function sig = truss2s(nodes,n1,n2,A,E,bigd)
%
%  Calculate stress in a truss element, 2-dimensional
%
%  nodes -- list of all node coored.
%  n1, n2 -- node numbers for this elem
%  A -- cross-section area
%  E -- modulus of elasticity
%  bigd -- global displacement vector
%
%  sig -- element stress
%
error(nargchk(6,6,nargin));
if n1==n2
   error('Distinct node numbers required')
end
x1 = nodes(n1,1);
y1 = nodes(n1,2);
x2 = nodes(n2,1);
y2 = nodes(n2,2);
dx = x2-x1;
dy = y2-y1;
len = sqrt(dx^2 + dy^2);
if len==0
   error('Zero length element');
end
theta = atan2(dy,dx);
C = cos(theta);
S = sin(theta);
dof = 
d = bigd(dof);
Cp =   %% (eq. 3.5.8)
sig = Cp*d;