Question

This is a trivial problem, but I’m just starting with matlab, and haven’t got used to their way of thinking yet (and syntax).

What I’m asking will be obvious to anyone who’s ever done anything with FEM or the like.

How do you put together a big stiffness matrix from several small ones. Say, you got for (element 1) a local stiffness matrix 4×4, the same for (element 2) – only different matrix, of course, but still 4×4.

What is the easiest way to do this:

[|--------| 0  0 ]  
[|        | 0  0 ]  
[|     |--|-----|]  
[|-----|--|     |]  
[0  0  |        |]  
[0  0  |--------|]  

(a33+b11, a34+b12,
(a43+b12, a44+b22, ...)

i.e. make a ‘big one’ ?

Answer 1

I think your question is this:

A = 4×4
B = 4×4

C = final matrix where A and B overlap and should be summed in overlap.

Do this:

C = zeros(6);
C(1:4,1:4) = A;
C(3:6,3:6) = C(3:6,3:6) + B;