k=elmstf(x,y,Area,Emod,inode,jnode)

function k=elmstf(x,y,Area,Emod,inode,jnode)
%
% k=elmstf(x,y,Area,Emod,inode,jnode)
% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
% This function forms the stiffness matrix 
% for a truss element.
%
% x,y         - global nodal coordinate vectors
% Area        - member area vector
% Emod        - modulus of elasticity vector
% inode,jnode - indices of the member ends
%
% k           - member stiffness matrix
%
% User m functions called:  none
%----------------------------------------------

i=inode; j=jnode;
xx=x(j)-x(i); yy=y(j)-y(i); L=norm([xx,yy]);
c=xx/L; s=yy/L; cc=c*c; cs=c*s; ss=s*s;
k=[[cc,cs];[cs,ss]]; k=Area*Emod/L*[k,-k;-k,k];