TR.31.2.9 IS:1893 (Part 1) 2002 & Part 4 (2005) Codes  Lateral Seismic Load
This feature enables one to generate seismic loads per the IS:1893 specifications using a static equivalent approach. Both Part 1 (2002) for building structures and Part 4 (2005) for industrial and stacklike structures are available.
The seismic load generator can be used to generate lateral loads in the X and Z directions only. Y is the direction of gravity loads. This facility has not been developed for cases where the Z axis is set to be the vertical direction (See the SET Z UP command in TR.5 Set Command Specification).
General Format
DEFINE 1893 ( ACCIDENTAL ) LOAD ( PART4 )
ZONE f1 { 1893specpart1  1893specpart4 }
weightdata
Refer to Common Weight Data for information on how to specify structure weight for seismic loads.
(CHECK SOFT STORY)
where
1893specpart1 = RF f2 I f3 { SS f4  SA f11 } (ST f5) ( { DM f6  DF f12 } ) (PX f7) (PZ f8) ( { DT f9  GL f10 } )
1893specpart4 = RF f2 I f3 { SS f4  SA f11 } ST f5 ( { DM f6  DF f12 } ) (PX f7) (PZ f8) ( { DT f9  GL f10 } ) ( CS f13 ) ( AX f14 ) ( ES f15 ) CV f16 DV f17
where
Notes

If the ACCIDENTAL option is specified, 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 ACC 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 GUID82BB33A358104CC0A6D65EF0440C202B#GUID82BB33A358104CC0A6D65EF0440C202B
To consider horizontal torsion in cases where a floor diaphragm is present in the model, the ACCIDENTAL option should not be specified. Instead, dynamic eccentricity along with accidental eccentricity should be provided in the 1893 seismic primary load case (i.e., 1893 LOAD X / Z f1 DEC f2 ACC f3 ). For equivalent seismic analysis, f2 is 1.5 and f3 is 0.05 as per IS 1893 code. f1 is always positive or zero, however f2 can be negative. If f2 is 0.0, only accidental torsion will be considered for this particular load case.

By default STAAD calculates natural periods of the structure in both X and Z directions respectively which are used in calculation for base shear. 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 depending upon the empirical expression given in IS: 1893 (Part 1)2002 or IS: 1893 (Part 4)2005.

In the case where no rigid floor diaphragm is present, STAAD identifies columns and shear walls (without openings) as vertical components for the purpose of computing lateral stiffness of the story.
The lateral stiffness of a column is calculated as:
where12EI / L^{3}  E
=  Young’s modulus
 I
=  moment of inertia
 L
=  length of the column
The lateral stiffness for a shear wall (without opening) is calculated as:
$\frac{1}{\frac{P{h}^{3}}{12EI}+\frac{1.2Ph}{AG}}$Which is the summation of inverse of flexural stiffness and inverse of shear stiffness, obtained as deflection of a cantilever wall under a single lateral load, P, at its top.
where h
=  height
 A
=  crosssectional area
 G
=  shear modulus of the wall
The summation of lateral stiffnesses of all columns and shear walls at a particular floor level constitutes the total lateral stiffness of that particular story or floor level. The program checks for a soft story of a building along both global X and Z directions respectively. This computation is valid only for those structures whose floors are treated as rigid diaphragm
Example
DEFINE 1893 LOAD ZONE 0.36 RF 5 I 1 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
Methodology
The design base shear is computed by STAAD.Pro for building structures as per IS: 1893 (Part 1) 2002 equation 7.5.3 or for industrial structures as per (Part 4) 2005:
V = A_{h.}W
Where:
When site specific spectra is used per IS 1893 (Part 4) 2005, then:
For stacklike structures, the design base shear is computed as per IS 1893 (Part 4) 2005 as:
V = C_{v}A_{h.}W·D_{v}
 You provide seismic zone coefficient and desired 1893 specs through the DEFINE 1893 LOAD command. Use the PART 4 command option to specify using IS: 1893 (Part 4) 2005.
 The program calculates the structure period, T.
 The program calculates S_{a}/g utilizing T.
 The program calculates V from the above equation. 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 LOAD command.
 The total lateral seismic load (base shear) is then distributed by the program among different levels of the structure per the IS: 1893 procedures.
See TR.32.12 Generation of Loads for additional information.
Soft Story Checking
As per the IS18932002 code Clause 7.1, to perform well during an earthquake a building must have simple and regular configuration, adequate lateral strength, stiffness and ductility. This is because a building with simple regular geometry and uniformly distributed mass and stiffness in plan as well as in elevation, will suffer much less damage than buildings with irregular configurations.
According to this standard, a building can be considered irregular, if at least one of the conditions given in Table 4  Plan Irregularities and Table 5  Vertical Irregularities, of IS18932002 is applicable.
For IS 1893 2002, STAAD.Pro has implemented the methodology to find vertical stiffness irregularities, as given in IS 18932002 Table 5 Sl No. (1) i) a) and Sl No. (1) i) b), in the form of soft story checking.
 Stiffness Irregularities: Soft Story – As per this provision of the code, a soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average lateral stiffness of the three story above.
 Stiffness Irregularities:
Extreme Soft Story – As per this provision of the code, a extreme soft story is
one in which the lateral stiffness is less than 60 percent of that in the story
above or less than 70 percent of the average lateral stiffness of the three
story above.
Thus, if any story of a building is found to be soft or extremely soft, the building is likely to suffer much damage in an earthquake than a similar type of building but has more regular vertical stiffness.