# How to Apply Lateral Boundary Attachment for a 1D Model in FLAC2D 9.4?

**URL:** <https://forum.itascainternational.com/t/how-to-apply-lateral-boundary-attachment-for-a-1d-model-in-flac2d-9-4/2417>\
**Category:** Uncategorized\
**Created:** [October 5, 2026, 3:13am UTC](https://forum.itascainternational.com/t/how-to-apply-lateral-boundary-attachment-for-a-1d-model-in-flac2d-9-4/2417 "2026-10-05T03:13:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Miguel\_S](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@Miguel\_S](https://forum.itascainternational.com/u/Miguel_S)\
**Post date:** [October 5, 2026, 3:13am UTC](https://forum.itascainternational.com/t/how-to-apply-lateral-boundary-attachment-for-a-1d-model-in-flac2d-9-4/2417/1 "2026-10-05T03:13:27Z")

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Hi everyone,

I am migrating a 1D dynamic analysis from **FLAC 8.1 to FLAC2D 9.4** and I am having difficulty reproducing the lateral boundary attachment condition.

In my FLAC 8.1 model, the two lateral boundaries were attached to each other using:

attach aside from 1,1 to 1,\_mj bside from 2,1 to 2,\_mj

This was used to make the corresponding gridpoints of the two lateral boundaries have the same horizontal motion, which is required to reproduce **1D soil-column behavior**.

For the dynamic stage, I also released the horizontal restriction using:

free x j 2 \_mj

n FLAC2D 9.x, I understand that the `attach` command has been replaced/modified, but I am not sure what is the correct syntax to reproduce this specific condition between the two lateral boundaries of a 1D model.

**What is the recommended FLAC2D 9.x code to attach the corresponding gridpoints of the two lateral boundaries in a 1D model, equivalent to the FLAC 8.1 command above?**

I am specifically looking for the **lateral attachment between the two sides** , rather than using free-field boundaries.

Any example or guidance would be greatly appreciated. Thank you!

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**Author:** ![EmirhanAltinok](https://avatars.discourse-cdn.com/v4/letter/e/58f4c7/32.png) [@EmirhanAltinok](https://forum.itascainternational.com/u/EmirhanAltinok)\
**Post date:** [October 5, 2026, 10:26am UTC](https://forum.itascainternational.com/t/how-to-apply-lateral-boundary-attachment-for-a-1d-model-in-flac2d-9-4/2417/2 "2026-10-05T10:26:59Z")

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Hello Miguel\_S

I am using following function to attach side boudary nodes :

fish define PeriodicBC(gPnt,gpGroup,gpSlot)  
global M\_xMin, M\_xMax, \_Attach\_List  
global ModelSize = M\_xMax-M\_xMin  
if gp.isgroup(gPnt,gpGroup,gpSlot)  
local pos1 = gp.pos(gPnt)  
local pos2 = pos1 + vector(ModelSize,0)  
local id1 = gp.id(gp.near(pos1))  
local id2 = gp.id(gp.near(pos2))  
command  
zone attach gridpointid [id2] to-gridpointid [id1]  
endcommand  
endif  
end

zone gridpoint group ‘bound=left’ range p-x [M\_xMin] tol 1e-3  
zone gridpoint group ‘bound=right’ range p-x [M\_xMax] tol 1e-3  
[PeriodicBC(::gp.list,‘left’,‘bound’)]

You can use this function by changing [M\_xMin] and [M\_xMax] model boundaries for both 1D and 2D analyses.