TR.32.10.1.13 Response Spectrum Specification per IBC 2018
General Format
SPECTRUM combmethod IBC 2018 (TORSION) (DECCENTRICITY f20) (ECCENTRICITY f21) *{ X f1  Y f2  Z f3 } ACCELERATION
{DAMP f5  CDAMP  MDAMP } ( {LIN  LOG} ) (MIS f6) (ZPA f7) ({ DOMINANT f10  SIGN }) (SAVE) (IMR f11) (STARTCASE f12)
The command is completed with the following data which must be started on a new line:
{ZIP f8  LAT f9 LONG f13  SS f14 S1 f15 } SITE CLASS (f16) (FA f17 FV f18) TL f19
Where:
Parameter  Default Value  Description 

DECCENTRICITY f20  1.0  Factor to be multiplied with static eccentricity (i.e., eccentricity between center of mass and center of rigidity). 
ECCENTRICITY f21  0.05  Factor for accidental eccentricity. Positive values indicate clockwise torsion and negative values indicate counterclockwise torsion. 
X f1, Y f2, Z f3  0.0  Factors for the input spectrum to be applied in X, Y, & Z directions. Any one or all directions can be input. Directions not provided will default to zero. 
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. 
ZIP f11  The zip code of the site location to determine the latitude and longitude and consequently the S_{s} and S_{1} factors. (IBC 2018, ASCE 716 Chapter 22)  
LAT f12  The latitude of the site used with the longitude to determine the S_{s} and S_{1} factors. (IBC 2018, ASCE 716 Chapter 22)  
LONG f13 
The longitude of the site used with the latitude to determine the S_{s} and S_{1} factors. (IBC 2018, ASCE 716 Chapter 22) 

SS f14  Mapped MCE for 0.2s spectral response acceleration. (IBC
2018, ASCE 716 Section 11.4.1) This is obtained using the USGS web service. 

S1 f15  Mapped spectral acceleration for a 1second period. (IBC
2018, ASCE 72016 Section 11.4.1) This is obtained using the USGS web service. 

CLASS f16  Enter A through F for the Site Class as defined in the IBC code. (IBC 2018 ASCE 72016 Section 20.3)  
FA f17  Optional ShortPeriod site coefficient at 0.2s. Value must be provided if SCLASS set to F (i.e., 6). (IBC 2018, ASCE 72016 Section 11.4.3)  
FV f18  Optional LongPeriod site coefficient at 1.0s. Value must be provided if SCLASS set to F (i.e., 6). (IBC 2018, ASCE 72016 Section 11.4.3)  
TL f19  LongPeriod transition period in seconds. (IBC 2018, ASCE 716 Chapter 22) 
combmethod = { SRSS  ABS  CQC  ASCE  TEN  CSM  GRP } are methods of combining the responses from each mode into a total response.
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.
 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).
IBC 2018 indicates that the spectrum should be calculated as defined in the IBC 2018 specification.
 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.
 ACCELERATOIN
 indicates that the Acceleration spectra will be entered.
 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
 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.
When FILE filename is entered, the interpolation along the damping axis will be linear.
 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).
Methodology
The methodology for calculating the response spectra is defined in ASCE 72016, section 11.4. The following is a quick summary:
 Input S_{s} and S_{1} (this could have been searched from database or entered explicitly)

Calculate
S_{ms}= F_{a} × S_{s}
and
S_{m1} = F_{v} × S_{1}
Where:
F_{a} and F_{v} are determined from the specified site classes A – E and using tables 11.41 and 11.42. For site class F, the values must be supplied. These are required to be provided by the user. You may also specify values for F_{a} and F_{v} in lieu of table values.

