[solved] Deflection result in FEM on a simply supported beam

Post here for help on using FreeCAD's graphical user interface (GUI).
Forum rules
and Helpful information
IMPORTANT: Please click here and read this first, before asking for help

Also, be nice to others! Read the FreeCAD code of conduct!
Post Reply
PMac
Posts: 135
Joined: Sat Sep 02, 2017 10:44 pm

[solved] Deflection result in FEM on a simply supported beam

Post by PMac »

Hi, as a FEM novice tried to model a simply supported beam.

Length (span) 2900 mm, 150x50 mm on edge. (separately calculated, Ix = 14062500 mm4, Sx = 187500 mm3)
C24 Timber Wood material using average MoE E of 10.8GPa. 450 kg/m3 density (self-weight not constrained for this test)
Fixed constraints on bottom outer edges, using 0.86kN/m UDL along upper face. This UDL is converted to a point load (860 N/m * 2.9m = 2494 N) and applied as a 2494 N force constraint at midspan on upper face.

GMSH mesh (50 max mesh) and Calculix appear to function OK.

External calculations seem to give a max deflection of around 5.24mm but this FEM result shows a 2.33 mm max deflection. What am I doing wrong?

Model attached.
Attachments
A00260002a FEM Simple Beam.FCStd
(891.79 KiB) Downloaded 34 times
Last edited by PMac on Wed May 04, 2022 9:45 pm, edited 1 time in total.
User avatar
NewJoker
Veteran
Posts: 3020
Joined: Sun Oct 11, 2020 7:49 pm

Re: Deflection result in FEM on a simply supported beam

Post by NewJoker »

You should get the desired result if you replace the fixed boundary conditions with FEM ConstraintDisplacement and select just the three translations for one edge and two translations (without the axial one to simulate roller support) for the other edge.

A00260002a FEM Simple Beam_fixed.FCStd
(887.43 KiB) Downloaded 38 times

By the way, this topic should be placed in the FEM subforum.
PMac
Posts: 135
Joined: Sat Sep 02, 2017 10:44 pm

Re: Deflection result in FEM on a simply supported beam

Post by PMac »

Thanks NewJoker. That fixed the deflection issue.

Have attached the now modified file for others to view the changes.
Attachments
A00260002b FEM Simple Beam.FCStd
(888.72 KiB) Downloaded 35 times
dinodano
Posts: 11
Joined: Mon Jun 06, 2022 5:42 am

Re: [solved] Deflection result in FEM on a simply supported beam

Post by dinodano »

Hi sorry, I'm trying to run a similar model and I can't seem to get my desired max deflection.

Dimensions: 100 x 9 x 35mm
Ix = 32156.25 mm4
Material: Graphite
E = 11.7 MPa
Density 1750kg/m3
I actually wanted to simulate self-weight and observe the deflection but for the purpose of replicating closely to your model and using the formula 5/384 wl4/EI. I used a constraint force of 0.54N (rounded from 0.54077625)
I used an auto G-mesh (should i not?) (what is the recommended mesh max size with N-mesh?)

External calculations seem to give a max deflection of around 18.7 micrometer but this FEM result shows a 38.27 (z direction) micrometer max deflection. What am I doing wrong?
DeflectionCalculation.xlsx
(10.86 KiB) Downloaded 19 times

Model attached.
trial6_deflection.FCStd
(205.02 KiB) Downloaded 18 times
Also, if I were to have both supports not at the ends but maybe 10mm away from both sides each, how do i go about it? I have been slicing the model at the location i want to put my supports to be at, and then putting displacement constraints at the edges of the sliced part. Is this the correct way of doing so?

Thank you.

Code: Select all

OS: Windows 10 Version 2009
Word size of FreeCAD: 64-bit
Version: 0.20.29177 (Git)
Build type: Release
Branch: releases/FreeCAD-0-20
Hash: 68e337670e227889217652ddac593c93b5e8dc94
Python 3.8.10, Qt 5.15.2, Coin 4.0.1, Vtk 8.2.0, OCC 7.6.2
Locale: English/Singapore (en_SG)
Installed mods: 
  * 3D_Printing_Tools
  * Manipulator 1.4.5
  * MeshRemodel 1.8919.0
PMac
Posts: 135
Joined: Sat Sep 02, 2017 10:44 pm

Re: [solved] Deflection result in FEM on a simply supported beam

Post by PMac »

dinodano wrote: Thu Jul 07, 2022 2:45 am Hi sorry, I'm trying to run a similar model and I can't seem to get my desired max deflection.
Hi dinodano,

Very rusty on this.

Had a quick squiz yesterday at your model and calculations. I could not see any obvious reason for observed differences - I confirm your results. I did wonder why you applied the load as a UDL constraint rather than a material of the given density and a self-weight constraint.

A few other things:
About choosing another mesh. Don't know anything about that, such matters as why one might choose a differently sized mesh or even a different mesher are well above my pay grade as a FEM novice.

About moving the supports to 10mm away from the current support positions at 100mm apart. If I've read that correctly you want to have a piece of graphite 100mm long with no immediate supports and a 10mm air-gap at each end to the supports at 120mm apart. Somehow even as a FEM novice I think that isn't going calculate. I would suspect the computer says No! Why not make your piece of graphite 120mm long to do the FEM deflection analysis?

Or did you mean 9mm width plus 2 x 10mm air-gaps to supports acting on the side faces? Can you show a sketch of what you're trying to achieve with this 10mm support increase?

One thing I remember observing in FEM was that it didn't like editing of constraints on the fly. IIRC, the only way I could get consistent results was to construct any model from scratch with correctly defined constraints. If I started to modify constraints after building a model, doing a 'what if' scenario, the numbers were not consistent.

Busy at present, but will likely revisit this in a few days, in the interim, maybe someone else could offer a solution.

And like someone suggested to my question, should your question be in the FEM section?
Post Reply