Blog of eng. Xosé Carreira (since 2007)

Structures, water, computers, languages and people (not necessarily in this order)

Resonance in practice

Second bending mode of vibration.
Twisting mode.
Putting all together, we have something like a Dutch roll.

on Thursday, September 06, 2007

11 comments:

  1. David Gonzalez-RodriguezFriday, September 07, 2007

    That looks sexy!

    ReplyDelete
  2. Suso ListaTuesday, September 11, 2007

    Hostia! vasme facer aprender inglés, eu que xa o creía superado da ,miña faceta de mariño mercante. Saludos.

    ReplyDelete
  3. Xosé Manuel CarreiraWednesday, September 12, 2007

    Ola Susiño, benvido! (hello Suso, Welcome!)

    ReplyDelete
  4. AnonymousThursday, September 13, 2007

    Caro xm, parabéns pelo blog. é uma referência para mim. podes-me indicar o link do opencourseware da universidade da california em berkeley, que indicaste no meu blog detrolhaaengenheiro.wordpress.com

    ReplyDelete
  5. Suso ListaThursday, September 13, 2007

    Jelou XM, jaguar llu. Ai si llu in mai blog an giar, je je.

    ReplyDelete
  6. Xosé Manuel CarreiraFriday, September 14, 2007

    @ Nelson Costa: Obrigado. Thank you for the compliments but I must admit that your blog is much useful than mine.

    You will find the link to Berkeley OCW in Top 10 Unis open courses. See you.

    ReplyDelete
  7. AnonymousFriday, September 14, 2007

    Oh! Resonance in bridges! Sure you have seen the video of the mighty Bridge of Tacoma at School (I mean you "Superior Techincal School"). "Why did Tacoma Bridge fall?" was a rather sure question in our 1st course physics exam, but I enjoyed more when (in the same video where explained Tacoma collapse) the narrator recorded the sound of a little hit in a glass and broke it by playing a sound of the same frecuency with a sintetizer.

    ReplyDelete
  8. Xosé Manuel CarreiraFriday, September 14, 2007

    Engineers tend to use the term resonance (overdamping) also as a synomim of self-excitation of structures. This is not very correct but it is a quite common mistake. Unfortunately, I also tend to talk about resonance for any dynamic instability, i.e. the example of the Dutch roll of the Millenium footbridge is not exactly (or not only) a problem of resonance.

    The collapse of the Old Tacoma Narrows Bridge in the 40s has been referred as a classical example of resonance, but this description is not fully correct. It is more correct to say that it failed due to the action of self-excitation of the deck under the action of the wind. The original video can be watched here. Amazing!

    ReplyDelete
  9. AnonymousThursday, September 20, 2007

    Could you give more detail on your last comment? What is the difference between resonance and self-excitation? Let's talk about the case of the Tacoma bridge, although I am not a structural engineer and my knowledge of dynamical systems is limited. My understanding is that a bridge is most susceptible to respond to certain frequencies of the external forcing, which correspond to its natural frequencies. Once the bridge gets excited in the dominant natural frequency, the oscillation gets amplified due to the coupling between forcing and oscillation (resonance). Therefore, I would have said that the Tacoma bridge accident was a case both of self-excitation and resonance, since both phenomena seem to be related. Is this correct?

    ReplyDelete
  10. Xosé Manuel CarreiraFriday, September 21, 2007

    This abstract explain it better than me. Basically self-excitation theory takes into account that the wind load depends on the exposed area; the exposed area depends on the displacements, and the deformed geometry depends on the wind load.

    Even from a classical point of view, bridges have a very nonlinear behaviour (because of their geometry and their materials) so the stiffness matrix is not constant (and the natural frequencies can vary with the applied load). A mere coincidence of the frequency of the load and the eigenvalues of the structure (resonance, negative damping) does not imply a rapid collapse, but, for obvious reasons, the situation must be avoided.

    ReplyDelete
  11. David Gonzalez-RodriguezFriday, September 21, 2007

    Thanks for the reference, I'll take a look at the article. It sounds like they will be offering an interesting explanation, since they don't only look at the structural response of the bridge, but also to the behavior of the air flow past the deck.

    ReplyDelete

Subscribe to: Post Comments (Atom)