## fcFEM - FEA from start to finish

About the development of the FEM module/workbench.

Moderator: bernd

HarryvL
Posts: 1052
Joined: Sat Jan 06, 2018 7:38 pm

### 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 264 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 264 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 264 times

and another representation of the same facts:

fcFEM_Simply_Supported_Plate_Stn_Conc_Deformation_PV.png (97.64 KiB) Viewed 264 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: 1052
Joined: Sat Jan 06, 2018 7:38 pm

### 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 186 times
PrzemoF
Posts: 2655
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: 8465
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: 1052
Joined: Sat Jan 06, 2018 7:38 pm

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

Ah yes of course. I will go back and ask them