Section 4 - Model Check

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4 – 1

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� Mistakes in engineering judgment

� Approximations to physical behavior

� Engineering theory

� Finite element theory

� Finite element implementation

� Modeling

� Bolted connection

� Welded connection

� Corners

� Transitions

4 – 2

COMMON TYPES OF ERRORS (Cont.)

� Modeling errors

� Connections

� Beam to plate

� Beam to solid

� Plate to solid

� Beam orientation

� Beam releases

� Loading (how well do you know the loading yourself?)

� Finite element error

� Round-off error (can be disastrous when it occurs)

� Computers use binary arithmetic (If you enter .1, internally it may be.099999999998)

� Program bugs (nobody’s perfect)

� A list of known errors is maintained and distributed

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4 – 3

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� Plates not lining up = zipper

� Any connections depending on in-plane rotational stiffness ofplates, or any rotational stiffness on solids

� Instabilities—for example, releasing both ends of a beam in torsion

� Offsets of elements in wrong coordinate system (should be in theoutput coordinate systems of the grid points for Bars and Beams)

� Member properties wrong (beam orientation)—also plates—membrane only—left out bending

� Beam end releases—are they local or global? (in beam coordinates)

� Element force output is normally in element coordinate system

4 – 4

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PARAM Operation

AUTOSPC,EPPRT,MAXRATIO Check relative magnitudes of matrixterms

FIXEDB Solve superelements individually Statics = fixed-boundary solution Dynamics = calculated component modes

IRES Energy error

GRDPNT Check mass, CG

USETPRT Print set tables

SESEF Strain energy fractions (superelements—SOL 103)

TINY Minimum percentage value of element strain enery for printout (Values not printed are not available for post-processing)

4 – 5

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4 – 6

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� Prints matrix trailers as the matrices are created

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File Operations

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� Sample printout using DIAG 8

DAY TIME ELAPSED I/O MB DEL_MB CPU SEC DEL_CPU SUB_DMAP/DMAP_MODULE MESSAGES

14:16:23 0:16 18.6 .0 3.5 .0 SEMG 28 EMG BEGN

*8** Module DMAP Matrix Cols Rows F T NzWds Density BlockT StrL NbrStr BndAvg BndMax NulCol

EMG 28 KELM 1 300 2 2 600 1.00000D+00 3 300 1 300 300 0 *8**

14:16:24 0:17 18.6 .0 3.5 .0 SEMG 111 EMA BEGN

*8** Module DMAP Matrix Cols Rows F T NzWds Density BlockT StrL NbrStr BndAvg BndMax NulCol

EMA 111 KJJZ 48 48 6 2 48 2.50000D–01 3 3 192 22 45 24 *8**

� Definitions

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� Use GPSTRESS, ELSDCON, or GPSDCON toselect surfaces and volumes.

� Use “OUTPUT (POST)” or “SETS DEFINITION” with

� “SURFACE” data to define surfaces

� “VOLUME” data to define volumes

SURFACE Entry Definition

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� Recommendations for discontinuous structures

� Use tolerance and branch tests to handle discontinuous stresses.

� Use local coordinate systems for orientation.

� Use “GEOMETRIC” method when element sizes differ.

� Try dividing into smaller “surfaces.”

� Use several options in one run and compare them.

� Remember that the elements are isoparametric, that is, “ideal”elements are mapped onto the real elements in the model. If thegrid point stresses differ when different options are used (or if thediscontinuities are too large), it may indicate any of the followingconditions:

� Mesh too coarse

� Elements badly shaped

� Modeling errors

4 – 11

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Pre-Analysis

� Understand the structure and the elements

� Make small models—understand the problem

� Use pilot models in areas of uncertainty

� If you are not familiar with using the element type or SOLution youexpect to use, make simple models and compare the answers totheoretical results (with a simple model, you should be able toobtain excellent correlation with theoretical results).

