The ultimate capacity of beams prestressed with external slipping tendons cannot be evaluated by a local analysis of the critical sections and a nonlinear analysis of the whole beam-tendon structural system is required. Since this type of analysis is complex, simplified approaches are of great assistance to engineers in the design process. In this work a new simplified method is introduced for evaluating the tendon traction increment at collapse and consequently the beam flexural strength without requiring a nonlinear analysis of the whole beam-tendon structural system. A detailed description of the proposed approach is given and some applications of practical interest to externally prestressed steel–concrete composite beams are discussed.