TR.32.10.1.8 Response Spectrum Specification per IS: 1893 (Part 4)2015
This command may be used to specify and apply the RESPONSE SPECTRUM loading as per IS: 1893 (Part 4)2015 for dynamic analysis of industrial and stacklike structures.
General Format
The data in the following format can be contained all on a single line or broken into two or three lines, so long as the second and third lines start with the ACC and DOMINANT or SIGN commands.
SPECTRUM compmethod IS1893 2015P4 LOAD ( TORSION (DECCENTRICITY f8) (ECCENTRICITY f9) ) *{ X f1  Y f2  Z f3 }
{ ACCELERATION  DISPLACEMENT } (SCALE f4) {DAMP f5  CDAMP  MDAMP } (MISSING f6) (ZPA f7) (IGNORE f13) ({ DOMINANT f10  SIGN }) (IMR f11) (STARTCASE f12)
The following command (SOIL TYPE parameter or response spectra data pairs) must be in a separate line.
{ SOIL TYPE f11  *{ P1,V1; P2,V2; P3,V3;…PN,VN } }
Where:
Parameter  Default Value  Description 

DECCENTRICITY f8  When ECC > 0, DEC
defaults to 1.5 When ECC < 0, DEC defaults to 1.0 
(Optional input) It is a factor which when multiplied with static eccentricity (i.e., eccentricity between center of mass and center of rigidity) gives dynamic eccentricity. Since the applied load is acting at the center of mass, the effect of inherent torsion arising due to static eccentricity is included in the analysis. Note: The torsion arising due to dynamic
eccentricity (i.e,. static eccentricity multiplied by dynamic
amplification factor) between center of mass and center of
rigidity is applied along with accidental torsion, as per the
recommendations of Cl. 7.8.2 of the IS 1893 Part 1
specification. The dynamic eccentricity is automatically
calculated by the program while you can specify the amount of
accidental eccentricity (if not specified, the default of 5% of
lateral dimension of the floor in the direction of the
earthquake will be considered). For details See Torsion
Methodology.

ECCENTRICITY f9  0.05 
It is a factor which indicates the extent of accidental eccentricity. For all buildings this factor is to be provided as 0.05. However, for highly irregular buildings this factor may be increased to 0.10. This factor is to be externally provided to calculate design eccentricity. Since accidental eccentricity can be on either side, you must consider lateral force acting at a floor level to be accompanied by a clockwise or a counterclockwise accidental torsion moment. If the f9 value is positive, it indicates clockwise torsion whereas a negative value indicates counterclockwise torsion. 
X f1  Y f2  Z f3  0  Factors for the input spectrum to be applied in X, Y, & Z directions. These must be entered as the product of [(Z/2)×(I/R)] for standard specific spectra. Any one or all directions can be input. Directions not provided will default to zero. Based on Clause7.3 of IS 1893(Part 4): 2015. 
SCALE f4  1  Linear scale factor by which design horizontal acceleration spectrum will be multiplied. This factor signifies that the structures and foundations, at which level base shear will be calculated, are placed below the ground level. 
DAMP f5  0.05 
The damping ratio.
Specify a value of exactly 0.0000011 to ignore damping.

MISSING f6 
Optional parameter to use "Missing Mass" method. The static effect of the masses not represented in the modes is included. The spectral acceleration for this missing mass mode is the f6value entered in length/sec^{2} (this value is not multiplied by SCALE). If f6is zero, then the spectral acceleration at the ZPA f7frequency is used. If f7is zero or not entered, the spectral acceleration at 33Hz (Zero Period Acceleration, ZPA) is used. The results of this calculation are SRSSed with the modal combination results. 

ZPA f7  33 [Hz]  The zero period acceleration value for use with MISSING option only. Defaults to 33 Hz if not entered. The value is printed but not used if MISSING f6 is entered. 
DOMINANT f10  1 (1st Mode)  The dominant mode method. All results will have the same sign as mode number f10 alone would have if it were excited then the scaled results were used as a static displacements result. Defaults to mode 1 if no value entered. If a 0 value entered, then the mode with the greatest % participation in the excitation direction will be used (only one direction factor may be nonzero). 
IMR f11  1  The number of individual modal responses (scaled modes) to be copied into load cases. Defaults to one. If greater than the actual number of modes extracted (NM), then it will be reset to NM. Modes one through f11 will be used. Missing Mass modes are not output. 
STARTCASE f12  Highest Load Case No. + 1  The primary load case number of mode 1 in the IMR parameter. Defaults to the highest load case number used so far plus one. If f12 is not higher than all prior load case numbers, then the default will be used. For modes 2 through NM, the load case number is the prior case number plus one. 
SOIL TYPE f11  2  The soil type present. Depending upon timeperiod, types of soil and damping, average response acceleration coefficient, Sa/g is calculated from reference to Fig.1 of this code where:

