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 use ANSYS 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, 0.0
n, 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.1
mp, 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 zero
d, 3, uy, 0.             ! Displacement at node 3 in y-dir is zero
d, 2, ux, 0.
d, 2, uy, 0.

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

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.

 

(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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