How to assemble matrices?

I am trying to assemble local stiffness matrix to global one. Each node has 3Dof. The problem is I dont know how to shift the matrices at last part while adding process. Thanks everyone. The code is

clc;clear all
el_tot=3; %number of elements
n_tot=3*(el_tot+1); %number of nodes
% LOCAL - TO - GLOBAL MAPPING
EL_N = [1:3:n_tot-3; 6:3:n_tot];

% INITIALIZE GLOBAL STIFFNESS MATRIX AND GLOBAL FORCE VECTOR
K  = zeros(n_tot);  % 2nd same size superfluous , n_tot );  --dpb edit
shift=0;
% ELEMENT LOOP
for iel = 1: el_tot
  % GET NODE INDICES FOR ELEMENT iel
  el_nodes = EL_N :,iel);
  % DEFINE ELEMENT STIFFNESS MATRIX AND VECTOR
  Kloc =ones(6,6);
  % INSERT / SUM LOCAL ELEMENT STIFFNESS MATRICES
  % INTO GLOBAL STIFFNESS MATRIX
  K(el_nodes,el_nodes) = K(el_nodes,el_nodes) + Kloc ;
end*

Answer 1

[KSSV]

It is a FEM problem. You cannot expect a FEM solution in MATLAB community. You have to assemble stiffness matrices based on your nodal connectivity. YOu may refer the below link to understand how to assemble the local stiffness matrix to global matrix.

http://in.mathworks.com/matlabcentral/fileexchange/32029-plate-bending/content/Plate%20Bending/assemble.m

You can extend the analogy to your 3D.

[utsdf]

Thank you so much Dr. Siva Srinivas Kolukula.