How to program node 3. It is a 3element (spring assembly) 4 element

P2 = 1000
P4 = 4000
kel1 = 1000*[1 -1;-1 1]
kel2 = 1000*[1 -1;-1 1]
kel3 = 1000*[1 -1;-1 1]
k = [1000,1000,1000];
elem_con = [1 2;2 3;3 4]
elem = size(elem_con,1)
nodes = elem+1
K = zeros(nodes)
%Forces
F = [0;1000;0;4000]
%Assembly
for i=1:elem
    ke = k(i)*[1 -1;-1 1]
    node1 = elem_con(i,1);
    node2 = elem_con(i,2);
    node3 = elem_con(i,3);
    K(node1,node1) = K(node1,node1) + ke(1,1)
    K(node1,node2) = K(node1,node2) + ke(1,2)
    K(node2,node1) = K(node2,node1) + ke(2,1)
    K(node2,node2) = K(node2,node2) + ke(2,2)
    K(node3,node2) = K(node3,node2) + ke(3,2)
    K(node3,node3) = K(node3,node3) + ke(3,3)
end
d = zeros(nodes,1)
d1 = 0;
d(2:4) = K(2:4,2:4)\F(2:4)
F = K*d
del1 = d(1:2)
del2 = d(2:3)
del3 = d(3:4)
fel1 = kel1*del1
fel2 = kel2*del2
fel3 = kel3*del3

[Venkat Siddarth]

Hi @Karlo,

In order to better assist you.Please elaborate your query.

Regards

Venkat Siddarth V

Answer 1

[Gyan Vaibhav]

Hi Karlo,

It seems like you are trying to assemble the global stiffness matrix K for a 3-element spring assembly system and solve for the displacements d and forces F. However, there seems to be a mistake in your code.

You are assuming a 3-node element, but in your element connectivity matrix “elem_con”, you have defined only 2 nodes per element.

Given the provided information, I will assume you have a 1D spring assembly system with 4 nodes and 3 spring elements between them.

You can modify your code as follows:

P2 = 1000;
P4 = 4000;
kel1 = 1000*[1 -1;-1 1];
kel2 = 1000*[1 -1;-1 1];
kel3 = 1000*[1 -1;-1 1];
k = [1000,1000,1000];
elem_con = [1 2;2 3;3 4];
elem = size(elem_con,1);
nodes = elem+1;
K = zeros(nodes);
%Forces
F = [0;P2;0;P4];  % Updated force vector
%Assembly
for i=1:elem
    ke = k(i)*[1 -1;-1 1];
    node1 = elem_con(i,1);
    node2 = elem_con(i,2);
    K(node1,node1) = K(node1,node1) + ke(1,1);
    K(node1,node2) = K(node1,node2) + ke(1,2);
    K(node2,node1) = K(node2,node1) + ke(2,1);
    K(node2,node2) = K(node2,node2) + ke(2,2);
end
d = zeros(nodes,1);
d(2:4) = K(2:4,2:4)\F(2:4);  % Solve for displacements
F = K*d;  % Compute forces
del1 = d(1:2);
del2 = d(2:3);
del3 = d(3:4);
fel1 = kel1*del1;  % Compute forces in element 1
fel2 = kel2*del2;  % Compute forces in element 2
fel3 = kel3*del3;  % Compute forces in element 3

I hope these suggestions help you resolve the issue.

Best regards,

Gyan