TR.31.2.12 IS:1893 (Part 4) 2015 Codes  Lateral Seismic Load
All industrial (plant and auxiliary) structures like main plants, process plants, water systems, and maintenance workshops are recognized as industrial structures.
General Format
DEFINE IS1893 2015 ( ACCIDENTAL ) LOAD PART4
18932015P4spec
weightdata
Refer to Common Weight Data for information on how to specify structure weight for seismic loads.
where
18932015P4spec = ZONE f1 RF f2 I f3 { SS f4  SA f11 } (ST f5) ( { DM f6  DF f12 } ) (PX f7) (PZ f8)
Notes

If the ACCIDENTAL option is specified for an ST 4 structure, the accidental torsion will be calculated per the IS 1893 specifications. The value of the accidental torsion is based on the center of mass for each level. The center of mass is calculated from the SELFWEIGHT, JOINT WEIGHT, and MEMBER WEIGHT commands you have specified. The ACCIDENTAL option is not applicable to other structure types.
The ACCIDENTAL option along with accidental eccentricity factor (generally 0.05 as per IS 1893 code) needs to be provided in the 1893 seismic primary load case (i.e., 1893 LOAD X / Z f1 ACC f3 ). f2 can be negative. See TR.32.12.2 Generation of Seismic Loads

By default, STAAD.Pro calculates natural periods of the structure in both X and Z directions respectively which are used in calculation for seismic force at each node. If PX and PZ are included, the program will consider these values for calculation of average response acceleration coefficient. If ST is used instead of PX and PZ values, then the program will calculate natural period using the Rayleigh method.
Example
DEFINE IS1893 2015 LOAD PART4 ZONE 0.36 RF 5 I 1.25 SS 1 ST 1 DM 0.05 JOINT WEIGHT 39 60 80 WEIGHT 100 LOAD 1 LOADTYPE Seismic TITLE SS_(+X) 1893 LOAD X 1 LOAD 2 LOADTYPE Seismic TITLE SS_(+Z) 1893 LOAD Z 1 LOAD 3 LOADTYPE Seismic TITLE SS_(+Y) 1893 LOAD Y 1
Methodology
The seismic force at each node is computed by STAAD.Pro for building structures as per IS: 1893 (Part 4) 2015:
V = A_{h.}W 
= 
 
= 
 You provide seismic zone coefficient and desired 1893 specs through the DEFINE 1893 2015 LOAD PART4 command.
 The program calculates the structure period, T using the Rayleigh method.
 The program calculates S_{a}/g utilizing T.
 Calculate Ah from section 7.3.
 The program calculates V from the above equation at each node corresponding to the product of the seismic coefficient and mass at each node. W is obtained from mass table data entered via SELFWEIGHT, JOINT WEIGHT(s), MEMBER WEIGHT(S), and/or REFERENCE LOAD you provide through the DEFINE 1893 2015 LOAD PART4 command.
See TR.32.12 Generation of Loads for additional information.