Time-history function for linear and nonlinear time-history analyses

The initial condition for time function is handled slightly different for linear and nonlinear time-history load cases as described below:

• Linear cases always start from zero, so the function is assumed to start from zero.
• Nonlinear cases may continue from a previous case. It is assumed that the function is also continuing from the previous value, so the function is really treated as being relative to the starting value. If you want to start at zero, the function should be defined to start at zero value. This enables you to take a long record and break it into multiple sequential analyses using a single function and arrival times, without having to create multiple modified functions.