TR.31.2.14 IBC 2012 Seismic Load Definition
The specifications of the seismic loading chapters of the International Code Council 2012 code and the ASCE 710 code for seismic analysis of a building using a static equivalent approach have been implemented as described in this section. Depending on the definition, equivalent lateral loads will be generated in the horizontal direction(s).
General Format
Lateral loads generated by seismic loading are specified in two stages. The first is to define the IBC 2012 loading, as detailed here. The second is to include that specification in one or more load cases.
There are two stages of command specification for generating lateral loads. This is the first stage and is activated through the DEFINE IBC 2012 LOAD command.
DEFINE IBC 2012 (ACCIDENTAL) LOAD
mapspec ibc12spec
(optionalweightspecs)
weightdata
Refer to Common Weight Data for information on how to specify structure weight for seismic loads.
Where:
Parameter  Description 

mapspec  This identifies the mapped spectral accelerations at a given location by either ZIP
(postal) code, latitude and longitude, or direct input. The ZIP code and
latitude & longitude options are for use in mainland US only:
{ ZIP f1  LAT f2 LONG f3  SS f4 S1 f5 } 
ibc12spec  This identifies the parameters needed to determine the lateral loading to be applied in a
load case which references this definition.
{ RX f6 RZ f7 I f8 TL f9 SCLASS f10 (CTX f11) (CTZ f12) (PX f13) (PZ f14) (XX f15) (XZ f16) (FA f17) (FV f18) } 
The optional seismic weight option is provided for old files. It is recommended that instead the weights are defined using a one or more reference load cases of type MASS, (refer to TR.31.6 Defining Reference Load Types). If the older format is used, refer to TR.31.2 Definitions for Static Force Procedures for Seismic Analysis for the range of available weight commands that can be used.
The ACCIDENTAL option is used to include an additional torsional moment taken as the lateral load is applied at a horizontal eccentricity as 5% of the building dimension at each level. This option should not be used if the option to include the natural torsion in the application of seismic load (refer to GUID6D9B5C489FFF4548BFB4ACAE1E973743 for additional details).
mapspec parameters:
ibc12spec parameters:
Implementation in STAAD.Pro
The seismic load generator can be used to generate lateral loads in the X & Z directions for Y up or X & Y for Z up. Y up or Z up is the vertical axis and the direction of gravity loads (See the SET Z UP command in TR.5 Set Command Specification). All vertical coordinates of the floors above the base must be positive and the vertical axis must be perpendicular to the floors.
The rules described in section 1613 of the ICC IBC2012 code (except 1613.5.5) have been implemented. This section directs the engineer to the ASCE 72010 code. The specific section numbers of ASCE 7 —those which are implemented, and those which are not implemented— are shown in the table below.
Implemented sections of IBC 2012 (ASCE 710) 
Omitted sections of IBC 2012 (ASCE 710) 

11.4  12.8.4.1 
11.5  12.8.4.3 and onwards 
12.8 
Steps used to calculate and distribute the base shear are as follows:

The Time Period of the structure is calculated based on section 12.8.2.1 of ASCE 710 (IBC 2012). This is reported in the output as Ta.

The period is also calculated in accordance with the Rayleigh method. This is reported in the output as T.

You may override the Rayleigh based period by specifying a value for PX or PZ (Items f7 and f8) depending on the direction of the IBC load.

The governing Time Period of the structure is then chosen between the above two periods, and the additional guidance provided in section 12.8.2 of ASCE 710 (IBC 2012). The resulting value is reported as "Time Period used" in the output file.

The Design Base Shear is calculated based on equation 12.81 of ASCE 710 (IBC 2012). It is then distributed at each floor using the rules of clause 12.83, equations 12.811, 12.812 and 12.813 of ASCE 710.

If the ACCIDENTAL option is specified, the program calculates the additional torsional moment. The lever arm for calculating the torsional moment is obtained as 5% of the building dimension at each floor level perpendicular to the direction of the IBC load (section 12.8.4.2 of ASCE 710 for IBC 2012). At each joint where a weight is located, the lateral seismic force acting at that joint is multiplied by this lever arm to obtain the torsional moment at that joint.

The amplification of accidental torsional moment, as described in Section 12.8.4.3 of the ASCE 710 code, is not implemented.

The story drift determination as explained in Section 12.8.6 of the ASCE 710 code is not implemented in STAAD.
Methodology
The design base shear is computed in accordance with the following equation (equation 12.81 of ASCE 710):
V = C_{s}W
The seismic response coefficient, C_{s}, is determined in accordance with the following equation (equation 12.82 of ASCE 710):
C_{s} = S_{DS}/[R/I_{E}]
 C_{s} = S_{D1}/[T⋅(R/I)] for T ≤ T_{L}
 C_{s} = S_{D1} · T_{L}/[T^{2}(R/I)] for T > T_{L}
However, C_{s} shall not be less than (equation 12.85 of ASCE 710):
C_{s} = 0.044 · S_{DS} · I ≥ 0.01
In addition, per equation 12.86 of ASCE 710, for structures located where S_{1} is equal to or greater than 0.6g, C_{s} shall not be less than
C_{s} = 0.5 · S_{1}/(R/I)
For an explanation of the terms used in the above equations, please refer to the IBC 2012 and ASCE 710 codes.
Example 1
DEFINE IBC 2012 LAT 38.0165 LONG 122.105 I 1.25 RX 2.5 RZ 2.5 SCLASS 4  TL 12 FA 1 FV 1.5 SELFWEIGHT JOINT WEIGHT 51 56 93 100 WEIGHT 650 MEMBER WEIGHT 151 TO 156 158 159 222 TO 225 324 TO 331 UNI 45
Example 2
The following example shows the commands required to enable the program to generate the lateral loads. Refer to TR.32.12 Generation of Loads for this information.
LOAD 1 (SEISMIC LOAD IN X DIRECTION) IBC LOAD X 0.75 LOAD 2 (SEISMIC LOAD IN Z DIRECTION) IBC LOAD Z 0.75
Using SS and S1 for mapspec
DEFINE IBC 2018 SS 2.451 S1 0.882 – I 1.25 RX 2.5 RZ 2.5 SCLASS 4 TL 12 FA 1 FV 1.5