Question about Dirichlet Boundary Conditions

Hello, everyone
I am very confused about Dirichlet Boundary Conditions.
Such as the example given by the manual:
bc = [mybc]
label = group A
bc_dof = [2] ; fixed displacement in z direction
db_initial = spatialdata.spatialdb.SimpleDB
db_initial.iohandler.filename = disp_A.spatialdb
db_initial.query_type = nearest ; change query type to nearest point algorithm
db_rate = spatialdata.spatialdb.UniformDB
db_rate.values = [displacement-rate-z] = [1.0e-06 * m/s] ; velocity is 1.0e-06 m/s

From the manual, bc_dof = [2] (fixed displacement in z direction) means the displacement in z direction is fixed, but it laterly says group A moves at a speed of 1.0e-06 m/s in the z direction. How could this happen? Group A is fixed and moves at a speed of 1.0e-06m/s in z direction at the same time? I mean if Group A is fixed, it cannot move, right?

Hope someone can give a detailed explanation.
Best regards,
Tu Xiang

The displacement in z is fixed on this boundary. It is is fixed to a time-dependent value prescribed by the spatial database.

Being fixed means that it does not respond to the internal forces, but rather is determined completely by the spatialdb.


Matt Knepley