Mechanical APDL Structural Analysis Guide

Choose a sufficient number of modes to characterize the structure's response and cover the fre-

quency range spanned by the spectrum. The accuracy of the solution depends on the number of

modes used. The larger the number of modes used, the higher the accuracy will be.

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 computes the mode
  coefficients according to the empirical equations given in the Mechanical APDL Theory Reference.
  
  
  • No damping needs to be specified for solution because it is implied by the ADDAM and VDDAM com-
    mands. If damping is specified, it is used for mode combinations, but ignored for solution.
  
  
  
  


• 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.

Choose one of the following mode combination methods offered for the single-point response

spectrum analysis:

• Square Root of Sum of Squares (SRSS)


• Complete Quadratic Combination (CQC)


• Double Sum (DSUM)


• Grouping (GRP)


• Naval Research Laboratory Sum (NRLSUM)


• Rosenblueth (ROSE)

The NRLSUM method is typically used in the context of the Dynamic Design and Analysis Method

(DDAM) spectrum analysis.

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 [ 411 ]).

The following commands are used to invoke different methods of mode combinations:

Command(s):SRSS,CQC,DSUM,GRP,NRLSUM,ROSE

GUI: Main Menu> Solution> Analysis Type> New Analysis> Spectrum

Main Menu> Solution> Analysis Type> Analysis Opts> D.D.A.M

Main Menu> Load Step Opts> Spectrum> Mode Combine

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Spectrum Analysis