6.7. Performing a DDAM Spectrum Analysis

The general process for performing a DDAM spectrum analysis consists of four primary steps:

If you wish to perform mode selection that includes and expands only the significant modes in a DDAM spectrum analysis, refer to Mode Selection Based on the DDAM Method.

The first two steps for a DDAM analysis are the same steps described for a modal analysis (see Step 1: Build the Model and Step 2: Obtain the Modal Solution). The procedure for the remaining steps is explained below:

 

6.7.1. Step 3: Obtain the Spectrum Solution

In this step, the program uses mode shapes extracted by the modal solution to calculate the DDAM solution. The following requirements apply:

  • The mode shape file (Jobname.mode) must be available.

  • The database must contain the same model from which the modal solution was obtained.

  • The Jobname.esav file must be available.

  • The Jobname.full file must be available for the participation factors calculation.

Note:  If the DDAM analysis is not performed in the same directory as the modal analysis, remote modal files usage must be activated (MODDIR). In this case, the DDAM information is stored in the Jobname.prs file and the modal analysis files in the modal analysis directory are not modified. Also, if element results calculation based on element modal results is activated (Elcalc = YES on the SPOPT command), the DDAM results file only contains DDAM results.

  1. Enter SOLUTION (/SOLU).

  2. Define the analysis type and analysis options. Mechanical APDL offers the following analysis options for a spectrum analysis. Not all modal analysis options and eigenvalue extraction techniques work with all spectrum analysis options.

    Table 6.7: Commands to Define a Single-point Response Spectrum Analysis

    Description / Notes Command
    Select Spectrum (SPRS) as the analysis type. ANTYPE, SPECTR
    Select Dynamic Design Analysis Method (DDAM) as the analysis type, and specify the number of modes (enter a value for NMODE).[a] SPOPT,DDAM,NMODE

    [a] Select enough modes to cover the frequency range spanned by the spectrum and to characterize the structure's response. The accuracy of the solution depends on the number of modes used. The larger the number, the higher the accuracy.


  3. Specify load step options.

    • The global excitation direction is specified using the SED command. Only excitation specified using SED is supported in a DDAM analysis.

    • Use the ADDAM and VDDAM commands to specify the spectrum values and types. Based on the coefficients specified in the ADDAM and VDDAM commands, the program calculates the mode coefficients according to the empirical equations given in Dynamic Design Analysis Method in the Mechanical APDL Theory Reference.

    • You can also specify the spectrum values using the DDASPEC command. Based on the ship type, mounting location, deformation type, and minimum acceleration value, spectrum coefficients are calculated based on NRL-1396. These coefficients are then used to calculate the mode coefficients according to the empirical equations given in Dynamic Design Analysis Method in the Mechanical APDL Theory Reference.

    • No damping needs to be specified for solution because it is implied by the ADDAM and VDDAM commands. If damping is specified, it is used for mode combinations, but ignored for solution.

    • Use the /UNITS command before defining the shock spectrum to perform the DDAM spectrum analysis in a units system other than BIN (default). Valid unit systems are SI, MKS, μMKS, CGS, MPA, BFT, and BIN.

    • Spectrum Options:

      The following options are included:

      • Residual Vector (RESVEC)

        The residual vectors reduce the error caused when higher modes are neglected in the analysis.

        Missing Mass and Rigid Response are not supported in a DDAM analysis.

  4. Combine the modes. A maximum of 10,000 modes can be combined.

    Select one of the following mode combination methods offered for the single-point response spectrum analysis listed in the table below.

    Table 6.8: Mode Combination Method Commands

    Mode Combination Method Command
    Square Root of Sum of SquaresSRSS
    Complete Quadratic CombinationCQC
    Double SumDSUM
    GroupingGRP
    Naval Research Laboratory Sum[a], [b]NRLSUM
    RosenbluethROSE

    [a] The NRLSUM method is typically used in the context of the Dynamic Design and Analysis Method (DDAM) spectrum analysis.

    [b] The Closely Spaced Modes (CSM) Method is also available using the NRLSUM,,,CSM command. It uses the NRL summation to process any out-of-phase modal responses with closed form treatment (NAVSEA  [408]).


    These commands allow computation of three different types of responses that you specify via their Label argument as listed in the table below.

    Table 6.9: Responses Specified by Label on the Mode Combination Method Commands

    Response TypeLabel
    Displacement

    (displacements, stresses, forces, etc.)

    		DISP
    Velocity

    (velocities, "stress velocities," "force velocities," etc. )

    		VELO
    Acceleration

    (accelerations, "stress accelerations," "force accelerations," etc.)

    		ACEL


    These commands can also be used to specify the type of modal forces used in the combination. ForceType = STATIC (default) combines the modal static forces (that is, stiffness multiplied by mode shape forces, both of which are stress-causing forces). ForceType = TOTAL combines the summed modal static forces and inertia forces (that is, stiffness and mass forces, both of which forces are seen by the supports).

    The DSUM method also accommodates time-duration input for earthquake or shock spectrum.

    Postprocessing can be done in one of two ways:

    For large models, direct postprocessing significantly reduces the total computation and postprocessing time of spectrum analysis, and even greater gains are achieved using remote read-only modal analysis files (MODDIR) and distributed-memory parallel processing (see Running Multiple Spectrum Analyses). The required conditions and settings that determine what is calculated and written during solution to Jobname.mcom and Jobname.rst are described below.