� Model checks before the analysis

� Geometry

� Pre–processor (or Undeformed plots)

� Look at connections between different element types

� Based on knowledge of elements

� Based on loads

� Look at corners (QUAD plates)

� Shrink plots

4 – 12

� ��

� Elements

� Beam and bar

� Check that I1 and I2 have proper orientation and values

� Check all end releases (in member coordinates)

� Verify all offsets (in output coordinate system of GRIDs)

� Material—need E, � (or G), and �

� Plates and shells

� Check aspect ratios, taper, and warpage

� Check orientation—Z, surfaces consistent

� Check attachments—especially any depending onin-plane rotational stiffness, any corners, and “shells”

� Verify any offsets (in element coordinate system )


� Property entry—be sure to get the correct properties.(One of the most commonly made errors is notspecifying MID2 for “bending” plates)

4 – 13

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� Solids

� Check aspect ratios

� Check taper

� Check attachments. If any attachments depend onrotational stiffness, special modeling effort is required


� Mass properties

� Check � on MATx entries

� Check NSM on property entries

� Bars, beams = mass/unit length

� Plates = mass/unit area

� Submit with PARAM,GRDPNT, xxxx

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� Always check center of gravity and total weight (mass)versus known values

4 – 14

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� Loadings:

� Verify they are correct (OLOAD RESULTANT)

� Constraints:

� Verfy that they are defined (often they are forgotten)

� Verify they are correct (location and orientation—in outputcoordinate system of the GRID points)

� Verify that they are applied (SPC CASE CONTROLcommand)

� Static Checks – ALWAYS RUN STATICS FIRST!!!

� Apply 1-G in X, Y, and Z directions independently

� Check load paths (GPFORCE)

� Check reactions (SPCFORCE)

� Does total = applied load?

� Are the reactions at the correct locations and dothey have the correct orientation?

� For dynamics, approximate frequency � ��

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4 – 15

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� Equilibrium check—verify model is not over-constrained

� Run free-free. Remove known constraints and check forunconstrained motion under applied loads or imposeddisplacements

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� Use DMAP Alters to check for over-constrained systems(shown later in this section)

� Thermal equilibrium check—if thermal loads are to be considered

� Check � on MATx entries

� Check for unconstrained thermal expansion – on a copy ofyour model:

� Apply a determinate set of constraints

� Use the same � for all materials

� Apply a uniform �T to the structure. It should expand“freely,” that is, with no reactions, element forces, orstresses

4 – 16

� ��

After the Analysis

� Statics

� Check EPSILON and MAXRATIO

� EPSILON > 10-9 may indicate trouble

� MAXRATIO > 105 may indicate trouble

� Check reactions. Do they equal the applied loads (� applied loadsare printed as “OLOAD RESULTANT” in superelement solutions)?

� Check load paths—use grid point force balance to “trace” loads

� Check stress contours for “consistency”

� “Sharp” corners indicate bad modeling

� Use different options (i.e., topological and geometric)and compare results

� Check stress discontinuities

� Compare values to “hand calc” or small model results

4 – 17

� ��

� Dynamics—normal modes

� Check frequencies. Are they in the expected range? (Did youforget WTMASS???)

� If free-free, are there six “rigid-body” (f=0.0) modes?

� Are there any mechanisms (f=0.0)?

� More than six “rigid-body” modes in free-free?

� Any “rigid-body” modes in constrained modes?

� Check mode shapes. Identify modes

� Plots and/or animation

� Effective weight and kinetic energy (sssalter checka.v705)help to identify “significant” modes

4 – 18

� ��

� The model (stiffness and mass matrices) should be checked toverify that elements are not (obviously) bad and that the model isnot over-constrained

� Sample—CELASi between non-coincident points or CELASi toground

� This check can be performed at various stages during theanalysis— at each stage, a different potential problem is checked

� G-set—at this stage of the solution, the elements (includingGENELs and K2GG) are checked for grounding

� N-set—at this stage, the MPC equations are checked

� A-set—(free-free only) check that the SPC’s do not over-constrainthe structure

� Although these checks can be perfromed manually, a DMAP alterwill make them easier to perform

