Moving Load Train Analysis - Automatic Time Step Selection#

The automatic time step selection serves to verify and find a suitable time step size Δt by the following steps:

Calculation of the start value of the time step size#

The start value of the time step size Δts is set as 1/20 of the oscillation period of the highest relevant frequency f_max. The value for f_max has to be defined by the user based on the specifications of the design code and the eigenfrequencies of the structure.

Verification of time step size by a convergence study#

In this procedure, the specified result-time curve is first computed using Δts. Then, Δts is multiplied by the factor for the reduction of time step size (default 0.5) and the result-time curve is recomputed using the smaller time step size. The verification is considered successful if the result does not change by more than the given tolerance for convergence (default 5%). Otherwise, the time step size is further reduced and checked again for convergence.

Assessment of time step sizes larger than the start value Δts#

In order to reduce computation time, it is possible to check, if time steps larger than Δts also yield accurate enough results. In this case the results obtained for those larger time step sizes are compared with the final iteration of the convergence study. If the results fullfill the tolerance as well, the larger time step size is taken for the calculation.