% evaluates derivative of the shape functions (in physical coordinates) at point xt 
 function B = Bmatrix1D(xt,xe)
include_flags;

    if nen == 2       % derivative of linear shape functions
        B =  1/(xe(1)-xe(2))*[-1 1];
    elseif nen == 3   % derivative of quadratic shape functions
        B(1)=(2*xt-xe(2)-xe(3))/((xe(1)-xe(2))*(xe(1)-xe(3)));
        B(2)=(2*xt-xe(1)-xe(3))/((xe(2)-xe(1))*(xe(2)-xe(3)));
        B(3)=(2*xt-xe(1)-xe(2))/((xe(3)-xe(1))*(xe(3)-xe(2)));
    end