In a previous series of papers (Benamar 1990 Ph.D. ¹hesis, ;niversity of Southampton; Benamar et al. 1991 Journal of Sound and <ibration 149, 179}195; 164, 399}424 [1}3]) a general model based on Hamilton's principle and spectral analysis has been developed for non-linear free vibrations occurring at large displacement amplitudes of fully clamped beams and rectangular homogeneous and composite plates. The results obtained with this model corresponding to the "rst non-linear mode shape of a clamped}clamped (CC) beam and to the "rst non-linear mode shape of a CC plate are in good agreement with those obtained in previous experimental studies (Benamar et al. 1991 Journal of Sound and <ibration 149, 179}195; 164, 399}424 [2, 3]). More recently, this model has been re-derived (Azar et al. 1999 Journal of Sound and <ibration 224, 377}395; submitted [4, 5]) using spectral analysis, Lagrange's equations and the harmonic balance method, and applied to obtain the non-linear steady state forced periodic response of simply supported (SS), CC, and simply supported}clamped (SSC) beams. The practical application of this approach to engineering problems necessitates the use of appropriate software in each case or use of published tables of data, obtained from numerical solution of the non-linear algebraic system, corresponding to each problem. The present work was an attempt to develop a more practical simple &&multi-mode theory'' based on the linearization of the non-linear algebraic equations, written on the modal basis, in the neighbourhood of each resonance. The purpose was to derive simple formulae, which are easy to use, for engineering purposes. In this paper, two models are proposed. The "rst is concerned with displacement amplitudes of vibration = K?V /H, obtained at the beam centre, up to about 0)7 times the beam thickness and the second may be used for higher amplitudes = K?V /H up to about 1)5 times the beam thickness. This new approach has been successfully used in the free vibration case to the "rst, second and third non-linear modes shapes of CC beams and to the "rst non-linear mode shape of a CSS beam. It has also been applied to obtain the non-linear steady state periodic forced response of CC and CSS beams, excited harmonically with concentrated and distributed forces.