To specify a response spectrum for seismic loading per the IBC 20005 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 Custom in the Code list.
4. Generate the IBC 2000 values for the table:
   Note: In order to use the generated spectrum values, you must leave the Spectrum Type selection as Acceleration.
   1. Click Generate IBC Spectrum - 2000. The Spectrum Parameters dialog opens.
   2. Specify the mapped acceleration values by either:

      select the Zip code from the drop-down list, click Find Lat / Long, and then click Calculate S1 / SS

      or

      enter the Latitude and Longitude and then click Calculate S1 / SS

      or

      enter the S1 and SS values directly

   3. Select a Site Class for the location.
      Note: If you select "F" as the Site Class, then you must provide both the Fa and Fv values.
      The Fa and Fv values for the site location are displayed.
   4. Type the Start, End, and Interval values for the period.
   5. Click Generate Spectrum. The Generated Spectrum: IBC 2000 dialog opens. The values are tabulated and the spectrum is plotted. You can use the settings on this dialog to review the data.
   6. Click Close.
   The response spectrum values and plot is generated.
5. 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.
6. (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.

7. 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.

8. Select the Damping Type to use and, if explicit Damping is selected, type the damping ratio to be used for all modes.
9. Select the directions which spectrum is applied and for each, type a factor (0-1.0).
10. (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.
11. Click Add. The new response spectrum is added to the currently selected load case.