Boundary

*Boundary, [amplitude=<amplitude name>, <option>]
<node set name>, <degree of freedom>, <value>

This command is used to prescribe boundary conditions on nodes or node sets. It is possible to define more than one *Boundary command per step. However, the defined boundary conditions are only active in the step for which they are defined.

• amplitude=<amplitude name>

  The optional amplitude parameter allows for the specification of an amplitude by which the boundary values are scaled (mainly used for non-linear static and dynamic calculations). This only makes sense for non-zero boundary values. Thus, in that case, the values entered on the sublines of the *Boundary command are interpreted as reference values to be multiplied with the (time dependent) amplitude value to obtain the actual value.

• <option>

  The second optional keyword <option> can be used to prescribe more "advanced" boundary conditions. Use velocity or acceleration to apply the first or second time derivative of the prescribed degree of freedom as a boundary condition. The option increment only prescribes the increment (i.e. the change) of a variable such that its original value at the start of the step is kept. To prescribe a hydrostatic (linear increase with depth) distribution, e.g. for pore water pressure, use hydrostatic as <option>. Notice that if hydrostatic is used, the subsequent line differs from the standard one.

• Per default, the subsequent lines take the flowing form:
  <node set name>, <degree of freedom>, <value>. They can be repeated as often as needed.

  • <node set name> = name of the node set to which the boundary condition is applied

  • <degree of freedom> = the degree of freedom to be constrained. The following degrees of freedom can be modified:

    Degrees of freedom	Description
    u1	Displacement in x1-direction
    u2	Displacement in x2-direction
    u3	Displacement in x3-direction
    pw	Pore-water pressure
    pa	Pore-air pressure
    w1	Water displacement in x1-direction
    w2	Water displacement in x2-direction
    w3	Water displacement in x3-direction

  • <value> = set this parameter equal to the value to be prescribed to the boundary condition. Notice that this value is interpreted as a reference value, which is multiplied with the amplitude value (in case an amplitude is assigned to the boundary condition).

• If hydrostatic is used to define the boundary condition, the subsequent line differs from the one above and takes the form:
  <node set name>, <degree of freedom>, <value>, <zero level>

  • <node set name> = name of the node set to which the boundary condition is applied

  • <degree of freedom> = the degree of freedom to be constrained.

  • <value> = inclination of the hydrostatic distribution, e.g. unit weight of water in case of a hydrostatic distribution pore water pressure.

  • <zero level> = coordinate (y-coordinate in 2-D and z-coordinate in 3-D) where the distribution is zero, e.g. location of the water table in case of a hydrostatic distribution of pore water pressure