Question

If I want to model free vibration, this is, saying that the structure should start vibrating with some known initial conditions (initial displacement and initial velocity at t=0) is there any simple way to introduce these conditions in SAP and start the time history from those conditions?

Answer

The initial conditions for a time-history load case can be either zero initial conditions or they can continue from a previous nonlinear state. It is not possible to directly specify initial conditions for free vibration analysis, but you could use end conditions for other nonlinear load case as initial condition for the free vibration analysis.