Load Case Data - Response Spectrum

When the Load Case Type is Response Spectrum , use the form to view and change the definition of a response-spectrum analysis case.

Note: If a modal analysis case has not yet been defined, first define the modal analysis case and then define the response-spectrum case.

Note: Please refer to the Load Case Data Form topic for information related to the Load Case Name edit box, the Load Case Type and Load Case Subtype drop-down lists, and the Notes Modify/Show and Design buttons.

Loads Applied - Begin using this area of the form by clicking the Add button.
Load Type Only built-in ground acceleration loads are permitted, although the load directions can be changed using the Load Name drop-down list.
Load Name drop-down list. Choose acceleration direction U1, U2, or U3. See Applying Acceleration Loads for more information.

Note: Specify only one acceleration load (or none) in each direction.

- Function drop-down list. Select the name of a previously defined response-spectrum function that specifies the magnitude of pseudo-spectral acceleration response vs. structural period.
- Scale Factor edit box. Enter a scale factor that multiplies each acceleration load. This scale factor has units of acceleration, and should be consistent with the length units currently in use.
- Advanced check box. Check this box to reveal the following additional columns in the applied-loads table.

- - Angle edit box. Specify an angle by which U1 and U2 are rotated from UX and UY in the specified coordinate system. See Applying Acceleration Loads for more information.

Important: Accelerations U1 and U2 should use the same angle. The angle is not used for direction U3.

- - - Add button. Click this button to display the Load Type, Load Name, and Function drop-down lists, and the Scale Factor and Angle,deg edit boxes. Use these items to specify the load to be applied in the load case.
    - Delete button.
    1. 1. Highlight the load pattern to be deleted in the set of applied loads.
      2. Click the Delete button.

Modal Load Case drop-down list. Response-spectrum analysis is based on modal superposition. Select the name of the modal load case whose modes are to be used as the basis for the response-spectrum analysis.

Tip: Most commonly, only one modal load case will have been defined, and its name will already be selected by default.

Modal Combination Method options. Select the type of statistical modal combination to use:
- CQC - Complete Quadratic Combination option. A modal combination technique that accounts for modal damping. It is the same as SRSS if damping is zero. Enter the characteristic frequencies f1 and f2 as defined in ASCE 4 for GMC.
- SRSS - Square Root of Sum of Squares option. A modal combination technique that does not account for modal damping or cross coupling. Enter the characteristic frequencies f1 and f2 as defined in ASCE 4 for GMC.
- Absolute option. Summation of the absolute values of the modal results.
- GMC - General (or Gupta) Modal Combination option. A modal combination technique that takes into account modal damping, and assumes higher correlation between modes at higher frequencies. Enter the characteristic frequencies f1 and f2 as defined in ASCE 4 for GMC.
- NRC 10 Pct - the Ten Percent method of the U.S. Nuclear Regulatory Commission Regulatory Guide 1.92 option. The Ten Percent method assumes full, positive coupling between all modes whose frequencies differ from each other by 10% or less of the smaller of the two frequencies. Modal damping does not affect the coupling. Enter the characteristic frequencies f1 and f2 as defined in ASCE 4 for GMC.
- Double Sum - the Double Sum method of the U.S. Nuclear Regulatory Commission Regulatory Guide 1.92 option. The Double Sum method assumes a positive coupling between all modes, with correlation coefficients that depend upon damping in a fashion similar to the CQC and GMC methods, and that also depend upon the duration of the earthquake. Specify this duration as parameter td as part of the load cases definition.
- - For all combinations except ABS, enter the characteristic frequencies f1 and f2 as defined in ASCE 4 for GMC.
  - For all combinations except ABS, use the options available in Periodic+Rigid drop-down list to control how rigid response is handled in the modal combination.

Directional Combination Type options. Select the type of combination to use for the different directions of acceleration loading:
- SRSS - Square Root of Sum of Squares option. A directional combination technique that is independent of the direction of loading.
- Absolute - Absolute Summation option. Summation of the absolute values of the results caused by different directions of loading. Specify an ABS Scale Factor smaller than one to consider scaled sums. For example, if a value of 0.3 is specified, the program will consider the worst of 100% loading in one direction plus 30% in the other direction(s), and so on for each of the two or three loaded directions.
- CQC3 - An extension of the SRSS method for finding the maximum response when the horizontal (U1 and U2) directions of loading use the same response spectrum function but have different scale factors. The critical angle of loading is determined automatically independent of the angle specified for the loading. The vertical response is combined with the maximum horizontal response using the SRSS method. If different response-spectrum functions are used for U1 and U2, the results must be interpreted carefully by the engineer.

Modal Damping display box and Modify/Show button. Modal damping is only used to determine the amount of cross-coupling between the modes for CQC and GMC types of modal combination. Click the Modify/Show button to access the Modal Damping form and specify the modal damping to use for the analysis.
Diaphragm Eccentricity display box and Modify/Show button. The diaphragm eccentricity option has meaning only if the model has diaphragms. ETABS ignores eccentricities where diaphragms are not present. Where diaphragms are present, the program calculates a maximum width of the diaphragm perpendicular to the direction of the seismic loading. This width is calculated by finding the maximum and minimum X or Y coordinates (depending on direction of load considered) of the points that are part of the diaphragm constraint and determining the distance between these maximum and minimum values. After the appropriate diaphragm width has been determined, ETABS applies a moment that is equal to the specified percent eccentricity times the maximum width of the diaphragm perpendicular to the direction of the seismic loading times the total lateral force applied to the diaphragm. This moment is applied about the diaphragm center of mass to account for the eccentricity.

Click the Modify/Show button to display the Eccentricities - Response Spectrum Analysis form and modify eccentricities on a diaphragm-by-diaphragm basis.