Comparison of FNA and direct integration
From CSI Wiki
Generally speaking, the FNA is more accurate than direct integration. User must have enough good modes for the FNA to be accurate, just as the user must have small time-steps for direct-integration to be accurate.
The two methods handle damping differently. Direct-integration uses mass- and stiffness-proportional damping, which means that damping at very low and very high frequencies can exceed critical, whereas the FNA method limits proportional damping to 0.99995 of critical at the frequency extremes. For direct-integration, the stiffness proportional damping is proportional to the current stiffness, whereas for FNA, the stiffness proportional damping is fixed and is based on the effective stiffness assigned to the link elements. With care, the effects can be similar, but not identical.
Certain models, and perhaps the physical structures, can be be fairly sensitive to loading and physical parameters, and hence also to the analysis method. This was observed before for certain structures with sliding isolators. As such, the user may expect to see different results with and without the foundation springs, and with or without the substructure. The user may want to check this using the FNA due to its efficiency.
It is recommended to use convergence tolerance smaller than about 0.001 for direct-integration. For FNA, using the default tolerances is recommended.
| Time History Analysis e | |
| Topic | Overview · Tips · FAQ |
| Analysis Types | Nonlinear modal time history analysis (Fast Nonlinear Analysis, FNA) |
| Miscellaneous | Progressive collapse analysis · Comparison of FNA and direct integration |
| Related Categories | Time History Analysis |
