8/31/2014 Using ANSYS Simple truss problem Truss1

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Using ANSYS - Natural Frequencies of a Simple Truss

Objective: To prepare a text file defining the ANSYS FEM model for the elementary truss problem shown below and useANSYS to solve for natural frequencies and modes.

1. Prepare a text file containing the following information. The 1000 lbf load is not needed.

/FILNAM,truss/title, Simple Truss

/prep7

n, 1, 25.0, 0.0, 0.0 ! Node 1 is located at (25.0, 0.0, 0.0)n, 2, 0.0, 18.0, 0.0n, 3, 0.0, 0.0, 0.0

et, 1, link1 ! Element type; no.1 is link1

!Beginning with ANSYS Release 13 link1 has been replaced with link180

mp, ex, 1, 3.e7 ! Material Properties, E for material no. 1 mp, prxy, 1, 0.3 ! poisson's ratio for material no.1mp, dens, 1, 0.736e-3 ! mass density for material no. 1

r, 1, 0.5 ! 'Real Constant' number 1 is 0.5

! (Cross sectional area)en, 1, 3, 1 ! Element Number 1 connects nodes 1& 2

en, 2, 2, 1

d, 3, ux, 0. ! Displacement at node 3 in x-dir is zerod, 3, uy, 0. ! Displacement at node 3 in y-dir is zerod, 2, ux, 0.d, 2, uy, 0.

2. Start ANSYS and select 'Run Interactive' Select the Working Directory

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3. Select File > Read Input From (locate and select the file truss.txt that you prepared)

4. Solution > Analysis Type > New Analysis > Modal > OK

5. Analysis Options > Block Lanczos

No. of Modes to Extract 2 Expand mode shapes yes

NMODE No. modes to expand 2

OK

FREQB 0 FREQE 0

Normalize mode shapes To mass matrix

OK

6. Solve > Current LS > OK

7. General Postprocessor > Results Summary (Shows frequencies calculated.)

8. Read Results > First Set

9. Plot Results > Deformed Shape > Def. + Undef.

The first mode of vibration is shown in the figure below.

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(Use PlotCtrls > CaptureImage to print or save your plots. Can also use Animate to visualize motion.)

To list the nodal deformation values for the mode use

List Results > Nodal Solution > DOF solution > All DOFs DOF

10. For the next mode: Read Results > Next Set

11. Plot Results > Deformed Shape > Def. + Undef. etc.

Last update 5-2-03

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