� In the sssalter directory, the DMAP alter checka.v705 is provided formodel checkout

4 – 19

� ��

� Sample Model 1—Cantilever Beam

987654321

ÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇÇ

� Properties:I1 = 10I2 = 10J = 5A = 1E = 10,000,000.� = .3� = .1WTMASS = .002588

4 – 20

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� Input File sample1a.dat

SOL 103TIME 5include ’SSSALTERDIR:checka.v705’CENDTITLE = Practical Dynamics Seminar Sample Problem 1aSUBTITLE = Cantilever BeamLABEL = Perform Model Checks DISP(plot) = ALLSUBCASE 1 SPC = 1 METHOD = 1BEGIN BULKPARAM GRDPNT 0PARAM WTMASS .002588PARAM AUTOSPC YESGRID 1 0 0.0 0.0 0.0 0 GRID 2 0 1.25 0.0 0.0 0 GRID 3 0 2.5 0.0 0.0 0 GRID 4 0 3.75 0.0 0.0 0 GRID 5 0 5. 0.0 0.0 0 GRID 6 0 6.25 0.0 0.0 0 GRID 7 0 7.5 0.0 0.0 0 GRID 8 0 8.75 0.0 0.0 0 GRID 9 0 10. 0.0 0.0 0 CBAR 1 1 1 2 1. CBAR 2 1 2 3 1.CBAR 3 1 3 4 1.CBAR 4 1 4 5 1.CBAR 5 1 5 6 1.CBAR 6 1 6 7 1.CBAR 7 1 7 8 1.CBAR 8 1 8 9 1.SPC 1 1 123456 0.0 PBAR 1 1 1. 10. 10. 5. MAT1 1 1.+7 . 3 .1 EIGRL 1 –1. 5 ENDDATA

OUTPUT FROM SAMPLE 1A

4-21

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1 PRACTICAL DYNAMICS SEMINAR SAMPLE PROBLEM 1 JANUARY 7, 1999 MSC/NASTRAN 4/28/98 PAGE 9

CANTILEVER BEAM

0 PERFORM MODEL CHECKS

O U T P U T F R O M G R I D P O I N T W E I G H T G E N E R A T O R

0 REFERENCE POINT = 0

M O

* 1.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 *

* 0.000000E+00 1.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 5.000000E+00 *

* 0.000000E+00 0.000000E+00 1.000000E+00 0.000000E+00 –5.000000E+00 0.000000E+00 *

* 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 *

* 0.000000E+00 0.000000E+00 –5.000000E+00 0.000000E+00 3.359375E+01 0.000000E+00 *

* 0.000000E+00 5.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 3.359375E+01 *

S

* 1.000000E+00 0.000000E+00 0.000000E+00 *

* 0.000000E+00 1.000000E+00 0.000000E+00 *

* 0.000000E+00 0.000000E+00 1.000000E+00 *

DIRECTION

MASS AXIS SYSTEM (S) MASS X–C.G. Y–C.G. Z–C.G.

X 1.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00

Y 1.000000E+00 5.000000E+00 0.000000E+00 0.000000E+00

Z 1.000000E+00 5.000000E+00 0.000000E+00 0.000000E+00

I(S)

* 0.000000E+00 0.000000E+00 0.000000E+00 *

* 0.000000E+00 8.593750E+00 0.000000E+00 *

* 0.000000E+00 0.000000E+00 8.593750E+00 *

I(Q)

* 0.000000E+00 *

* 8.593750E+00 *

* 8.593750E+00 *

Q

* 1.000000E+00 0.000000E+00 0.000000E+00 *

* 0.000000E+00 1.000000E+00 0.000000E+00 *

* 0.000000E+00 0.000000E+00 1.000000E+00 *

OUTPUT FROM SAMPLE 1A (CONT)

4-22

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CANTILEVER BEAM

0 PERFORM MODEL CHECKS

^^^

^^^ NO VALUE PROVIDED FOR PARAMETER CHECKTOL

^^^

^^^ CALCULATED VALUE OF CHECKTOL = 1.228800E–01

^^^

^^^ MODEL CHECKING IS INVOKED – MSC RECOMMENDS THAT A SEPARATE RUN USING PARAM,CHECKOUT,YES SHOULD ALSO

BE DONE TO INSURE MODEL ACCURACY.

