Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finite Element Analysis - Author J.N Reddy Book

  1. Sep 14, 2012 #1

    Is there anyone out there familiar with 'An introduction to the Finite Element Method' by J.N. Reddy?

    I am struggling to decipher what is happening on page 129 as shown in the attachment. If some-one is willing to help I will reply with a more specific query on that page. Thanks

    Attached Files:

  2. jcsd
  3. Sep 14, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    I think this would be easier to understand if you write it in matrix form.
    Suppose you have two linear elements joined end to end, so there are 3 nodes.
    If the stiffness matrices of the two elements are
    k^1_{11} & k^1_{12} \\
    k^1_{21} & k^1_{22}
    and $$\begin{bmatrix}
    k^2_{11} & k^2_{12} \\
    k^2_{21} & k^2_{22}
    The assembled stiffness matrix is
    k^1_{11} & k^1_{12} & 0 \\
    k^1_{21} & k^1_{22}+k^2_{11} & k^2_{12} \\
    0 & k^2_{21} & k^2_{22}
    And similarly for the right hand side vectors.

    You probably first met this idea in a dynamics course, setting up the equations of motion for mass-and-spring systems.
  4. Sep 15, 2012 #3
    Thanks.Actually further down the page the matrix form is shown (see attached). However, I dont see how the assembled matrix you have shown for 2 linear elements can be derived 'explicitly' from the matrix attached. Ie, the second row of attached contains ##k_{11}^3## which does not exist for a system of 2 elements....? Of course we know it does not exist hence we can simply not write it in but....

    Attached Files:

  5. Sep 15, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper

    I think the matrix in the book is slightly misleading. There should also be a vertical dotted line showing that some columns are missing, lile
    \color{red}{ k^1_{11}} & \color{red}{k^1_{12}} & & \vdots \\
    \color{red}{k^1_{21}} & \color{red}{k^1_{22} + k^2_{11}} & \color{blue}{k^2_{12}} & \vdots \\
    & \color{blue}{k^2_{21}} & \color{blue}{k^2_{22} + k^3_{11}} & \vdots \\
    \cdots & \cdots & \cdots & \ddots & \cdots & \cdots \\
    & & & \vdots & k^{N-1}_{22} + k^N_{11} & \color{red}{K^N_{12}} \\
    & & & \vdots & \color{red}{k^N_{21}} & \color{red}{K^N_{22}}

    When N = 1, you just have the first and last rows and columns forming a 2x2 matrix.

    When N = 2, you have the first second and last rows and columns forming a 3x3 matrix, i.e. the matrix entries shown in red.

    When N = 3, you have the entries shown in red and blue.
    Last edited: Sep 15, 2012
  6. Sep 15, 2012 #5
    In two elements K11^3 = 0 as there is no U4 either. Its just the notations that are generally written for more than two elements
  7. Sep 17, 2012 #6


    User Avatar
    Science Advisor
    Gold Member

    As a mechanical engineer I found it much easier to lay out the matrices as AlephZero has done (i.e. graphically), the reason the author of the book lays them out in equation or linear algebra form is basically that's what you would need if you were writing your own FEA software.

    Great posts AlephZero! That's some impressive application of TEX!
  8. Sep 18, 2012 #7
    THanks to all and particularly AlephZero. His matrix notation greatly clarifies things for me.

  9. Sep 18, 2012 #8


    User Avatar
    Science Advisor
    Homework Helper

    Actually, there's a better technology than TeX for doing this. It's called "some big sheets of paper and a pack of colored pens." :smile:

    (But Mech_Engineer probably knew that already.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Finite Element Analysis - Author J.N Reddy Book