## fcFEM - FEA from start to finish

About the development of the FEM module/workbench.

Moderator: bernd

HarryvL
Posts: 1071
Joined: Sat Jan 06, 2018 7:38 pm
Location: Netherlands

### Re: fcFEM - FEA from start to finish

I spent some time quality checking the Mohr Coulomb model with tension cut off. Here the results for a challenging analysis of a thin simply supported concrete plate (10x10x0.15m) loaded by a uniform load of 10kPa (1.0Te/m2).

An elastic analysis gives a reinforcement ratio of approximately 4% at the center of the plate. This is high but still realistic. The elastic deformation of the plate under the design load is 41mm, which is L/250 and therefore on the high side of what is allowable.

fcFEM_Simply_Supported_Plate_Stn_Conc_Elastic_Deformation.png (112.72 KiB) Viewed 482 times

Next I used fcFEM to load the plate to collapse. The material parameters are as follows (including the tough assumptions of full non-associated plasticity, psi=phi-30, and zero allowable tension):

Code: Select all

``````yield strength steel = 315.0 MPa
compressive strength concrete = 15.75 MPa
friction angle (phi) = 30.0 degrees
dilation angle (psi) = 0.0 degrees
material: Mohr-Coulomb
tension cut off: principal stress
tension strength concrete = 0.0 MPa
reinforcement ratio: automatic from elastic analysis
minimum reinforcement ratio = 0.001
``````

The load-deflection curve shows a 10% margin between the elastic design load and the collapse load of the plate:

fcFEM_Simply_Supported_Plate_Standard_Concrete_L_u.png (32.69 KiB) Viewed 482 times

Note that the much larger elastic deformations are a result of assuming zero tension in the concrete from the start. The analysis should only be used to judge collapse load. When the incremental deformation of the plate at collapse (below) is compared to the elastic deformation (above) then it is clear that a collapse mechanism has developed (straight vs curved plate segments plus a small zone of rotation ... a plastic hinge):

fcFEM_Simply_Supported_Plate_Stn_Conc_Deformation.png (157.24 KiB) Viewed 482 times

and another representation of the same facts:

fcFEM_Simply_Supported_Plate_Stn_Conc_Deformation_PV.png (97.64 KiB) Viewed 482 times

Although its still early days and much more testing and (speed and memory) optimising is required I think fcFEM holds promise as a robust (conceptual) design tool.

Next I will work on an idea for an alternative material model that should be faster and more stable. Stay tuned in.
HarryvL
Posts: 1071
Joined: Sat Jan 06, 2018 7:38 pm
Location: Netherlands

### Re: fcFEM - FEA from start to finish

... meanwhile, somewhere on a Thai beach 🏝 ... if only they had used FEM work bench they would have known to put the rafter upside down.
CD795816-C1E0-4814-A381-5AA4A90771C4.jpeg (491.05 KiB) Viewed 404 times
PrzemoF
Posts: 3048
Joined: Fri Jul 25, 2014 4:52 pm
Contact:

### Re: fcFEM - FEA from start to finish

HarryvL wrote:
Mon Jul 01, 2019 7:34 am
... meanwhile, somewhere on a Thai beach 🏝 ... if only they had used FEM work bench they would have known to put the rafter upside down.
[..]
For me this will be the post of the month!!!
bernd
Posts: 10819
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

### Re: fcFEM - FEA from start to finish

you never know ... they might have no snow but strong wind suction and FreeCAD FEM did gave them the result to put it upside down ...
HarryvL
Posts: 1071
Joined: Sat Jan 06, 2018 7:38 pm
Location: Netherlands

### Re: fcFEM - FEA from start to finish

Ah yes of course. I will go back and ask them
bernd
Posts: 10819
Joined: Sun Sep 08, 2013 8:07 pm
Location: Zürich, Switzerland

### Re: fcFEM - FEA from start to finish

bernd wrote:
Sat Mar 30, 2019 6:50 pm
HarryvL wrote:
Sat Mar 30, 2019 5:12 pm
Something else ... I just pulled and built the latest FC source from femconcrete and it broke my fcFEM macro It looks like an additional parameter was added to the FemInputWriter constructor? I simply put another None at the initiation call of the relevant object and now it works again, but it makes me worry that any call to FC API is vulnerable? Also, it would mean that the macro would either work with 0.17/0.18 or 0.19, but not both. Any suggestions on how to manage this? Thanks, Harry
This is some new problem we gone face in FEM. On one side we still do a lot devellopment, but on the other side people start to use FEM API. I will change it in a way it still runs and you need not to adapt it. But we need to think about this. As soon as a new FEM constraint object arises, we will brake it again. The funny thing is we are compatible with 0.17 just because there was no new object in 0.18 ...
No change has been made so far because still there was no new object. But now we have new ones. I changed the signature of FemInputWriter. See topic https://forum.freecadweb.org/viewtopic.php?f=18&t=43092 especially post https://forum.freecadweb.org/viewtopic. ... 10#p370183 But from now we gone have a stable API for this class which I do not intend to change anymore.

BTW: collecting all analysis member is just one line of code