Calculate
S_{ds} = (2/3) S_{ms}
and
S_{d1} = (2/3) S_{m1}
The spectrum is generated as per section 11.4.5.
 Exceptions 1 in Clause 11.4.8 of ASCE 716 is implemented. In exception 1, for Site class E and S_{s} > 1.0, Fa value is taking the same for site class C. In this case, there is slight reduction in the seismic force generated.
 The exception of Clause 2 in 11.4.8 of ASCE 716 is not implemented in STAAD.Pro for response spectra.
 The vertical ground motion for seismic design as described in clause 11.9 is not implemented as the program cannot determine the vertical time period, T_{v}.
 ASCE 72016 Clause 12.9.1.4 stipulates that if the combined response for modal base shear, V, is less than that of the calculated base shear from the equivalent lateral force procedure, V_{t}, then the response spectra values should be scaled up by V/V_{t}. STAAD.Pro does not automatically check this nor increase the response quantities in this situation. You can check this by simply analyzing the static seismic load case and comparing the base shear values. The X, Y, or Z factors for the response spectrum can be then increased accordingly if required.
Inherent and Accidental Torsion
In response spectrum analysis all the response quantities (i.e., joint displacements, member forces, support reactions, plate stresses, etc.) are calculated for each mode of vibration considered in the analysis. These response quantities from each mode are combined using a modal combination method (either by CQC, SRSS, ABS, TEN PERCENT, etc.) to produce a single positive result for the given direction of acceleration. This computed result represents a maximum magnitude of the response quantity that is likely to occur during seismic loading. The actual response is expected to vary from a range of negative to positive value of this maximum computed quantity.
No information is available from response spectrum analysis as to when this maximum value occurs during the seismic loading and what will be the value of other response quantities at that time. As for example, consider two joints J2 and J3 whose maximum joint displacement in global X direction come out to be X1 and X2 respectively. This implies that during seismic loading joint J1 will have X direction displacement that is expected to vary from X1 to +X1 and that for joint J2 from X2 to +X2. However, this does not necessarily mean that the point of time at which the X displacement of joint J1 is X1, the X displacement of joint J2 will also be X2.
For the reason stated above, torsional moment at each floor arising due to dynamic eccentricity along with accidental eccentricity (if any) is calculated for each mode. Lateral story shear from this torsion is calculated forming global load vectors for each mode. Static analysis is carried out with this global load vector to produce global joint displacement vectors for each mode due to torsion. These joint displacements from torsion for each mode are algebraically added to the global joint displacement vectors from response spectrum analysis for each mode. The final joint displacements from response spectrum along with torsion for all modes are combined using specified modal combination method to get final maximum possible joint displacements. Refer to the steps explained below.
See TR.32.10.1.1 Response Spectrum Specification  Custom for additional details on IMR load case generation.
Steps
For each mode following steps are performed to include Torsion provision.
 Lateral story force at each floor is calculated.

At each floor design eccentricity is calculated.
Thus, design eccentricity e_{di} = f20×e_{si} + f12×b_{i} where f20 = 1.0 and f21 = (±) 0.05
Where:
 e_{si} = static eccentricity between center of mass and center of rigidity at floor i.
 b_{i} = floor plan dimension orthogonal to the direction of earthquake loading.

Torsional moment is calculated at each floor.
M_{ik} = Q_{ik} × e_{di} at floor i for mode k
 The lateral nodal forces corresponding to torsional moment are calculated at each floor. These forces represent the additional story shear due to torsion.
 Static analysis of the structure is performed with these nodal forces.
 The analysis results (i.e., member force, joint displacement, support reaction, etc) from torsion are algebraically added to the corresponding modal response quantities from response spectrum analysis.
 Steps 1 through 6 are performed for all modes considered and missing mass correction (if any). Finally, the peak response quantities from the different modal responses are combined as per the specified combination method (e.g., SRSS, CQC, TEN, etc.)
Dynamic Eccentricity
 the rotational component of ground motion about the vertical axis,
 the difference between computed and actual values of the mass, stiffness, or strength, and
 uneven live mass distribution.
e_{di} = DEC×e_{si} + ECC×b_{i} 
=  
=  
= 
Refer to Cl. 12.9.2.2.2 for the requirements of accidental torsion per the ASCE 716 code.