How can I calculate Global stiffness matrix?

I have two basic matrix and I want to sum them in C matrix like below picture. I added A matrix but I couldn't add B matrix in that way. What can ı do?

C = zeros (6,6)
A = [ 1 2 3 4 ;
       4 5 6 7 ;
       7 8 9 10 ;
       10 11 12 13 ]

B =  [ 5 7 8 4 ;
       4 5 6 7 ;
       7 8 9 10 ;
       10 11 12 13 ]
 for x = 1:1:6 
C(x:2*x+2,x:2*x+2)=C(x:2*x+2,x:2*x+2)+A(x:2*x+2,x:2*x+2) %+B ?
 end

Answer 1

[Cameron]

I don't remember everything from my FEA classes, but I think this is what you're looking for

C = zeros(6,6);
A = [ 1 2 3 4 ;
4 5 6 7 ;
7 8 9 10 ;
10 11 12 13 ];
B =  [ 5 7 8 4 ;
4 5 6 7 ;
7 8 9 10 ;
10 11 12 13 ];
C(1:4,1:4) = A;
C(3:6,3:6) = C(3:6,3:6) + B;
disp(C)

     1     2     3     4     0     0
     4     5     6     7     0     0
     7     8    14    17     8     4
    10    11    16    18     6     7
     0     0     7     8     9    10
     0     0    10    11    12    13

[Merve Ala]

Thank you so much it helped a lot :)