The Load Case Data - Response Spectrum form is used to view and change the definition of a response-spectrum analysis case. Click the Analysis > Type > Response Spectrum > New command to display the Load Case Data - Response Spectrum form.
Note: If a modal analysis case has not yet been defined, first define the modal analysis case and then define the response-spectrum case.
Load Case Name edit box. Use the name shown or type a new name in this edit box. It should be unique among all load cases of all types. Click the Set Def Name button to use a default name for the load case.
Notes Modify/Show button. Click this button to access the Load Case Notes form. Use the form to add notes to the model file specific to this load case.
Design button. Click this button to access the Design Load Type form. Choose Program Determined, or User Specified and then a design type from the drop-down list.
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. 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.
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. Enter the characteristic frequencies f1 and f2 as defined in ASCE 4 for GMC.
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 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.
Modal Load Case and Use Modes from this 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. Select the "Standard" option for acceleration loading and the "Advanced" option for the displacement inertia loading.
Tip: Most commonly, only one modal load 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.
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.
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.
Load Applied
Load Type This is non-editable. It can be "Accel" or "Load Inertia," depending on the loading type specified in the Modal Load Case.
Load Name drop-down list. For an "Accel" Load Type, select the acceleration direction(s): U1, U2, U3, R1, R2, and R3. Specify only one acceleration load (or none) in each direction. See Applying Acceleration Loads for more information. For "Load Inertia" select a previously defined load pattern.
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.
See Also