Analysis Case Data - Response Spectrum

Access the Analysis Case Data - Response Spectrum form as follows:

Click the Define menu > Analysis Cases command to access the Analysis Cases form.
Click the Add New Case button, or highlight a previously defined case and click the Add Copy of Case or Modify/Show Case button, to display the Analysis Cases Data form.
Select Response Spectrum from the Analysis Case Type drop-down list.

The Analysis Case Data - Response Spectrum form is used 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.

Modal Combination 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.
SRSS - Square Root of Sum of Squares option. A modal combination technique that does not account for modal damping or cross coupling.
ABS - 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.
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.
Dbl 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 Analysis Cases definition.

Modal Analysis Case and Use Modes from this Modal Analysis Case drop-down list. Response-spectrum analysis is based on modal superposition. Select the name of the modal analysis case whose modes are to be used as the basis for the response-spectrum analysis.

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

Directional Combination 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.
ABS - 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.
Modified SRSS option. A modified SRSS method compliant with Chinese building codes.

Diaphragm Eccentricities The diaphragm eccentricity option has meaning only if the model has diaphragms. SAP2000 ignores eccentricities where diaphragms are not present. Where diaphragms are present, SAP2000 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, SAP2000 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 Override button to display the Override Diaphragm Eccentricities form and modify eccentricities on a diaphragm-by-diaphragm basis.

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.
Show Advanced Load Parameters check box. Check this box to reveal the following additional columns in the applied-loads table.

Coordinate System drop-down list. Specify a coordinate system in which the acceleration directions are measured. See Applying Acceleration Loads for more information.

Important: All accelerations should use the same coordinate system.

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. To add a load to the set of applied loads, enter the load name (direction) and other parameters at the top of the table, then click Add button.
Modify button. To modify a load in the set of applied loads, click on the load in the table to select it, make any changes to the load name and other parameters at the top of the table, then click Modify button.
Delete button. To remove a load from the set of applied loads, click on the load in the table to select it, then click Delete button.

Modal Damping display 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.