University of Florida/Eml4507/s13.team3.Guzy

The free body diagram is defined by the matrix

 EDU>> Edof=[1 1 2;2 2 3;3 2 3];

Next, create the load force matrix

 EDU>> f=[0 100 0];

Next each element stiffness matrix is created

 EDU>> K= [0 0 0;0 0 0;0 0 0];
 EDU>> k=1500;
 EDU>> ep1=k;
 EDU>> ep2=2*k;
 EDU>> Ke1=springle(ep1);
 EDU>> Ke2=springle(ep2);

Now we create the global stiffness matrix

 EDU>> K=assem(Edof(1,:),K,Ke2);
 EDU>> K=assem(Edof(2,:),K,Ke1);
 EDU>> K=assem(Edof(3,:),K,Ke2);

Boundary conditions are now applied

 EDU>> bc=[1 0; 3 0];
 EDU>> [a,r]=solveq(K,f,bc);

Displacements are found:

 EDU>> ed1=extract(Edof(1,:),a)
 ed1 =
     0      0.0133
 EDU>> ed2=extract(Edof(2,:),a)
 ed2 =
     0.0133      0 
 EDU>> ed3=extract(Edof(3,:),a)
 ed3 =
     0.0133      0 

Spring forces are evaluated

 EDU>> es1=spring1s(ep2,ed1)
 es1 =
     40
 EDU>> es2=spring1s(ep1,ed2)
 es2 =
    -20
 EDU>> es3=spring1s(ep2,ed3)
 es3 =
    -40

By hand:

${\displaystyle \displaystyle f=kd}$

${\displaystyle \displaystyle f=force}$

${\displaystyle \displaystyle k=stiffness}$

${\displaystyle \displaystyle d=displacement}$ In matrix form: ${\displaystyle {\begin{bmatrix}f_{1x}\\f_{2x}\\\end{bmatrix}}={\begin{bmatrix}k&-k\\-k&k\\\end{bmatrix}}{\begin{bmatrix}d_{1x}\\d_{2x}\\\end{bmatrix}}}$ First we create a matrix to show the systems Degree of Freedoms. The first column represents each spring(#1,#2,#3). The second and third column represent the connectivity of each node. ${\displaystyle DOF={\begin{bmatrix}1&1&2\\2&2&3\\3&2&3\\\end{bmatrix}}}$ The following is a matrix of the Force vector ${\displaystyle F={\begin{bmatrix}F_{1x}\\F_{2x}\\F_{3x}\\\end{bmatrix}}}$ Next we need each element stiffness matrix for every spring: Element stiffness matrix for Spring #1: ${\displaystyle k_{1}={\begin{bmatrix}3000&-3000\\-3000&3000\\\end{bmatrix}}={\begin{bmatrix}3000&-3000&0\\-3000&3000&0\\0&0&0\\\end{bmatrix}}}$ Element stiffness matrix for Spring #2: ${\displaystyle k_{2}={\begin{bmatrix}1500&-1500\\-1500&1500\\\end{bmatrix}}={\begin{bmatrix}0&0&0\\0&1500&-1500\\0&-1500&1500\\\end{bmatrix}}}$ Element stiffness matrix for Spring #3: ${\displaystyle k_{3}={\begin{bmatrix}3000&-3000\\-3000&3000\\\end{bmatrix}}={\begin{bmatrix}0&0&0\\0&3000&-3000\\0&-3000&3000\\\end{bmatrix}}}$ To get the global stiffness matrix, we add up each element stiffness matrix: ${\displaystyle K={\begin{bmatrix}3000&-3000&0\\-3000&3000&0\\0&0&0\\\end{bmatrix}}+{\begin{bmatrix}0&0&0\\0&1500&-1500\\0&-1500&1500\\\end{bmatrix}}+{\begin{bmatrix}0&0&0\\0&3000&-3000\\0&-3000&3000\\\end{bmatrix}}={\begin{bmatrix}3000&-3000&0\\-3000&7500&-4500\\0&-4500&4500\\\end{bmatrix}}}$ Plugging each matrix into Hooke's Law we obtain: ${\displaystyle {\begin{bmatrix}3000&-3000&0\\-3000&7500&-4500\\0&-4500&4500\\\end{bmatrix}}{\begin{bmatrix}d_{1x}\\d_{2x}\\d_{3x}\\\end{bmatrix}}={\begin{bmatrix}F_{1x}\\F_{2x}\\F_{3x}\\\end{bmatrix}}}$ Because node one and three are attached to the wall, their displacements are zero: ${\displaystyle \displaystyle d_{1x}=0}$ ${\displaystyle \displaystyle d_{3x}=0}$ Now we solve the follwing equation: ${\displaystyle {\begin{bmatrix}3000&-3000&0\\-3000&7500&-4500\\0&-4500&4500\\\end{bmatrix}}{\begin{bmatrix}0\\d_{2x}\\0\\\end{bmatrix}}={\begin{bmatrix}F_{1x}\\F_{2x}\\F_{3x}\\\end{bmatrix}}}$ Solving the System we obtain the following equations: ${\displaystyle \displaystyle 3000*0-3000*d_{2x}+0*d_{3x}=F_{1x}}$ ${\displaystyle \displaystyle -3000*0+7500*d_{2x}-4500*d_{3x}=F_{2x}}$ ${\displaystyle \displaystyle 0*0-4500*d_{2x}+4500*d_{3x}=F_{3x}}$ Simplifying the above equations we obtain: ${\displaystyle \displaystyle -3000*d_{2x}=F_{1x}}$ ${\displaystyle \displaystyle 7500*d_{2x}-4500*d_{3x}=F_{2x}}$ ${\displaystyle \displaystyle -4500*d_{2x}+4500*d_{3x}=F_{3x}}$ Solving we obtain: ${\displaystyle \displaystyle F_{1x}=40}$ Plugging this value in ${\displaystyle \displaystyle {\frac {40}{-3000}}=d_{2x}=-0.01333}$ Using substitution for the remaining values, we obtain magnitudes of: ${\displaystyle d_{1x}=0}$ ${\displaystyle d_{2x}=0.01333}$ ${\displaystyle d_{3x}=0}$ ${\displaystyle F_{1x}=40}$ ${\displaystyle F_{2x}=20}$ ${\displaystyle F_{3x}=40}$

1. ↑ http://upload.wikimedia.org/wikiversity/en/b/b1/Fead.s13.sec53a.djvu Obtained from calfem34: p.53-2b]
2. ↑ http://upload.wikimedia.org/wikiversity/en/b/b1/Fead.s13.sec53a.djvu Obtained from calfem34: p.53-2b]