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modal analysis of simply supported beam

i have done experimental, analytical and numerical modal analysis of simply supported beam. the analytical and experimental results are in corellation.

i have a problem with numerical analysis. i modeled the beam in ansys and solved the problem with the follong boundary conditions:

  1. restricted the DOF's along the length at both ends
  2. restricted the DOF's along the thickness of the beam at both ends

the modal analysis of the above model gave me 0 Hzs as the first natural frequency which is not the case with exp and analytical methods.

can some one help me in this regard are my boundary condition's wrong ???

Reply to
thota.manoj
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A few questions........

How did the higher modes correlate? Did oyu animate the mode shapes?

What were the BC's for the experimental work? Clamped at both ends? A knife edge or roller with a clamp a one end?

The BC's described for the ANSYS model do not strike me as BC's for an SS beam

Each end of the beam has 6 DOF's in 3D model ...Is this a 3D model or a 2D model?

Which DOF's did restrian? for a 2D model........one end gets two fixed DOF's....the other only one

For a 3D model you need more restraints.

A "zero" Hz model can indicate a rigid body mode.....inadequate restraints.

I would suggest a simplier program than ANSYS to double check your work.

cheers Bob

Reply to
BobK207

the corellation was between theoritical and experimental work. i used a FFT analyser to find the transfer function. and using that function i found the first 3 natural frequencies. i used the formulas given in harris hand book and found the same. the values are in correlation.

i clamped the beam on both the sides using knife edges. thus i was able to constrain 5 dof except the rotation about the axis perpendicular to the thickness at both the ends

Reply to
thota.manoj

mr bob

after reading ur post i came to know the mistake with regards to the dof. this time i solved the problem with 5 dof constraied (simulating the experimental conditions) and i was sucessful this time.

thank u bob.

Reply to
thota.manoj

btw u are talking about some simpler method to cross chk my work !!! wht's that ??

Reply to
thota.manoj

You are welcome. Its been a long time for me since any ANSYS work

but I has been my experience that a 0Hz mode and bad correlation is usually a BC problem.

With complex computer codes & 3D models it is easy to get the BC's wrong.

ANSYS is a very powerful complex FEM code ...over kill for a 2D beam.

I was thinking of a beam analysis program like Multiframe.......again my experience is that the simpliest tool that will do the job is better but

if you're just testing out ANSYS until you try a more complicated model then you have other reasons for using ANSYS.

Multiframe has a free demo version for download.

cheers Bob

I hope my English isn't too hard for you to understand. Where are you posting from?

Reply to
BobK207

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