^^^

^^^ RESULTS OF RIGID BODY CHECKS OF MATRIX KGG FOLLOW

^^^

^^^ ALL 6 DIRECTIONS ARE CHECKED, ONLY THOSE DOFS WHICH FAIL WILL BE PRINTED

^^^

^^^MATRIX KGG PASSED RIGID–BODY CHECKS. THE STRAIN ENERGY IN EACH DIRECTION WAS LESS THAN 1.228800E–01

^^^

^^^NO M–SET FOR THIS SUPERELEMENT – KNN NOT CHECKED

^^^

^^^RESULTS OF CHECK OF MGG

0 MATRIX WGHTG (GINO NAME 101 ) IS A DB PREC 6 COLUMN X 6 ROW SQUARE MATRIX.

0COLUMN 1 ROWS 1 THRU 1 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

1) 1.0000D+00

0COLUMN 2 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 1.0000D+00 0.0000D+00 0.0000D+00 0.0000D+00 5.0000D+00

0COLUMN 3 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) 1.0000D+00 0.0000D+00 –5.0000D+00

0COLUMNS 4 THRU 4 ARE NULL.

0COLUMN 5 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) –5.0000D+00 0.0000D+00 3.3594D+01

0COLUMN 6 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 5.0000D+00 0.0000D+00 0.0000D+00 0.0000D+00 3.3594D+01

0THE NUMBER OF NON–ZERO TERMS IN THE DENSEST COLUMN = 2

0THE DENSITY OF THIS MATRIX IS 25.00 PERCENT.

MASSWT CSP 3.863988E+02 0.000000E+00

GRDPNT INT 0


4-23

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CANTILEVER BEAM

0 PERFORM MODEL CHECKS SUBCASE 1

^^^

^^^ RESULTS OF RIGID BODY CHECKS OF MATRIX KAA FOLLOW

^^^


^^^

^^^


^^^


^^^

^^^LARGEST STRAIN ENERGY OF 6.144000E+08 EXCEEDS PROVIDED LIMIT OF 1.228800E–01

^^^

^^^ IF THE G– OR N– CHECKS HAVE FAILED, THEY SHOULD BE RESOLVED BEFORE LOOKING AT THE A– SET CHECKS

^^^

^^^ THE FOLLOWING DIRECTIONS HAVE FAILED THE TEST

^^^

^^^

^^^DIRECTION 1 HAS STRAIN ENERGY = = 8.000000E+06

^^^


^^^


^^^


^^^


^^^


^^^

^^^ IF THE MODEL HAS PASSED THE PREVIOUS G AND N CHECKS POSSIBLE REASONS FOR FAILING THIS TEST ARE –

^^^

^^^ 1) THE STRUCTURE IS NOT INTENDED TO BE FREE–FREE

^^^ – IF THIS IS THE CASE (A CONSTRAINED STRUCTURE) FAILING THIS CHECK IS A SIGN THAT THE STRUCTURE

^^^ IS PROPERLY CONSTRAINED TO GROUND

^^^

^^^ 2) PARAM,GRDPNT IS LOCATED FAR FROM THE CG OF THE MODEL. MSC RECOMMENDS THAT GRDPNT SHOULD BE LOCATED

^^^ AS CLOSE AS POSSIBLE TO THE GEOMETRIC CENTER OF THE MODEL (SEE GPWG OUTPUT)

^^^

^^^ 3) AUTOSPC CONSTRAINING NEAR–SINGULAR DOF.

^^^ POSSIBLE CORRECTIVE ACTIONS:

^^^ – INCLUDE PARAM,K6ROT,100. OR SET PARAM,SNORM,20.

^^^ CURRENT MSC RECOMMENDATION IS TO USE PARAM,SNORM


4-24

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1 PRACTICAL DYNAMICS SEMINAR SAMPLE PROBLEM 1 JANUARY 7, 1999 MSC/NASTRAN 4/28/98

CANTILEVER BEAM


^^^STRAIN ENERGY FOR DIRECTION 1 = 8.000000E+06

^^^NORMALIZED REACTIONS FOR THAT CHECK FOLLOW

^^^ONLY TERMS LARGER THAN 1.000000E–01 ARE PRINTED

^^^

^^^ USE PARAM,PRINTOL TO CHANGE THIS VALUE

0 REACCOLA

POINT VALUE POINT VALUE POINT VALUE POINT VALUE POINT VALUE

COLUMN 1

2 T1 1.00000E+00




^^^


0 REACCOLA


COLUMN 1

2 T2 1.00000E+00 2 R3 –6.25000E–01




^^^


0 REACCOLA


COLUMN 1

2 T3 1.00000E+00 2 R2 6.25000E–01


4-25

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^^^


0 REACCOLA


COLUMN 1

2 R1 1.00000E+00




^^^


0 REACCOLA


COLUMN 1

2 T3 –1.00000E+00 2 R2 –4.16667E–01




^^^


0 REACCOLA


COLUMN 1

2 T2 1.00000E+00 2 R3 –4.16667E–01


4-26

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^^^RESULTS OF CHECK OF MAA


CANTILEVER BEAM


0 MATRIX WGHTA (GINO NAME 101 ) IS A DB PREC 6 COLUMN X 6 ROW SQUARE MATRIX.

0COLUMN 1 ROWS 1 THRU 1 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

1) 9.3750D–01

0COLUMN 2 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 9.3750D–01 0.0000D+00 0.0000D+00 0.0000D+00 5.0000D+00

0COLUMN 3 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) 9.3750D–01 0.0000D+00 –5.0000D+00


0COLUMN 5 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) –5.0000D+00 0.0000D+00 3.3594D+01

0COLUMN 6 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 5.0000D+00 0.0000D+00 0.0000D+00 0.0000D+00 3.3594D+01



0 MASSWT CSP 3.863988E+02 0.000000E+00

0 GRDPNT INT 0

4-27

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� Grid Point Weight Output (GPWG module)

� The scale factor entered with parameter WTMASS is applied to theassembled element mass before GPWG. The GPWG module,however, converts mass back to the original input units that existedprior to the scaling effect of the parameter WTMASS

� GPWG is performed on the g-size mass matrix, which is the massmatrix prior to the processing of the rigid elements, MPCs, andSPCs

� Any masses at scalar points and fluid-related masses are notincluded in the GPWG calculation

� GPWG for a superelement does not include the mass fromupstream superelements. Therefore, GPWG for the residualstructure includes only the mass of the residual (not any upstreamsuperelements). The center of gravity location is also based on themass of the current superelement only

� The output from the GPWG is for information purposes only and isnot used in the analysis

� The rigid-body mass matrix [M0] is computed with respect to thereference grid point in the basic coordinate system. The Grid pointto be used is specified using PARAM, GNDPNT

� For further information see the MSC/NASTRAN Linear StaticAnalysis User’s Guide (V68), Appendix B

4-28

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� Stiffness Check Output

� These checks are performed by multiplying the stiffness matrix by aset of rigid-body vectors(Rb) which are based on the geometry(calculated about PARAM,GRDPNT)

� The rigid-body strain energy checks are calculated as (note that thefactor of 1/2 is not included in the calculation)

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� This check is performed on the G-, N-, and A- set matrices (i inCHKKii is the set being checked)

� If any term in the resulting “CHK” matrix exceeds the value ofPARAM,CHECKTOL (default value is calculated base on thestiffness of your model), the results of the check are printed

� “Reaction forces” are calculated, normalized to a maximum of 1.0,filtered, and printed (if CHECKTOL is exceeded )

