The analysis of externally prestressed composite beams involves some difficulties related to the coupling between the local strain of the cable and the global deformation of the structure. The collapse modalities are governed both by the nonlinear behavior of materials and by second order effects. The authors present a model that includes geometric and material nonlinearities, based on the theory of small strains and moderate rotations obtained from the exact nonlinear theory. Some aspects of interest are illustrated by means of numerical applications.