To set response spectrum parameters
To set the parameters that apply to all response spectrum definitions and loads in your structure model, use the following procedure.
The program uses some parameters for all response spectrum load definitions. These are set in the program Options dialog.
 Select the File ribbon tab and then select Options from the backstage menu. The Options dialog opens.
 Select the Dynamics tab.

Set the Modal Analysis options used for all dynamic loads:

Select the Eigen Method option:
 Subspace – the default subspace iteration method
 Arnoldi/Lanczos – Instructs the solver to use Arnoldi/Lanczos method for extraction of eigen vectors.
 Ritz Vector – Use load dependent Ritz vectors method for extraction of eigen vectors.
 (Optional) To specify the highest frequency to be considered for the dynamic analysis, check the Set Number of Modes option and then type the Value in cycles/sec.
 (Optional) (for when Set Number of Modes is checked) To specify a number of eigen vectors per shift, check the Shift Modes option and type the integer Value for the shift. This option is used when the solver fails to extract the Set Number of Modes due to insufficient memory.
 (Optional) (for Arnoldi/Lanczos method only) To specify the frequency to be used as an initial shift, check the Set Shift Frequency and then type the Value in Hertz. The solver will look for eigenvalues close to the shift. The eigenvalues found may not necessarily be the smallest values (i.e., closest to zero). If a full scale eigen solution is required, then this option should not be used.
 (Optional) To set the highest frequency (cycle/sec) to be considered for dynamic analysis, check the Set Max Frequency and then type the Value in Hertz.

Select the Eigen Method option:

Set the Response Spectrum options:
 Select the method of Modal Combination used for combined results. If the ASCE method is selected, enter the f1 and f2 frequency values to use. Also, note that Missing Mass is automatically selected for this method.
 (Optional) Check the Missing Mass option to use the missing mass method to include the static effect of the masses not represented in the modes or not.

If missing mass is to be considered:
Option Description Set Spectral acceleration Check this option to specify the spectral acceleration to use for the missing mass mode in the Value field. Set Zero Period Acceleration Frq Check this option to specify a frequency at which the corresponding spectral acceleration value is used for the missing mass mode in the Value field. 
Select the method of determining the Sign of Results.
 Unsigned – No sign is assigned to the combined results.
 Largest Effect – his option results in the creation of signed values for all results. The sum of squares of positive values from the modes are compared to sum of squares of negative values from the modes. If the negative values are larger, the result is given a negative sign. This command is ignored for ABS option.
 Highest Participation – All results will have the same sign as the mode with the greatest percent participation in the excitation direction.
 Dominant – The dominant mode method. All results will have the same sign as the Mode number specified alone would have if it were excited then the scaled results were used as a static displacements result. If not specified, the first mode is used. Must be an integer greater than zero.

Set the Damping options:
 Select the Modal Damping method to use.
 For Single, Specify, and Evaluate options, specify the damping values or parameters to use.
 Click OK.