� ��

� Note that “full” data recovery is not performed, and if a dof whichdoes not belong to the remaining set is constrained, the nearestpoint (by connection) in the remaining set is indicated. See resultsfor CHKKAA—point 1 is constrained, but does not belong to theA-set, therefore, the constraint shows up at point 2

4-29

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� Mass Check Output

� These checks are performed by multiplying the mass matrix by aset of rigid-body vectors(Rb) which are based on the geometry(calculated about PARAM,GRDPNT)

� The calculation is similar to that performed on the stiffness matrix

� The results at the G-set should match Grid Point Weight Generator

� The checks at the N- and A-set check if MPCs and constraintsremove (or re-distribute) mass

4-30

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� Same model as before, only now connect a CELAS2 elementbetween dof 2 at Grid Points 2 and 3 (this will cause “grounding”),as the direction of the stiffness terms is not along the lineconnecting the GRID points)

� Samples of CELASi elements which cause “grounding”

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4-31

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� Input File – sample1b.dat

ID MSC–XL, MSC–NASTRANSOL 103TIME 5include ’SSSALTERDIR:checka.v705’CENDTITLE = Practical Dynamics Seminar Sample Problem 1bSUBTITLE = Cantilever BeamLABEL = Perform Model Checks – bad CELAS2 element in fileDISP(plot) = ALLSUBCASE 1 SPC = 1 METHOD = 1BEGIN BULKPARAM GRDPNT 0PARAM WTMASS .002588PARAM POST 0PARAM AUTOSPC YES$$ add a CELAS2 which is incorrectly specified$CELAS2 99 1000. 2 2 3 2$GRID 1 0 0.0 0.0 0.0 0 GRID 2 0 1.25 0.0 0.0 0 GRID 3 0 2.5 0.0 0.0 0 GRID 4 0 3.75 0.0 0.0 0 GRID 5 0 5. 0.0 0.0 0 GRID 6 0 6.25 0.0 0.0 0 GRID 7 0 7.5 0.0 0.0 0 GRID 8 0 8.75 0.0 0.0 0 GRID 9 0 10. 0.0 0.0 0 $CBAR 1 1 1 2 1. CBAR 2 1 2 3 1.CBAR 3 1 3 4 1.CBAR 4 1 4 5 1.CBAR 5 1 5 6 1.CBAR 6 1 6 7 1.CBAR 7 1 7 8 1.CBAR 8 1 8 9 1.SPC 1 1 123456 0.0 PBAR 1 1 1. 10. 10. 5. MAT1 1 1.+7 .3 .1 EIGRL 1 –1. 5 ENDDATA

OUTPUT FROM SAMPLE 1B

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1 PRACTICAL DYNAMICS SEMINAR SAMPLE PROBLEM 1B JANUARY 7, 1999 MSC/NASTRAN 4/28/98 PAGE 10

CANTILEVER BEAM

0 PERFORM MODEL CHECKS – BAD CELAS2 ELEMENT IN FILE

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^^^LARGEST STRAIN ENERGY OF 1.562500E+03 EXCEEDS LIMIT OF 1.228801E–01

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^^^ SOME POSSIBLE REASONS FOR THIS ARE –

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^^^ 1) CELASI ELEMENTS CONNECTING TO ONLY ONE GRID POINT

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^^^ 2) CELASI ELEMENTS CONNECTION TO NON–COINCIDENT POINTS

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^^^ 3) CELASI ELEMENTS CONNECTING TO NON–COLINEAR DOF

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^^^ 4) IMPROPERLY DEFINED DMIG MATRICES

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^^^REACTION FORCES FOR THIS CHECK FOLLOW –

^^^ THESE TERMS ARE NORMALIZED TO A MAXIMUM VALUE OF

^^^1.OE+0. ONLY TERMS LARGER THAN 1.000000E–01 ARE PRINTED

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^^^

0 REACCOLG


COLUMN 1

2 T2 –1.00000E+00 3 T2 1.00000E+00

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� At the G-set, the structural matrices are grounded when the alterattempts to rotate the model about the z-axis

