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Boundary conditions in FLAC


Boundary Conditions in FLAC

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Boundary Conditions
The prescribed conditions applying to the boundary of the model can be: - Real (e.g., ground surface) - Artificial (e.g, symmetry axes, boundary lines)
Dam boundary (real)

Water table

Model boundary (artificial)

Horizontal boundary stress (artificial)

Excavation boundary (real) Fixed bottom boundary (artificial)

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Clasification of Boundary Conditions

Mechanical boundary conditions in FLAC can be: 1. Stress boundary conditions:
stresses are fixed using the APPLY command

2. Displacement boundary conditions:
gridpoint velocities are fixed using the FIX command

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Sign Convention for Stresses in FLAC
Normal Stress

(-)

(+)

(compression)

(tension)

(-) Surface Pressure

(+)

Shear Stress (+) (-)

(e.g., pore pressure)

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Commands used to apply Boundary Conditions (1)
APPLY keyword < value > < var vx vy > range
(The command applies a stress boundary condition to an external or internal boundary)

keyword

sxx syy sxy xf yf

xx component of the stress applied to the boundary of the model yy component of the stress applied to the boundary of the model xy component of the stress applied to the boundary of the model x component of the force applied to a gridpoint y component of the force applied to a gridpoint

nstress normal component of the stress sstress shear component of the stress value var value of the variable in the origin of the application range vx vy range i=i1,i2 lineal increment of the variable in the x direction, in the application range lineal increment of the variable in the y direction, in the application range j=j1,j2

< keyword > from i1,j1 to i2,j2 < keyword > long: long way of the range

short: short way of the range (by default)

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APPLY Command
Example 1
syy=1MPa

grid 10 10 mo el gen circle 5 5 2.5 mo null reg 5 5 gen adjust apply syy -1e6 j 1 apply syy -1e6 j 11 apply sxx -1e6 i 1 apply sxx -1e6 i 11 plo grid apply

syy=sxx=1MPa

Example 3
grid 10 10 mo el gen circle 5 5 2.5 mo null reg 5 5 syy=sxx=3MPa gen adjust apply syy -3e6 j 1 apply syy -1e6 j 11 apply sxx -3e6 var 0 2e6 i 1 apply sxx -3e6 var 0 2e6 i 11 plo grid apply

Example 2
grid 10 10 mo el gen circle 5 5 2.5 mo null reg 5 5 gen adjust apply syy -1e6 j 1 apply sxx -1e6 var 0 1e6 i 1 apply sxx -1e6 var 0 1e6 i 11 plo grid apply
syy=sxx=0

syy=sxx=1MPa

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Commands used to apply Boundary Conditions (2)
FIX keyword < mark > i = i1, i2 j = j1, j2
(The command applies displacement boundary conditions, fixing the value of the specified variable)

keyword

x y

fix the x component of velocity in the selected range fix the y component of velocity in the selected range

< mark > only the marked gridpoints of the selected range are fixed

This boundary condition can be deleted using the FREE command, which has the same syntax. Displacement boundary conditions can be alternatively applied using APPLY command (xvel, yvel; nvel, svel) using the syntax previously described for this command.

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FIX Command
Example grid 10 10 mo el gen circle 5.0 5.0 2.5 mo null reg 5 5 gen adjust fix x i 1 fix x i 11 fix y j 1 fix y j 11 plot grid fix hold
10.0

gp i=1

gp i=11 gp j=11

8.0

Y axis

6.0

4.0

2.0

0.0

gp j=1

0.0

2.0

4.0

6.0

8.0

10.0

X axis

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