$$M$$-$$\theta $$ relationship for a simply supported beam shown below is given by

Rotational stiffness-coefficient, $${K_{11}}$$ for the frame having two members of equal $$EI/L$$ is given by.

Bending moments at joint $$b$$ and $$C$$ of the portal frame are respectively,

Horizontal stiffness coefficient, $${K_{11}}$$ of bar $$ab$$ is given by,