� This is indicated by the large term in the CHKKGG matrix for dof 6

� By looking at the REACGNRM matrix—this matrix represents theforces (normalized to a maximum of 1.0) preventing the model frommoving as a rigid body. The column associated with dof 6(z–rotation) contains terms for dof 2 of GRID points 2 and 3,indicating that a modeling error exists in that area

� This is the location of the CELAS2

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� Same model as before, only now connect an MPC between dof 2 atGrid Points 2 and 3 (since the points are not coincident, this willcause “grounding”)

� The MPC states that the y-direction translation of Grid Point 2 mustequal the y-direction translation of Grid Point 3

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� Input File

SOL 103TIME 5include ’SSSALTERDIR:checka.v705’CENDTITLE = Practical Dynamics Seminar Sample Problem 1cSUBTITLE = Cantilever BeamLABEL = Perform Model Checks – MPC equationDISP(plot) = ALLSUBCASE 1 MPC = 2 SPC = 1 METHOD = 1BEGIN BULKMPC,2,2,2,1.,3,2,–1.PARAM GRDPNT 0PARAM WTMASS .002588PARAM POST 0PARAM AUTOSPC YESGRID 1 0 0.0 0.0 0.0 0 GRID 2 0 1.25 0.0 0.0 0 GRID 3 0 2.5 0.0 0.0 0 GRID 4 0 3.75 0.0 0.0 0 GRID 5 0 5. 0.0 0.0 0 GRID 6 0 6.25 0.0 0.0 0 GRID 7 0 7.5 0.0 0.0 0 GRID 8 0 8.75 0.0 0.0 0 GRID 9 0 10. 0.0 0.0 0 CBAR 1 1 1 2 1. CBAR 2 1 2 3 1.CBAR 3 1 3 4 1.CBAR 4 1 4 5 1.CBAR 5 1 5 6 1.CBAR 6 1 6 7 1.CBAR 7 1 7 8 1.CBAR 8 1 8 9 1.SPC 1 1 123456 0.0 PBAR 1 1 1. 10. 10. 5. MAT1 1 1.+7 .3 .1 EIGRL 1 –1. 5 ENDDATA

OUTPUT FROM SAMPLE 1C

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0 PERFORM MODEL CHECKS – BAD MPC EQUATION

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^^^MATRIX KGG PASSED RIGID–BODY CHECKS. THE STRAIN ENERGY IN EACH DIRECTION WAS LESS THAN 1.228800E–01

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^^^ RESULTS OF RIGID BODY CHECKS OF MATRIX KNN FOLLOW

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^^^LARGEST STRAIN ENERGY OF 1.920000E+09 EXCEEDS PROVIDED LIMIT OF 0.000000E+00

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^^^ SOME POSSIBLE CAUSES OF THIS ARE –

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^^^ 1) MPC EQUATIONS WHICH DO NOT SATISFY RIGID–BODY MOTION

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^^^ 2) RBE3 ELEMENTS FOR WHICH THE INDEPENDENT DOF CANNOT DESCRIBE ALL POSSIBLE RIGID–BODY MOTIONS

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0 REACCOLN


COLUMN 1

1 T2 –1.00000E+00 1 R3 –6.25000E–01 3 T1 1.00000E+00 3 R2 6.25000E–01

OUTPUT FROM SAMPLE 1C (CONT)

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^^^RESULTS OF CHECK OF MGG

0 MATRIX WGHTG (GINO NAME 101 ) IS A DB PREC 6 COLUMN X 6 ROW SQUARE MATRIX.

0COLUMN 1 ROWS 1 THRU 1 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

1) 1.0000D+00

0COLUMN 2 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 1.0000D+00 0.0000D+00 0.0000D+00 0.0000D+00 5.0000D+00

0COLUMN 3 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) 1.0000D+00 0.0000D+00 –5.0000D+00


0COLUMN 5 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) –5.0000D+00 0.0000D+00 3.3594D+01

