Reset, reaction forces, constraints and model change
Four small branches sit inside every step. They have little in common except that each one changes something between one step and the next: what the state is reset to, which forces are carried over, how degrees of freedom are tied together, and what the model is made of.
Reset
Reset sets chosen quantities back to zero at the start of the step. It is
No by default; setting it to Yes reveals four switches:
| Quantity |
|---|
| Displacement |
| Strain |
| Stress |
| Void ratio |
The usual use is after a geostatic step. That step establishes equilibrium under self-weight, and whatever small deformation it produces is a numerical artefact rather than something the model should carry forward. Resetting the displacement, and usually the strain, means the stages that follow measure movement from a clean zero.
Stress and void ratio are reset far less often — they are the state the soil is actually in, so clearing them discards the initial condition just established.
See Reset in the numgeo reference manual.
Reaction forces
This branch does two separate things, and both are needed for the strength reduction workflow.
Save reaction forces records the reactions at the constrained degrees of
freedom at the end of the step, per direction — x, y and z independently.
Use saved reaction forces applies previously saved reactions as loads in
this step, again per direction. Each direction takes a Loading rate of
instant or amplitude; with amplitude an amplitude is selected, and an
Edit amplitude command opens the
amplitude manager.
Together these let a boundary be released gradually. A geostatic step holds the model fixed and saves the reactions; the next step re-applies them and reduces them to zero along a ramp, so the model deforms into equilibrium rather than being released all at once.
See Reaction forces.
Constraints
Equal degree of freedom ties the chosen degree of freedom of a group of nodes together, so they all take the same value.
| Field | Options |
|---|---|
| Degree of freedom | u1, u2, u3, pw, pa, w1, w2, w3 |
| Penalty | The stiffness with which the constraint is enforced |
u1 to u3 are the solid displacements, pw and pa the pore water and pore
air pressures, and w1 to w3 the water displacements.
This is how a rigid loading platen is modelled without meshing one: tie the vertical displacement of every node under the footing, and they move as one.
See Equation.
Model change
Model change alters the model partway through the analysis. The Method
is Material, which replaces the material of a region with another chosen from
the material manager;
an Edit materials command opens it directly.
Changing material mid-analysis is how curing is represented — soil replaced by concrete once a wall is cast, or a soft fill given its final stiffness after consolidation.
See Change Material.