Prescribed conditions allow you to model nonzero initial conditions, boundary conditions, loads, and predefined fields. The following topics are discussed:

## Amplitude variations

Complex time- or frequency-dependent boundary conditions, loads, and predefined fields can be specified by referring to an amplitude curve in the prescribed condition definition. Amplitude curves are explained in Amplitude Curves.

In Abaqus/Standard if no amplitude is referenced from the boundary condition, loading, or predefined field definition, the total magnitude can be applied instantaneously at the start of the step and remain constant throughout the step (a “step” variation) or it can vary linearly over the step from the value at the end of the previous step (or from zero at the start of the analysis) to the magnitude given (a “ramp” variation). You choose the type of variation when you define the step; the default variation depends on the procedure chosen, as shown in Defining an analysis.

In Abaqus/Standard the variation of many prescribed conditions can be defined in user subroutines. In this case the magnitude of the variable can vary in any way with position and time. The magnitude variation for prescribing and removing conditions must be specified in the subroutine (see User Subroutines and Utilities).

In Abaqus/Explicit if no amplitude is referenced from the boundary condition or loading definition, the total value will be applied instantaneously at the start of the step and will remain constant throughout the step (a “step” variation), although Abaqus/Explicit does not admit jumps in displacement (see Boundary conditions in Abaqus/Standard and Abaqus/Explicit). If no amplitude is referenced from a predefined field definition, the total magnitude will vary linearly over the step from the value at the end of the previous step (or from zero at the start of the analysis) to the magnitude given (a “ramp” variation).

When boundary conditions are removed (see Boundary conditions in Abaqus/Standard and Abaqus/Explicit), the boundary condition (displacement or rotation constraint in stress/displacement analysis) is converted to an applied conjugate flux (force or moment in stress/displacement analysis) at the beginning of the step. This flux magnitude is set to zero with a “step” or “ramp” variation depending on the procedure chosen, as discussed in Defining an analysis. Similarly, when loads and predefined fields are removed, the load is set to zero and the predefined field is set to its initial value.

## Applying boundary conditions and loads in a local coordinate system

You can define a local coordinate system at a node as described in Transformed coordinate systems. Then, all input data for concentrated force and moment loading and for displacement and rotation boundary conditions are given in the local system.

## Loads and predefined fields available for various procedures

Table 1. Available loads and predefined fields.
Added mass (concentrated and distributed) Abaqus/Aqua eigenfrequency extraction analysis (Natural frequency extraction)
Base motion Procedures based on eigenmodes:
Transient modal dynamic analysis
Response spectrum analysis
Random response analysis
Boundary condition with a nonzero prescribed boundary All procedures except those based on eigenmodes
Connector motionConnector load All relevant procedures except modal extraction, buckling, those based on eigenmodes, and direct steady-state dynamics
Cross-correlation property Random response analysis
Current density (concentrated and distributed) Coupled thermal-electrical analysis
Fully coupled thermal-electrical-structural analysis
Current density vector Eddy current analysis
Electric charge (concentrated and distributed) Piezoelectric analysis
Equivalent pressure stress Mass diffusion analysis
Film coefficient and associated sink temperature All procedures involving temperature degrees of freedom
Fluid flux Analysis involving hydrostatic fluid elements
Fluid mass flow rate Analysis involving convective heat transfer elements
Flux (concentrated and distributed) All procedures involving temperature degrees of freedom
Force and moment (concentrated and distributed) All procedures with displacement degrees of freedom except response spectrum