Figure $5$ shows a frame structure containing four beams with bending stiffness of EI$.$ Find the buckling load by FEM using four beam elements. The beam's incremental stiffness matrix is

$[$n$]=\frac{P}{10L}\left[\begin{array}{cccc}12& L& -12& L\\ L& 4\frac{L^2}{3}& -L& -\frac{L^2}{3}\\ -12& -L& 12& -L\\ L& -\frac{L^2}{3}& -L& 4\frac{L^2}{3}\end{array}\right]$