0COLUMN 6 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 5.0000D+00 0.0000D+00 0.0000D+00 0.0000D+00 3.3594D+01



0 ^^^RESULTS OF CHECK OF MNN

0 MATRIX WGHTN (GINO NAME 101 ) IS A DB PREC 6 COLUMN X 6 ROW SQUARE MATRIX.

0COLUMN 1 ROWS 1 THRU 1 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

1) 1.0000D+00

0COLUMN 2 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 1.0000D+00 0.0000D+00 0.0000D+00 0.0000D+00 5.1563D+00

0COLUMN 3 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) 1.0000D+00 0.0000D+00 –5.0000D+00


0COLUMN 5 ROWS 3 THRU 5 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

3) –5.0000D+00 0.0000D+00 3.3594D+01

0COLUMN 6 ROWS 2 THRU 6 ––––––––––––––––––––––––––––––––––––––––––––––––––

ROW

2) 5.1563D+00 0.0000D+00 0.0000D+00 0.0000D+00 3.4180D+01



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� At the G-set, the structural matrices pass the rigid–body tests, sincethe CELAS2 which caused the problem in sample1b has beenremoved

� Now matrix KNN fails the rigid–body test due to theincorrectly–specified MPC equation (which does not satisfyrugud–body requirements)

� This is indicated by the large term in the CHKKNN matrix for dof 6

� By looking at the REACNCOL matrix—this matrix represents theforces (normalized to a maximum of 1.0) preventing the model frommoving as a rigid body. The 6th column contains terms for GRIDpoints 1 and 3, indicating that a modeling error exists in that area

� This is the location of the MPC (NOTE – since the test is performedon the N–set, GRID 2 dof 2 no longer exists, since it is in the M–setand has been removed)

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� Take the time to understand the structure and how it behaves underload. Perform hand analysis or use a simple model first.

� Take the time to understand MSC/NASTRAN (particularly theelements). Run small samples each time you try something new.

� Use independent checks (if available).

� Estimate the cost (labor and computer costs) before you start.

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� Identify your modes using one or more of the following:

� Plot your eigenvectors (either using the MSC/NASTRAN plotter orMSC/PATRAN) and identify them

� Try setting NORM=MAX on EIGRL entry and look at modalmasses. Small values may indicate singularities or local modes(not recommended)

� Use alters to print kinetic energy and modal effective mass (SeePractical Dynamic Analysis Seminar Notes).

� Watch for warnings on orthogonality checks.

� Look for extraneous low frequency modes – these often indicateincorrect modeling (for example plate elements without MID2 on thePSHELL entry).

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� ��

� In dynamic analysis, use normal modes as a diagnostic tool.

� Simulate statics in modal dynamic solutions and compare theresults to a static solution (this is a way to determine if your modesare capable of representing the solution).

� In Transient analysis, apply a constant loading

� In frequency response, apply the load at 0.0 hz

� Use sssalter “modevala.vxx” to see if your modes can represent thesolution if the loads are applied “statically” (although you arelooking at a dynamic solution, it is hard for the modes to representthe dynamic solution under loading if they cannot represent thestatic solution).

� Selecting time or frequency set selection can have a major impacton the solution accuracy.

� In Transient response, the accuracy is directly related to theintegration time step (A central difference is used to calculate thevelocity and acceleration). If you are using a direct solution, runusing different integration time steps to see if the answers change.

� In Frequency Response, the peak responses occur at resonance.Use a Modal solution with FREQ3, FREQ4, and/or FREQ5 entriesto guarantee that the solution is obtained at resonance.

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Stiffened Plate with Error in Modeling

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� �� !� ��

� understand your structure and how you expect it to perform

� Understand your loading.

� understand your model

� understand how to use the program

� understand the limitations of the method

� use small sample problems to understand the solution

� ALWAYS perform a static solution first, then progress to the morecomplicated solutions

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Section 4 - Model Check

Documents

plate beam

finite element error

bending beam end releasesare

use gpstress

solid plate

surface data

module messages

wrong coordinate system