custom P1,V1; P2,V2; P3,V3; … Pn,Vn  Data is part of input immediately following spectrum command for a "custom" response spectrum. Period  Value pairs (pairs separated by semicolons) are entered to describe the spectrum curve. Period is in seconds and the corresponding Value is acceleration (current length unit/ sec^{2}). If data is in g acceleration units then the factor by which spectra data will be multiplied is g to the current length unit (9.81, 386.4, etc).  
IGNORE f13  0.009 
(Optional input) It indicates the mass participation (in percent) of those modes to be excluded while considering torsion provision of IS1893. Depending upon the model it may be found that there are many local modes and torsional modes whose mass participation is practically negligible. These modes can be excluded without much change in the final analysis result. If not provided all modes will be considered. If none provided the default value of 0.009% will be considered. If IGN is entered on any one spectrum case it will be used for all spectrum cases. Note: If the value
of f14 is considerable it may lead to considerable variation of analysis result
from the actual one. Hence caution must be taken while using IGNORE command.
If the MODE SELECT command is provided along with the IGNORE command, the number of modes excluded from the analysis will be those deselected by the MODE SELECT command and also those deselected by the IGNORE command. 
1893 2015P4 indicates the analysis as per IS:1893(Part 4)2015 procedures.
combmethod = { SRSS  ABS  CQC  ASCE  TEN  CSM  GRP } are methods 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.
 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).
 TORSION
 indicates that the torsional moment (in the horizontal plane) arising due to eccentricity
between the center of mass and center of rigidity needs to be considered.
See Torsion for additional information.
Lateral shears at story levels are calculated in global X and Z directions. For global Y direction the effect of torsion will not be considered.
The Torsion methodology as per Clause 10.4 of IS 1893 (Part4): 2015 remains same as IS1893(Part1):2016 and is only applicable for ST=4.
 ACCELERATION or DISPLACEMENT
 DAMP, MDAMP, and CDAMP
 select source of damping input:
 DAMP indicates to use the f2 value for all modes
 MDAMP indicates to use the damping entered or computed with the DEFINE DAMP command if entered, otherwise default value of 0.05 will be used
 CDAMP indicates to use the composite damping of the structure calculated for each mode. You must specify damping for different materials under the CONSTANT specification
For response spectra analysis, the damping ratio coefficients for steel and concrete that are recommended (other than the default value of 5%) for industrial structures as per Clause 9.4 (Table 5) are as follows:Material DBE MCE Steel 0.02 0.04 Reinforced concrete 0.05 0.07 Prestressed concrete 0.03 0.05  LINEAR or LOGARITHMIC
 Select Linear or Logarithmic interpolation of the input Spectra versus Period curves for determining the spectra value for a mode given its period. Linear is the default. Since Spectra versus Period curves are often linear only on LogLog scales, the logarithmic interpolation is recommended in such cases; especially if only a few points are entered in the spectra curve.
 SIGN
 This 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.
 SAVE
 This option results in the creation of a acceleration data file (with the model file name and an .acc file extension) containing the joint accelerations in g’s and radians/sec^{2}. These files are plain text and may be opened and viewed with any text editor (e.g., Notepad).
Individual Modal Response Case Generation
Individual modal response (IMR) cases are simply the mode shape scaled to the magnitude that the mode has in this spectrum analysis case before it is combined with other modes. If the IMR parameter is entered, then STAAD.Pro will create load cases for the first specified number of modes for this response spectrum case (i.e., if five is specified then five load cases are generated, one for each of the first five modes). Each case will be created in a form like any other primary load case.
The results from an IMR case can be viewed graphically or through the print facilities. Each mode can therefore be assessed as to its significance to the results in various portions of the structure. Perhaps one or two modes could be used to design one area/floor and others elsewhere.
You can use subsequent load cases with TR.32.11 Repeat Load Specification combinations of these scaled modes and the static live and dead loads to form results that are all with internally consistent signs (unlike the usual response spectrum solutions). The modal applied loads vector will be omega squared times mass times the scaled mode shape. Reactions will be applied loads minus stiffness matrix times the scaled mode shape.
With the Repeat Load capability, you can combine the modal applied loads vector with the static loadings and solve statically with PDelta or tension only.
Refer to TR.32.10.1.1 Response Spectrum Specification  Custom for additional details on IMR load case generation.