The ultimate capacity of beams with external prestressing cables slipping at saddle points cannot be evaluated by a local analysis of the critical sections and a nonlinear analysis of the whole structure is required. In the past some simplified approaches were proposed for reinforced concrete beams. On the other hand no simplified procedures are available for steel-concrete composite beams. In this work a new simplified method is introduced for evaluating the traction increment in the cable and consequently the flexural strength of composite beams. The proposed approach is described in detail and some applications of practical interest to simply supported beams are discussed.