fcFEM - FEA from start to finish

About the development of the FEM module/workbench.

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HarryvL
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Re: fcFEM - FEA from start to finish

Postby HarryvL » Tue Jun 11, 2019 2:48 am

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
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
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
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
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.
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HarryvL
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Re: fcFEM - FEA from start to finish

Postby HarryvL » 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.
CD795816-C1E0-4814-A381-5AA4A90771C4.jpeg
CD795816-C1E0-4814-A381-5AA4A90771C4.jpeg (491.05 KiB) Viewed 186 times
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PrzemoF
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Re: fcFEM - FEA from start to finish

Postby PrzemoF » Mon Jul 01, 2019 9:12 am

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!!! :lol:
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bernd
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Re: fcFEM - FEA from start to finish

Postby bernd » Mon Jul 01, 2019 12:59 pm

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 ... :mrgreen:
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HarryvL
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Re: fcFEM - FEA from start to finish

Postby HarryvL » Mon Jul 01, 2019 1:58 pm

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