How to Extract Geogrid Tensile Force/Stress at Specific Coordinates in FLAC3D?

FLAC3D
1

Hello @everyone, @itascan
I hope you are doing well. I am working on an MSE wall model in FLAC3D with 10 geogrid layers, realistic interfaces, and staged construction. My goal is to extract tensile force and tensile stress in the geogrid at specific coordinates so I can compare the FLAC3D results with my theoretical FHWA calculations (Tmax = σh × Sv and σtensile = Tmax / t).

However, I am confused about which FLAC3D command correctly gives the geogrid tensile force per unit width and the actual tensile stress at a chosen location.
So far, I have tried using:

structure geogrid history resultant-nx position (x,y,z) surface-x (1,0,0)

and

structure geogrid history stress-xx position (x,y,z) depth-factor 0

These commands run without errors, but the results are much lower than my theoretical values, and I am unsure if these quantities match what FHWA defines as Tmax and σtensile.

What I am trying to understand

  • Does resultant-nx represent the correct membrane tensile force per unit width?

  • Does stress-xx give the axial tensile stress in the geogrid shell element ¶, or is another command more appropriate?

  • If these two commands are not the right way to get tensile force and tensile stress at specific coordinates, which commands should I be using?

  • Do I need to run structure geogrid recover or set surface-x before sampling histories?

I am confused about which FLAC3D command is the correct one to extract:

  • geogrid tensile force per unit width at a point, and

  • geogrid axial tensile stress at a point.

Could the Itasca experts please clarify which specific command(s) I should be using to obtain the proper tensile force and tensile stress at a given coordinate?

Thank you very much.

2

Hello @Sudipta

The resultant-nx will give you force per unit length in the x-direction, which in your case points in the global x-direction indicated by the surface-x (1,0,0) keyword you supplied in the structure geogrid history command. I couldn’t find much indication in the documentation about how to determine if the force is tensile or compressive - the best if found is here.

The stress-xx will give the tensile stress (positive value) given your surface coordinate system. See Membrane Action in the documentation for more info.

The surface coordinate system always needs to be set before recovering the stress resultants.

3

Hi @dblanksma

Thanks for your reply.

Could you kindly suggest which commands are most suitable for obtaining the tensile force and stress at specific points or coordinates in the geogrid for my case?

4

Hello @everyone, @itascan,

I am writing a brief follow-up because I urgently need clarification in order to proceed with my analysis.

I am trying to understand which exact FLAC3D command should be used to extract the correct geogrid tensile force per unit width and tensile stress at a specific points or co-ordinates in my MSE wall model. I previously tried:

structure geogrid history resultant-nx position (x,y,z) surface-x (1,0,0)
structure geogrid history stress-xx position (x,y,z) depth-factor 0

Any clarification, even brief, would be extremely helpful. I am currently stuck at this step, so I would be very grateful for the support.

Thank you again to the Itasca team and community members.