The paper introduces a discrete model to describe the buckling of a stiffened panel beam under a complex loading environment. The study begins by examining the existing load interaction equation for a continuous panel. Experimental and finite element investigations establish the validity of considering the critical panel of a more complex structure in isolation. The paper then devises a discrete model for this critical panel, which was validated for a range of boundary conditions using anti-optimisation. The numerical results show that the discrete model exhibits the buckling behaviour of a continuous panel under combined loading. Recent studies established that the truss-lattice configuration has stable post-buckling behaviour and derived fast analysis technique for such a structure. It is therefore concluded that the truss-lattice model introduced in the present paper can offer a fast analysis formulation for buckling (and potentially post-buckling) of multiple-panel beams suitable for optimisation.