    Setup for postprocessing via Jobname.mcom  —  By default (Elcalc = NO on the SPOPT command), the mode combination phase writes a file of POST1 commands (Jobname.mcom) that combine the maximum modal responses by the specified mode combination method to calculate the overall response of the structure. Read in Jobname.mcom in POST1 to do the mode combinations, using the results file (Jobname.rst) from the modal expansion pass.

    Setup for direct postprocessing  —  If element results calculation based on element modal results is activated (Elcalc = YES on the SPOPT command), the file Jobname.mcom is written but not used during postprocessing. Modal responses are combined and stored in the Jobname.rst file during the solution for which the mode combination method command is issued (SRSS, CQC,…). Element results and reaction forces are calculated only if they were also requested during the modal expansion pass (see OUTRES). Note that you must set MSUPkey = YES on the MXPAND command during the modal analysis to write elemental results to the Jobname.mode file. For available element results, see Option: Number of Modes to Expand (MXPAND).

    The mode combination method determines how the structure's modal responses are to be combined as detailed in the table below.

    Response TypeLabel How modal responses are combined:
    Displacement

    (displacements, stresses, forces, etc.)

    		DISPDisplacements and stresses are combined for each mode on the mode combination command.
    Velocity

    (velocities, "stress velocities," "force velocities," etc. )

    		VELOVelocities and stress velocities are combined for each mode on the mode combination command.
    Acceleration

    (accelerations, "stress accelerations," "force accelerations," etc.)

    		ACELAccelerations and stress accelerations are combined for each mode on the mode combination command.

    Element results calculation based on modal element results (Elcalc = YES on the SPOPT command) is not supported if the Closely Spaced Modes (CSM) method is selected for the mode combination method NRLSUM.

    Note:  A DDAM spectrum analysis may alternately be performed in two separate solves: first, the spectrum solution, and then the mode combination. The damping needed by the mode combination must be specified before the spectrum solve.

  5. Start solution (SOLVE).

    The output from the solution includes the Response Spectrum Calculation Summary. This table, which is part of the printed output, lists the participation factors, mode coefficients (based on lowest damping ratio), and the weight distribution for each mode. To obtain the response of each mode (modal response), multiply the mode shape by the mode coefficient (based on lowest damping ratio). You do this by retrieving the mode coefficient with the *GET command (Entity = MODE) and using it as a scale factor in the SET command.

    If needed, you can retrieve the frequencies, participation factors, mode coefficients and effective damping ratios with the *GET command (Entity = MODE).

    In addition to the above, the output from the solution also includes a DDAM Response Spectrum Calculation Summary. This table, which is part of the printed output, lists the shock design value and spectrum loading type for each mode.

    You can use the *GET command (Entity = DDAM) to retrieve the shock design values from the last DDAM spectrum solve.

  6. Repeat Step 3 to excite the structure along different directions if necessary. Note that solutions are not written to the Jobname.rst file at this time.

  7. Leave the SOLUTION processor.

    Command(s): FINISH
    GUI: Close the Solution menu.

    Note:  To calculate the effect of multiple spectra independently, see Running Multiple Spectrum Analyses.

 

6.7.2. Step 4: Review the Results

Results from a DDAM analysis are written to the mode combination file Jobname.mcom in the form of POST1 commands. These commands calculate the overall response of the structure by combining the maximum modal responses in some fashion (specified by one of the mode combination methods). The overall response consists of the overall displacements (or velocities or accelerations). If element result calculations were requested in the expansion pass and stored in the results file, the overall response also includes the overall stresses (or stress velocities or stress accelerations), strains (or strain velocities or strain accelerations), and reaction forces (or reaction force velocities or reaction force accelerations).

You can use POST1, the general postprocessor, to review the results. See Step 4: Review the Results for details about the postprocessing.

Note:  If the SPOPT command has been issued with Elcalc = YES, Jobname.mcom is not required in POST1 as results are present on the Jobname.rst file.

 

6.7.3. Example: DDAM Spectrum Analysis

!  Build the Model
/FILNAM,                        ! Jobname
/TITLE,                         ! Title
/PREP7                          ! Enter PREP7
 ...
 ...                            ! Generate model
 ...
FINISH
!
!  Obtain the Modal Solution
/SOLU                           ! Enter SOLUTION
ANTYPE, MODAL                   ! Modal analysis
MODOPT, LANB                    ! Block Lanczos method
MXPAND, ...                     ! Number of modes to expand, ...
D, ...                          ! Constraints
SOLVE                           ! Initiates solution
SAVE
FINISH
!
!  Obtain the Spectrum Solution
/SOLU                           ! Reenter SOLUTION
ANTYPE, SPECTR                  ! Spectrum analysis
SPOPT, DDAM, ...                ! Dynamic design analysis method  
                              
ADDAM, ...                      ! Acceleration spectrum computation constants
VDDAM, ...                      ! Velocity spectrum computation constants 

DMPRAT, ...                     ! Specify damping ratio required if CSM method is used
SED, ...                        ! Specify excitation direction of shock spectrum
NRLS,,, CSM,                    ! NRLSUM mode combinations with Closely Spaced Modes Method (CSM)
                                
SOLVE                           ! Initiates solution
FINISH
!

!  Review the Results
/POST1                          ! Enter the database results postprocessor
/INPUT,,MCOM                    ! Read mode combination file
...! Postprocess as desired 
...! (PRNSOL; PRRSOL; PRDI; PRNSOL; etc.)
...
FINISH