Assembling Global Stiffness Matrix

I am trying to make a global stiffness matrix using a for loop from several smaller (4X4) matrices. I was able to get a loop to run for 2X2 matrices, but when I increase the number of rows and colums in my smaller matrices it no longer works. I want the loop to run when the kn matrices is rand(4). This is what I have currently:

k1=rand(2);

k2=rand(2);

k3=rand(2);

C = {k1,k2,k3};

N = numel(C);

M = zeros(1+N,1+N);

for k = 1:N

M(k:k+1,k:k+1) = M(k:k+1,k:k+1)+C{k};

end

disp(M)

Answer 1

[Torsten]

This is analogous to your 2x2 code.

I don't know if it's the right way to code M.

k1=rand(4);
k2=rand(4);
k3=rand(4);
C = {k1,k2,k3}; 
N = numel(C);
M = zeros(3+N,3+N);
for k = 1:N
    M(k:k+3,k:k+3) = M(k:k+3,k:k+3)+C{k};
end
disp(M)

    0.6812    0.4903    0.5574    0.1710         0         0
    0.9156    1.2886    0.9432    0.6481    0.5245         0
    0.2805    1.4433    0.6168    1.3751    0.6339    0.7170
    0.1670    1.5541    0.9690    1.3091    0.4106    0.6047
         0    0.4948    0.9290    0.3031    0.9958    0.0024
         0         0    0.1797    0.8201    0.7794    0.7742

[Sharanya]

This is ouputting a 23X23 matrix, but I only want a 20x20 matrix. Each stiff(n) is a 4x4 matrix

k1=stiff(1);

k2=stiff(2);

k3=stiff(3);

k4=stiff(4);

k5=stiff(5);

k6=stiff(6);

k7=stiff(7);

k8=stiff(8);

k9=stiff(9);

k10=stiff(10);

k11=stiff(11);

k12=stiff(12);

k13=stiff(13);

k14=stiff(14);

k15=stiff(15);

k16=stiff(16);

k17=stiff(17);

k18=stiff(18);

k19=stiff(19);

k20=stiff(20);

C = {k1,k2,k3,k4,k5,k6,k7,k8,k9,k10,k11,k12,k13,k14,k15,k16,k17,k18,k19,k20};

N=numel(C)

M = zeros(3+N,3+N);

for k = 1:N

M(k:k+3,k:k+3) = M(k:k+3,k:k+3)+C{k};

end

disp(M)

[Torsten]

You will have to explore how the global stiffness matrix is computed from the C's.

Answer 2

[Arif Hoq]

% creating stiff matrix
for i=1:20
    stiff{i}=randi(100,4,4);
end

k1=stiff(1);
k2=stiff(2);
k3=stiff(3);
k4=stiff(4);
k5=stiff(5);
k6=stiff(6);
k7=stiff(7);
k8=stiff(8);
k9=stiff(9);
k10=stiff(10);
k11=stiff(11);
k12=stiff(12);
k13=stiff(13);
k14=stiff(14);
k15=stiff(15);
k16=stiff(16);
k17=stiff(17);
k18=stiff(18);
k19=stiff(19);
k20=stiff(20);
C = [k1,k2,k3,k4,k5,k6,k7,k8,k9,k10,k11,k12,k13,k14,k15,k16,k17,k18,k19,k20]; 
N=numel(C);
M = zeros(N,N);
for k = 1:N-3
    M(k:k+3,k:k+3) = M(k:k+3,k:k+3)+C{k};
end
disp(M)

     4     5    37    70     0     0     0     0     0     0     0     0     0     0     0     0     0     0     0     0
    70   112   133   126    50     0     0     0     0     0     0     0     0     0     0     0     0     0     0     0
    63   116   157   136    24    98     0     0     0     0     0     0     0     0     0     0     0     0     0     0
     6   107   167   135   245   149    49     0     0     0     0     0     0     0     0     0     0     0     0     0
     0    35   161   122   166   181    91    42     0     0     0     0     0     0     0     0     0     0     0     0
     0     0    46    98   103   176   149   110    43     0     0     0     0     0     0     0     0     0     0     0
     0     0     0    66   125   129   144   115   124     3     0     0     0     0     0     0     0     0     0     0
     0     0     0     0    87    77   157    83   165   153    63     0     0     0     0     0     0     0     0     0
     0     0     0     0     0    39   126   107   213   201    53    25     0     0     0     0     0     0     0     0
     0     0     0     0     0     0    24    92   190    87   155    74    68     0     0     0     0     0     0     0
     0     0     0     0     0     0     0    68   102    85   244   165   110    54     0     0     0     0     0     0
     0     0     0     0     0     0     0     0    45    90   155   241   204    87     7     0     0     0     0     0
     0     0     0     0     0     0     0     0     0    42   116   177   219   134    90    58     0     0     0     0
     0     0     0     0     0     0     0     0     0     0    40    34   238   245    96   147    49     0     0     0
     0     0     0     0     0     0     0     0     0     0     0    43    71   221   165   133   101    73     0     0
     0     0     0     0     0     0     0     0     0     0     0     0    19   145   115   142   158    61    11     0
     0     0     0     0     0     0     0     0     0     0     0     0     0    29    71   153   274   146    53    21
     0     0     0     0     0     0     0     0     0     0     0     0     0     0    88   141   119   116   173    67
     0     0     0     0     0     0     0     0     0     0     0     0     0     0     0    78    80   110    90    37
     0     0     0     0     0     0     0     0     0     0     0     0     0     0     0     0    93    39     5    96