×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Modal analysis of a simple cantilever beam

Modal analysis of a simple cantilever beam

Modal analysis of a simple cantilever beam

(OP)
Hi,
I ran a modal analysis of a hollow rectangular section cantilever beam with beam4, beam8 and beam188 elements. Beam4 and Beam44 produced the same frequencies for bending modes and match closely to hand calculations. But results with Beam188 differ by ~30%. Can anyone explain the reason for this behavior?

Thank You.

RE: Modal analysis of a simple cantilever beam

The difference you're seeing is due to the theory which goes into the element formulation.  Older beam elements such as BEAM4 and BEAM8 are based on Euler-Bernoulli beam theory.  Newer beam elements, such as BEAM188 are based on Timoschenko (shear-deformable) theory.  Do a little searching on the web or through an old text book if you'd like to know what those theories are based on exactly.  For most applications BEAM188's are probably a better choice...but that's just my opinion.

Good luck,
-Brian

RE: Modal analysis of a simple cantilever beam

(OP)
I knew about the theories behind the beam elements. I thought shear deformation does not play any role in modal analysis. But the results contradict this. Why?

Thanks.

RE: Modal analysis of a simple cantilever beam

Hi cspkumar,

are you shure, teh real constants of the beam8 and beam44 are correct?

Regards,
Alex

RE: Modal analysis of a simple cantilever beam

(OP)
The moment of inertia's are exactly the same for Beam4, Beam44 and Beam188 elements. Also, modeled with shell elements and the frequencies are different. If E, I and m are the same then, frequencies should be the same I thought, correct?

Thanks.

RE: Modal analysis of a simple cantilever beam

What your length to thickness ratio? If it is larger than say 10 then shear deformation isn't the reason for the difference...even though shear deformation is taken into account in modal analysis. It's a linear effect. Anyway are you sure your results are mesh converged? I didn't check but I think that beam188's use lower order polynomials in bending than beam4's so they may converge slower.

RE: Modal analysis of a simple cantilever beam

(OP)
The slenderness ratio is well above the minimum. Increasing the mesh density did not change the frequencies in beam188 elements.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources