,

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To compare with the theoretical result I guess the same strategy. Idealization of a pure pinned support and rotation around the x axis constrained.
I have tested on a 6meters UPN160 with distributed Self weight and agreement with the theory is reached in this case with just 4xB32R. Not bad at all.


PropSection, v1.0.4, is a free software that provides the shear center of any custom section.

I guess we just need to add an extra ofsset to locate properly the custom beam to avoid the twist. I will test later and post the result with and without correction. Let’s see if it works .

This open-source software for the calculation of section properties is interesting:

However, it doesn’t have any GUI.

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It is an excellent option, I’ve been using SectionProperties for at least 2 years and it has grown a lot since then. It offers many possibilities. Recommended.

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Too bad it doesn’t have even a simple GUI. Is rotating and merging built-in sections the only way to analyze non-standard shapes ? It would be great if there was any open-source software allowing users to import a drawing of the cross-section from CAD software like FreeCAD.

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In PropSection you can introduce the coordinates of any arbitrary section.

imagen.

Is SectionProperties also for Windows? Looks good and one can import dxf. I must give it a try

It’s only in French, right ?

Yes, it should work on Windows. It’s just a Python package.

Oui. As many other programs they give for free.

I used it in the past for the D-box structure of aircraft wings, the skin (leading edge structure) was imported from a *.dat file and merged onto an I-shaped member (stringers+spars). Today probably it is easier than before, but it was a relatively difficult task to do the first time. But the code was automated and parametrized so later changes were easier.

classical beam element ignoring shear center position and twist effect. however CalculiX does it properly, so only center of gravity is required to place the section plane of beam element.

finding center of gravity in sections does not necessary to use any external software, it can be simple steps to be done.

The beam can be placed in the horizontal position ussing a second Offset paramenter which clearly meakes a difference.

*BEAM SECTION,ELSET=Lower_Flange,MATERIAL=Flange_65_x_10.5,SECTION=RECT,OFFSET2=7.11904761904762,OFFSET1=0.191538462

0.065,0.0105
0,1,0

*BEAM SECTION,ELSET=Upper_Flange,MATERIAL=Flange_65_x_10.5,SECTION=RECT,OFFSET2=-7.11904761904762,OFFSET1=0.191538462
0.065,0.0105
0,1,0

*BEAM SECTION,ELSET=Web,MATERIAL=Web_139_x_7.5,SECTION=RECT,OFFSET2=2.173333333

0.139,0.0075
0,0,1

I have positioned two UPN160 one with respect to the center of gravity and the second with respect to the Shear Center.

I have not been able to completely avoid rotation without a minimum constrain (rotational spring).
That’s a good point for this technique as it seems able to model bending/twist. ¿Is that possible Xyont or am i reading incorrecftly the displacements?

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i did not see into detail since no loading input available, but it seems the loads come from self weight (gravity) or point load and simple supported.

if that’s true, then second figures as predicted to shown rotated movement due to eccentricity of the support. it will raised to unstable structure for no rotation restraint applied.

shear center and twist only apply with concentrated point loads or line loads along member at some eccentricity in position. so it’s related to loads not the beam element position itself.

below what i have got in 2016, may i need to look up back and investigate further of beam element,

it seems eccentricity and twist only considered by beam element in CalculiX when user defined the member distributed load transferred to webs. this assumption given correct assumption since any transversal load must resisted by stiff part.

above true assumption are ignored by many classical beam element. this what i mean before, another condition is required to test and validate.