In general, the solution accuracy of a linear dynamic analysis study depends on:
- Faithful representation of the dynamic environment like and loads and restraints.
- Number of normal modes considered in the solution. The highest frequency used in the analysis should be larger than the highest frequency of the loads. You can assess convergence by increasing the number of modes and comparing the results.
- Solution step. For modal time history, the step should be a fraction of the period of the highest load frequency used in the analysis.
- Other solution parameters depending on the analysis type.
As a general rule of thumb, the number of modes considered must contribute to a total mass participation factor of at least 80% of the system mass in the direction of excitation. Special cases may require more or less modes.
For random vibration studies, in addition to the 80% mass participation factor recommendation, the range of natural frequencies considered for the solution must cover the frequency spectrum of the excitation.