Mechanical APDL Structural Analysis Guide

Figure 5.1: Examples of Load-Versus-Time Curves

Load

Stepped(KBC,1)

1

2

3 4

5

Time

(a)

Load

Steady-state

analysis

Stepped(KBC,1)

Time

(b)

1

2

3

4

5

The first load step you apply is usually to establish initial conditions. You then specify the loads and
load step options for the second and subsequent transient load steps. For each load step, you need to
specify both load values and time values, along with other load step options such as whether to step
or ramp the loads, use automatic time stepping, and so on.You then write each load step to a file and
solve all load steps together. Establishing initial conditions is described below; the remaining tasks are
described later in this chapter.

The first step in applying transient loads is to establish initial conditions (that is, the condition at Time
= 0). A transient dynamic analysis requires two sets of initial conditions (because the equations being

solved are of second order): initial displacement (uo) and initial velocity (

ɺ

). If no special action is

taken, both uo and

ɺ


are assumed to be zero.Initial accelerations (

ɺɺ

) are always assumed to be zero,

but you can specify nonzero initial accelerations by applying appropriate acceleration loads over a small
time interval.

The following text describes how to apply different combinations of initial conditions:

The term initial displacement as it appears in the following text can be any combination of displacement

and force loads. Also, all load steps in the example input fragments that are run without applied transient

effects (TIMINT,OFF) should be converged.

Zero initial displacement and zero initial velocity -- These are the default conditions, that is, if uo =

ɺ

=

0, you do not need to specify anything. You may apply the loads corresponding to the first corner of
the load-versus-time curve in the first load step.

Nonzero initial displacement and/or nonzero initial velocity -- You can set these initial conditions with the
IC command.
Command(s):IC
GUI: Main Menu> Solution> Define Loads> Apply> Initial Condit'n> Define

Caution

Be careful not to define inconsistent initial conditions. For example, if you define an initial

velocity at a single DOF, the initial velocity at every other DOF will be 0.0, potentially leading

to conflicting initial conditions. In most cases, you will want to define initial conditions at

every unconstrained DOF in your model. If these conditions are not the same at every DOF,

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Performing a Full Transient Dynamic Analysis