Combine the Modes

Combine the modes in a separate solution phase. The procedure is as follows:

1. Enter SOLUTION.

Command(s): /SOLU

GUI: Main Menu> Solution 2. Define analysis type.

Command(s): ANTYPE

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

• Option: New Analysis [ANTYPE]

Choose New Analysis.

• Option: Analysis Type: Spectrum [ANTYPE]

Choose analysis type spectrum.

3. Choose one of the mode combination methods.

ANSYS offers five different mode combination methods 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)

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

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

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

GUI: Main Menu> Solution> Analysis Type> New Analysis> Spectrum Main Menu> Solution> Analysis Type> Analysis Opts> Single-pt resp

Main Menu> Load Step Opts> Spectrum> Spectrum-Single Point-Mode Combine These commands allow computation of three different types of responses:

• Displacement (label = DISP)

Displacement response refers to displacements, stresses, forces, etc.

• Velocity (label = ^VELO)

Velocity response refers to velocities, "stress velocities," "force velocities," etc.

• Acceleration (label = ACEL)

Acceleration response refers to accelerations, "stress accelerations," "force accelerations," etc.

The DSUM method also allows the input of time duration for earthquake or shock spectrum.

Note — You must specify damping if you use the Complete Quadratic Combination method of mode combination (CQC). In addition, if you use material-dependent damping [MP,DAMP,...], you must request that element results be calculated in the modal expansion. (^Elcalc = YES on the MXPAND command.)

Section 6.3: Steps in a Single-Point Response Spectrum (SPRS) Analysis

4. Start solution.

Command(s): SOLVE

GUI: Main Menu> Solution> Solve> Current LS

The mode combination phase writes a file of POST1 commands (Jobname.MCOM). Read in this file in POST1 to do the mode combinations, using the results file (Jobname.RST) from the modal expansion pass.

The file Jobname.MCOM contains POST1 commands that combine the maximum modal responses by using the specified mode combination method to calculate the overall response of the structure.

The mode combination method determines how the structure's modal responses are to be combined:

• If you selected displacement as the response type (label = ^DISP), displacements and stresses are combined for each mode on the mode combination command.

• If you selected velocity as the response type (label = ^VELO), velocities and stress velocities are combined for each mode on the mode combination command.

• If you selected acceleration as the response type (label = ^ACEL), accelerations and stress accelerations are combined for each mode on the mode combination command.

5. Leave the SOLUTION processor.

Command(s): FINISH

GUI: Close the Solution menu.

Note — If you want to compute velocity or acceleration in addition to displacement, repeat the mode combination step after postprocessing the displacement solution by using the ^VELO or ACEL label on the mode combination commands (SRSS, CQC, GRP, DSUM, NRLSUM). Remember that the existing Jobname.MCOM file is overwritten by the additional mode combination step(s).

6.3.6. Review the Results

Results from a single-point response spectrum 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) and, if placed on the results file during the expansion pass, 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.

Note — If you want a direct combination of the derived stresses (S1, S2, S3, SEQV, SI) from the results file, issue the SUMTYPE,PRIN command before reading in the Jobname.MCOM file. With the PRIN option, component stresses are not available.

Note that the command default (SUMTYPE,COMP) is to directly operate only on the unaveraged element com-ponent stresses and compute the derived quantities from these. Refer to Section 5.5.3: Creating and Combining Load Cases in the ANSYS Basic Analysis Guide. Also, see the ANSYS Commands Reference for a description of the SUMTYPE command.

1. Read the commands on Jobname.MCOM.

Command(s): /INPUT

GUI: Utility Menu> File> Read Input From For example, issue /INPUT with the following arguments:

/INPUT,FILE,MCOM!Assumes the default jobname FILE

2. Display results.

• Option: Display Deformed Shape Command(s): PLDISP

GUI: Main Menu> General Postproc> Plot Results> Deformed Shape

• Option: Contour Displays

Command(s): PLNSOL or PLESOL

GUI: Main Menu> General Postproc> Plot Results> Contour Plot> Nodal Solu or Element Solu

Use PLNSOL or PLESOL to contour almost any result item, such as stresses (SX, SY, SZ ...), strains (EPELX, EPELY, EPELZ ...), and displacements (UX, UY, UZ ...). If you previously issued the SUMTYPE command, the results of the PLNSOL or PLESOL command are affected by the particular SUMTYPE command option (SUMTYPE,COMP or SUMTYPE,PRIN) that you selected.

Use the PLETAB command to contour element table data and PLLS to contour line element data.

Caution: Derived data, such as stresses and strains, are averaged at the nodes by the PLNSOL command. This averaging results in "smeared" values at nodes where elements of different materials, different shell thicknesses, or other discontinuities meet. To avoid the smearing effect, use selecting (described in Chapter 7, “Selecting and Components” in the ANSYS Basic Analysis Guide) to select elements of the same material, same shell thickness, etc. before issuing PLNSOL.

You can view correct membrane results for shells (SHELL, MID) by using KEYOPT(8) = 2 (for SHELL181 or SHELL93) or KEYOPT(11) = 2 (SHELL63). These KEYOPTS write the mid-surface node results directly to the results file, and allow the membrane results to be directly operated on during squaring operations. The default method of averaging the TOP and BOTsquared values to obtain a MID value can possibly yield incorrect MID values.

• Option: Vector Displays Command(s): PLVECT

GUI: Main Menu> General Postproc> Plot Results> Vector Plot> Predefined

• Option: Tabular Listings

Command(s): PRNSOL (nodal results) PRESOL (element-by-element results) PRRSOL (reac-tion data)

GUI: Main Menu> General Postproc> List Results> Nodal Solution Main Menu> General Postproc> List Results> Element Solution Main Menu> General Postproc> List Results> Reaction Solution

• Other Capabilities

Many other postprocessing functions, such as mapping results onto a path, transforming results to different coordinate systems, and load case combinations, are available in POST1. See The General Postprocessor (POST1) in the ANSYS Basic Analysis Guide for details.

If you are using batch mode, note the following:

Section 6.3: Steps in a Single-Point Response Spectrum (SPRS) Analysis

• The modal solution and spectrum solution passes can be combined into a single modal analysis [AN-TYPE,MODAL] solution pass, with spectrum loads [SV, SVTYP, SED, FREQ].

• The mode expansion and mode combination solution passes can be combined into a single modal ana-lysis [ANTYPE,MODAL and EXPASS,ON] solution pass with a mode combination command.