How to transform a mass matrix de dim 12*12 into 6*6 ?

Hi I want to do a model of FEM of a cantilever beam divided into two element (Three node) . I have a part of program that

 estimate the mass matrix of 6 node of dim (12*12) but i want to transform this matrix into dim 6*6.

Please help me.

% node# unused xyz

node=[1 0 0 0 0 0 0;

      2  0 0 0  0 1 0;

      3  0 0 0  0 2 0;

      4  0 0 0  1 1 1];

elt=[

%node1 node2 pe ie refnode

     1   2    1  1   4       0    %longerons

     2   3    1  1   4       0];

% matid matType E nu rho

pe=[1 1 7.2e+10 3.0e-01 2.7e+03];

% secId SecType J i1 i2 A

ie=[1 1 5e-09 5e-09 5e-09 2e-05

    2     1      5e-09  5e-09  5e-09  2e-05

    3     1      5e-09  5e-09  5e-09  2e-09];

opt=[1 2 3];

% element matrix assembly

pe = pe(find(pe(:,1)==elt(1,3)),3:size(pe,2));

ie = ie(find(ie(:,1)==elt(1,4)),3:size(ie,2));

if length(pe)==3 pe=[pe 0]; end

if pe(4)==0 pe(4)=pe(1)/2/(1+pe(2)); end

x = [find(node(:,1)==elt(1,1));find(node(:,1)==elt(1,2))];

idof = [(1:6)/100+elt(1,1) (1:6)/100+elt(1,2)];

x = node(x,5:7);

l = norm(x(2,:)-x(1,:)); l2 = l^2;

% mass - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

if ~isempty([1 3]==opt(1))

   m = zeros(12);

   ind = [1 7];    m(ind,ind) = [1 .5;.5 1]/3 * ie(4);

   ind = [4 10];   m(ind,ind) = [1 .5;.5 1]/3*ie(1);

   ind = [2 6 8 12]; m(ind,ind) =  ie(4) * ...

    [78    11*l   27   -6.5*l; 11*l    2*l2    6.5*l -1.5*l2

     27    6.5*l  78   -11*l ; -6.5*l -1.5*l2 -11*l   2*l2]/210;

   ind = [3 5 9 11]; m(ind,ind) =  ie(4) * ...

    [78   -11*l   27    6.5*l;-11*l    2*l2   -6.5*l -1.5*l2

     27   -6.5*l  78    11*l ; 6.5*l  -1.5*l2  11*l   2*l2]/210;

   m = (pe(3) * l) * m;

else

   m = [];

end

Answer 1

[KSSV]

Hello.. This is purely a FEM question, how you can expect a FEM answer in MATLAB community? Any ways, it is quite a big code to go through. You may refer the following link for your reference:

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