How can any number of element matrices be assembled into a global matrix?

Hello all,

I'm trying to find a general MATLAB algorithm/code/script which assembles a global stiffness matrix from element stiffness matrices. I'm dealing with trusses, so the global stiffness matrix will have a number of rows and columns equal to the number of degrees of freedom in the respective truss, which is equal to twice the number of nodes. Needless to say, each element of the truss connects two nodes, and as such has a 4 x 4 element stiffness matrix. While I have been able to come up with algorithms for a specified number of degrees of freedom, I was wondering if there was a general algorithm I could use to combine the element stiffness matrices (Ke1, Ke2, etc.) into a single global stiffness matrix (K). I have already been able to create an algorithm which defines each element stiffness matrix using the element's area, modulus, and length, so all that's left is to figure out how to create the global matrix. Any and all help is appreciated.

[Stephen23]

"a general algorithm I could use to combine the element stiffness matrices (Ke1, Ke2, etc.) into a single global stiffness matrix (K)"

Allocate them using indexing. Do not try to do it dynamically: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Answer 1

[KSSV]

You may refer the below FEM examples, here I have assembled the element matrices into global matrices.

http://in.mathworks.com/matlabcentral/fileexchange/30970-natural-frequencies---buckling-loads-of-columns-using-finite-element-method

http://in.mathworks.com/matlabcentral/fileexchange/32029-plate-bending

[Anton Scotte]

Hi, I have the same issue. I have a bit trouble understanding what the index is in your function. "index - d.o.f. vector associated with an element".

How should this d.o.f. vector be constructed for each element? If each truss is connected with 2 nodes in each end (and is a 2D-problem) then I get a 4x4 stiffnes matrix, where each element has a dof of 4? In this case the d.o.f vector should have the length 4 if I understand your function correct. But what values should I populate this vector with?

Forgive me if this is trivial, I'm new to FEM in general as well.

Thank you