To specify a response spectrum for seismic loading per the IS 1893 code, use the following procedure.

1. Either:

   On the Loading ribbon tab, select the Load Items tool in the Loading Specifications group

   Tip: This will add the load item to the currently selected load group selected in the program status bar.

   or

   In the Load & Definition dialog, select a primary load case in the Load Cases Details list and then click Add.

   The Add New Load Items dialog opens.
2. Select the Response Spectrum tab.
3. Select IS-1893 in the Code list.
4. Specify the IS 1893 specific parameters:
   1. Select a Subsoil class for the location.

      Subsoil option	Description
      Custom	You must enter Period and Acceleration or Displacement value pairs in the table, based on the Interpolation Type selected. You can also select the Interpolation Type for values between data points.
      Hard, Medium, or Soft Soil	Average response acceleration coefficient is calculated. You will not need to select Spectrum Type or Interpolation Type for these options.

   2. (Optional) Select the Ignore mode(s) with mass participation (IGN) option and then enter a percentage value. Local and torsional modes whose mass participation is less than this percent value are then considered negligible and there fore will be excluded.
   The response spectrum plot is generated as these parameters are specified.
5. (Optional) Select the Use Torsion option and then enter the Dynamic Eccentricity (DEC) and Accidental Eccentricity (ECC) factors. The dynamic eccentricity factor is multiplied by the static eccentricity (i.e., the distance between the center of mass and center of rigidity) to get the dynamic eccentricity. This value should typically be 1.0 or higher.

   The accidental eccentricity is used to calculate the value of accidental eccentricity. This value is typically 0.05 but may be increased to 0.10 for highly irregular buildings. This value can be positive or negative to accommodate clockwise or counter-clockwise rotation.

6. Select the Combination Method to use to combine the responses from each mode into a total response.
   Note: For the ASCE method, there are two optional parameters, FF1 and FF2, which can be checked and specified.
7. (Optional) Check the Save option to create a file contain the joint accelerations in ratios of gravity and in radians/sec².

   The file name is saved in the same location as the STAAD input file, will have the same root filename as the STAAD input file but with an .acc file extension.

8. Select if the Interpolation Type is Linear or Logarithmic.

   The logarithmic option is recommended when only a few spectra data points are entered, as spectra vs period curves are typically only linear on a log-log scale.

9. Select the Damping Type to use and, if explicit Damping is selected, type the damping ratio to be used for all modes.
10. Select the directions which spectrum is applied and for each, type a factor (0-1.0).
11. (Optional) You can apply each of the following optional steps as needed:
    1. Select one of the Signed Response Spectrum Results Options if needed:

       check Dominant and type a Mode No to use for determine the signs of all modes. You can alternatively type 0 for the Mode No to have the program select the mode with the greatest percent participation in the excitation direction for the dominant mode.

       or

       check Sign to create signed values for all results by comparing the sum of the squares of positive versus negative results.

    2. To generate individual modal response load cases, check the Generate load case(s) for first option and then type the number of load cases to generate. You can also type the Load Case number to use for the first of the generated load cases. If the number of load cases requested is larger than the number of modes extracted, then only the number of modes extracted is used.
    3. Type a Scale factor to apply to the spectra data.
    4. Check the Missing Mass option to include the static effect of masses not represented in the modes. Type an optional spectral acceleration for this missing mass mode.
    5. If the Missing Mass option is used and you want to specify an acceleration corresponding to a frequency instead, check the ZPA option and type a frequency. If neither the missing mass acceleration value or ZPA frequency is specified, then the spectral acceleration at 33Hz is used to calculate the missing mass mode.
12. Click Add. The new response spectrum is added to the currently selected load case.