Time integration

Time integration controls how the step advances: the size of the increments, how long the step runs, and — for dynamic analyses — the scheme used to integrate the equations of motion.

Time stepping

Value	Unit	Default	Meaning
Initial time step	s	0.01	Size of the first increment
Step time (total)	s	1	Length of the step
Minimum time step	s	0.001	Smallest increment allowed
Maximum time step	s	0.1	Largest increment allowed

The step begins at the initial time step and adapts from there: increments that converge easily grow towards the maximum, and one that fails is cut back towards the minimum. The two limits are what keep that adaptation sensible.

If the calculation cuts back below the minimum time step it stops rather than continuing, so a step that ends early is usually a sign that something is wrong in the model rather than that the minimum is too large.

Note

In a Static analysis these times are load fractions, not durations. A step time of 1 means the load is fully applied, and an initial time step of 0.01 means the first increment applies one per cent of it. In Transient and Dynamic analyses the same fields are real seconds.

Time integration scheme

Shown for Dynamic analyses only.

Scheme
Hilber-Hughes-Taylor
Backward Euler

Choosing Hilber-Hughes-Taylor reveals the numerical damping parameter α, which must lie between -0.33 and 0:

α	Effect
0	No numerical damping — the undamped Newmark scheme
-0.05	The default, and the same value used by Abaqus and Ansys
-0.33	Significant numerical damping

Numerical damping suppresses spurious high-frequency response that the mesh cannot represent properly. It is not physical damping — that belongs to the material, under Rayleigh damping.

Factor of safety

Shown for Reduction analyses only. In a strength reduction analysis the strength of the materials is reduced step by step until the system fails, and the factor at which it fails is the factor of safety.

Value	Default	Meaning
FoS	1.0	Factor of safety the analysis starts from
ΔFoS	0.05	Increment by which the factor is raised
ΔFoS max	0.05	Largest increment allowed
ΔFoS min	0.0001	Smallest increment allowed

The increment adapts between the two limits in the same way the time step does: it shrinks as failure is approached, which is what fixes the factor of safety precisely. Raising ΔFoS min makes the analysis faster and the answer coarser.

A reduction step needs an equilibrium state to start from, so it follows the steps that apply self-weight and any other loads rather than opening the analysis.

Strength reduction is described under Strength reduction in the numgeo theory manual.