Some #structures can be modelled as multi-degree-of-frreeom or #MDOF systems for #dynamic #analysis. This is a simple #2DOF #model for a two-storey #PortalFrame with rigid floors and flexible columns and it thus has two #frequencies at which it `likes’ to #vibrate. The #fundamental #mode has the lowest frequency and there is one #harmonic mode in this example which, for the chosen values of masses and stiffnesses, vibrates at twice the rate of the fundamental mode.