Configuring a response spectrum procedure

1. Display the Edit Step dialog box following the procedure outlined in Creating a step (Procedure type: Linear perturbation; Response spectrum), or Editing a step.

2. On the Basic and Damping tabbed pages, configure settings such as a damping coefficient and the methods for combining multidirectional excitations as described in the following procedures.

1. In the Edit Step dialog box, display the Basic tabbed page.

2. In the Description field, enter a short description of the analysis step. Abaqus stores the text that you enter in the output database, and the text is displayed in the state block by the Visualization module.

3. Click the arrow to the right of the Excitations field, and select a directional summation method.

   • For the following options, Abaqus/Standard sums the directional excitation components first and then performs the modal summation:

     • Select Single direction to sum the directional excitation components for a single direction algebraically.

     • Select Multiple direction absolute sum to sum the directional excitation components for multiple directions algebraically.

   • For the following options, Abaqus/Standard performs the modal summation first and then sums the directional excitation components:

     • Select Multiple direction square root of the sum of squares to sum the directional excitation components for multiple directions using the square root of the sum of the squares.

     • Select Multiple direction thirty percent rule to sum the directional excitation components for multiple directions using the 30% rule.

     • Select Multiple direction forty percent rule to sum the directional excitation components for multiple directions using the 40% rule.

   See Directional summation methods, for more information.

4. Click the arrow to the right of the Summations field, and select a modal summation method. For information on each of the methods, see Modal summation methods.

5. On the First direction tabbed page (and on the Second direction and Third direction tabbed pages if applicable), do the following:

   1. In the Use response spectrum field, select the spectrum amplitude to use for calculating the response. Alternatively, you can click to create a new amplitude. (See Defining a spectrum,” for more information.)
   2. Enter Direction cosines X, Y, and Z.
   3. In the Scale factor field, enter the factor multiplying the magnitudes in the response spectrum.
   4. In the Time duration field, enter the time duration of the dynamic event from which the spectrum was created. This setting is applicable only when the Double sum combination modal summation method is specified.

   For multiple direction excitations, you have the option of toggling on Apply third direction on the Third direction tabbed page to include data for a third direction.

1. In the Edit Step dialog box, display the Damping tabbed page.

   (For information on displaying the Edit Step dialog box, see Creating a step, or Editing a step.)

2. Indicate how you want to provide damping values:

   • Choose Specify damping over ranges of Modes to provide damping values for specific mode ranges.

   • Choose Specify damping over ranges of Frequencies to provide damping values at specific frequencies. Abaqus/Standard interpolates the damping coefficient for a mode linearly between the specified frequencies

   If you omit damping data on the Damping tabbed page, Abaqus/Standard assumes zero damping values. For more information, see Specifying damping.

3. If you selected Modes in Step 2, select one or more of the following options for defining damping:

   • Display the Direct modal tabbed page to specify the fraction of critical damping, $\xi$, for a particular mode range. Do the following:

     1. Toggle on Use direct damping data.

     2. Enter the following in the data table:

        • Start Mode: the mode number of the lowest mode of a range.

        • End Mode: the mode number of the highest mode of a range.

        • Critical Damping Fraction: fraction of critical damping, $\xi$.

   • Display the Composite modal tabbed page to select composite modal damping using the damping coefficients calculated in the preceding frequency step. (The damping calculations performed in the frequency step are performed using damping data provided in the material definition.) Do the following:

     1. Toggle on Use composite damping data.

     2. Enter the following in the data table:

        • Start Mode: the mode number of the lowest mode of a range.

        • End Mode: the mode number of the highest mode of a range.

   • Display the Rayleigh tabbed page to define Rayleigh damping, and do the following:

     1. Toggle on Use Rayleigh damping data.

     2. Enter the following in the data table:

        • Start Mode: the mode number of the lowest mode of a range.

        • End Mode: the mode number of the highest mode of a range.

        • Alpha: mass proportional damping, ${\alpha }_M$.

        • Beta: stiffness proportional damping, ${\beta }_M$.

   For detailed information on how to enter data, see Entering tabular data.

4. If you selected Frequencies in Step 2, select one or both of the following options for defining damping:

   • Display the Direct modal tabbed page to specify the fraction of critical damping, $\xi$, for a particular frequency range. Do the following:

     1. Toggle on Use direct damping data.

     2. Enter the following in the data table:

        • Frequency: frequency value in cycles/time.

        • Critical Damping Fraction: fraction of critical damping, $\xi$.

   • Display the Rayleigh tabbed page to define Rayleigh damping, and do the following:

     1. Toggle on Use Rayleigh damping data.

     2. Enter the following in the data table:

        • Frequency: frequency value in cycles/time.

        • Alpha: mass proportional damping, ${\alpha }_M$.

        • Beta: stiffness proportional damping, ${\beta }_M$.

   For detailed information on how to enter data, see Entering tabular data.

5. If desired, repeat Steps 2–4 to create multiple damping definitions.

When you have finished configuring settings for the step, click OK to close the Edit Step dialog box.