Modal Analysis 
Setting  Description 

Calculate Eigen Solution 
Check this option to request a modal
analysis.Note: If a time history or response spectrum load
case has been added, then a modal analysis is required and
cannot be unchecked.

Eigen Method 
 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.

Set Number of Modes 
Check this option to specify the highest frequency (cycle/sec) to be considered for the dynamic analysis. Type the Value to use. 
Shift Modes 
(for when Set Number of Modes is checked) Check this option to specify a number of eigen vectors per shift. This option is used when the solver fails to extract the Set Number of Modes due to insufficient memory. 
Set Shift Frequency 
(for Arnoldi/Lanczos method only) Check this option to specify the frequency to be used as an initial shift. 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. 
Set Max Frequency 
Check this option to set the highest frequency (cycle/sec) to be considered for dynamic analysis. 

Time History 
Setting  Description 

Time Step 
Type the solution time step used in the stepbystep
integration of the uncoupled equations. Values smaller than
0.00001 will be reset to the default value of 0.001389
seconds. 
Include Missing Mass 
Check this option to include the missing mass procedure in the time history analysis. 
Set Max Time History 
Check this option to specify an ending time for a time history analysis in the Value field. If not specified, the time history will end when the last forcing function ends. 

Damping 
Setting  Description 

Modal Damping 
 Single – a single specified value used by all
modes.
 Specify – explicitly defined modal damping
ratios to use for some or all modes.
 Composite – based on values specified for each
material. No additional values are provided here.
 Evaluate – modal damping which is calculated for
all modes.

Damping Ratio 
For Single
damping, type the Damping Ratio to use
for the entire structure. A value of 0.05 is used by
default. 
Damping Ratios 
For explicit damping ratios for each
mode, click in the Damping Ratios to
display the modal damping table. Type the
Damping value to use with each mode.
A value of 0.05 is used by default for the first six modes. 
Interpolation 
Select the method of interpolation to use for Evaluate modal damping:
 Calculated – the values of modal damping are determined based on Minimum Damping Ratio and Maximum Damping Ratio provided.
 Specified – the values of the modal damping are
evaluated based on specified massproportional factor,
Alpha, and stiffnessproportional
factor, Beta. The minimum and maximum modal
damping values are used as lower and upper limits for
the evaluated damping values. Default values are
provided for both methods.


Response Spectrum 
Setting  Description 

Modal Combination 
Select the method of combining the responses from each mode into a total response.
 SRSS
 Square Root of Summation of Squares method.
 ABS
 Absolute sum. This method is very conservative and
represents a worst case combination.
 CQC
 Complete Quadratic Combination method (Default).
This method is recommended for closely spaced modes instead of SRSS.
Resultants are calculated as:
$F=\sqrt{\sum _{n}\sum _{m}{f}_{n}{\rho}_{nm}{f}_{m}}$
where  ρ_{nm}
 =   $\frac{8{\zeta}^{2}(1+r){r}^{2/3}}{{(1{r}^{2})}^{2}+4{\zeta}^{2}r{(1+r)}^{2}}$
  r
 =   ω_{n}/ω_{m} ≤ 1.0

Note: The
crossmodal coefficient array is symmetric and all terms are positive.
 ASCE
 NRC Regulatory Guide Rev. 2 (2006) Gupta method for
modal combinations and Rigid/Periodic parts of modes are used. The ASCE498
definitions are used where there is no conflict. ASCE498 Eq. 3.221 (modified
Rosenblueth) is used for close mode interaction of the damped periodic portion
of the modes.
 TEN
 Ten Percent Method of combining closely spaced modes.
NRC Reg. Guide 1.92 (Rev. 1.2.2, 1976).
 CSM
 Closely Spaced Method as per IS:1893 (Part 1)2002
procedures.
 GRP
 Closely Spaced Modes Grouping Method. NRC Reg. Guide
1.92 (Rev. 1.2.1, 1976).
The CQC and ASCE498 methods require
damping. ABS, SRSS, CRM, GRP, and TEN methods do not use damping unless spectraperiod
curves are made a function of damping (see File option below). CQC, ASCE, CRM,
GRP, and TEN include the effect of response magnification due to closely spaced
modal frequencies. ASCE includes more algebraic summation of higher modes. ASCE
and CQC are more sophisticated and realistic methods and are recommended.
For the ASCE method, type the f1 and f2 frequencies for ASCE 498.

Missing Mass 
Check this option to use the "Missing Mass" method to include the static effect of the masses not represented in the modes.
Note: For the ASCE combination method, this option is automatically checked.
Setting  Description 

Set Spectral Acceleration 
Check this option to specify the spectral
acceleration to use for the missing mass mode. Type
the Value
(with length/sec^{2} units). If
not specified, then the spectral acceleration at the
Zero Period Acceleration frequency is used. If the
ZPA frequency is also not specified, then the
spectral acceleration at a frequency of 33 hz is
used. 
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. For this option, the
spectral acceleration at the Value
(entered in Hertz) is used. If not specified, then a
frequency of 33 hz, and thus the corresponding
spectral acceleration at that frequency, is
used. 

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.

