Residual Mode

For several seismic codes there is a residual mode option available.

To calculate pseudostatic forces for a residual mode, do the following:

1. Select Residual mode in the dialog for defining a seismic case.
2. Define a vibration frequency of the last-calculated vibration mode or as a user defined value.
3. Define an increment coefficient for pseudostatic forces of the residual mode.

The aim of the residual mode (pseudomode) option is to consider the seismic analysis of the total structure mass without increasing the number of calculated vibration modes. The sum of mass participation in structure vibrations calculated in the modal analysis grows as the number of modes increases. However, to obtain 90-percent mass participation required by codes, it is necessary to calculate a large number of eigenmodes, lengthening calculations and result analysis.

Note: The Residual mode option works correctly only if the modal analysis, preceding the seismic analysis case, uses the lumped mass matrix without rotations.

Pseudomode determines an additional vibration mode, which includes the missing percentage of mass participation evaluated on the basis of the previous vibration modes. Such a procedure is allowed for by the French seismic codes.

Values of pseudostatic forces of the additional residual mode are calculated as follows:

1. Pseudostatic forces for successive modes are calculated according to code regulations for an assumed number of eigenvibration modes.
2. For each mode and at each node on each direction X, Y, Z, the the acting mass is calculated as a result of division of a pseudostatic force value by the acceleration corresponding to a given frequency in the diagram of vibration excitation provided in the code.
3. Masses at each node are summed for all the modes for each global direction X, Y, Z.
4. To obtain missing masses not participating in the calculated eigenvibration modes, the obtained mass sums are subtracted from the structure mass concentrated at nodes. For example, from the elements of the lumped mass matrix.
5. Additional masses are then multiplied by direction coefficients defined for a given seismic analysis case.
6. The user defines what frequency should correspond to the additional residual mode.
7. Pseudostatic forces for the residual mode are calculated as a result of multiplication of mass values at nodes for successive directions by the acceleration corresponding to a frequency in the diagram of vibration excitation provided in the code.
8. Pseudostatic forces for the residual mode are multiplied by the user-defined coefficient.

Results for the residual mode are calculated by finding a solution of a static case involving pseudostatic forces.

Note: If a residual mode is used in a seismic case, there will be no compatibility between sums of applied forces and reactions in the reaction table. The difference stems from pseudostatic forces applied in the residual mode.