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Structural Engineering -12x12 stiffness matrix analysis

  1. Oct 6, 2008 #1
    I am trying to write a MATLAB script to do an elastic analysis of 3D frame sturctures. The method I am using is the direct stiffness method, but I cannot find the equations for the 12x12 stiffness matrix for each member in the global system.

    If anyone has any information on 3D matrix analysis I would greatly appreciate the help. So far none of the texts I have found have explicitly given these formula.

    I have the 6x6 matracies for 2D analysis if anyone is interested.

    Thank You

  2. jcsd
  3. Oct 7, 2008 #2


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    I am assuming that you are using something along the lines of a truss element, but in a 3D structure, i.e. members can carry no bending moments.

    Take a look here :
    http://www.google.com/url?sa=t&source=web&ct=res&cd=6&url=http%3A%2F%2Fwww.engineer.tamuk.edu%2Fdepartments%2Fieen%2Ffaculty%2FDrLPeel%2FCourses%2FMeen5330%2Fstudent_presentations%2FFEA_F06.ppt&ei=bkvrSKziMdywmQf0mcHaCg&usg=AFQjCNFyXckTs9v56tKt3qI9Yc-c1jALjQ&sig2=k9yvGnIMuqDkYuKnyPc9kg [Broken]

    Towards the back of the presentation it talks about the 3D truss stiffness matrix. Both of my FEA books also discuss this as well. See if you can get a hold of a copy from your library or a fellow student.
    Last edited by a moderator: May 3, 2017
  4. Oct 7, 2008 #3
    Thank you for your reply. That is almost exactly what I am chasing, tho I need the 12x12 stiffness matraix equations because I am considering bending so I will have the 6 additional degree of freedom to account for due to the rotation of each node.

    If I find the equations I will post them here. Are you allowed to photo copy pages from text books and post them here or is that against forum rules?

    Thanks, Elbarto
  5. Oct 8, 2008 #4


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    Oh. So you're using some kind of beam element. I'll see if I can dig them up in my FEA book.
  6. Oct 8, 2008 #5
    Yes I am doing a full 3D frame element so I have to consider all 12 DOF's for each member. I had a look in my library today and think I managed to find the local member stiffness equations which will probably work ok if I play around with them a bit but I would appreciate it if you could check your text book out.

    Thanks alot
    Regards Elbarto
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