Collapse modalities of composite girders prestressed by external tendons

The authors analyze the non-linear behavior of composite concrete-steel beams prestressed by external tendons which are free to slip along their path defined by a finite number of saddle points with zero friction, in order to examine the failure modalities for the most common static schemes (simply supported beams and continuous beams) and for the most common constructive sequences (propped and unpropped beams, prestressing before and after the concrete slab casting). The formulation presented is based on a Kirchhoff model for the beam and the compatibility condition between beam and slipping external tendons is developed in the range of the infinitesimal deformation theory, for generic tendon paths. The balance condition is obtained by means of the virtual work principle for materials with nonlinear constitutive laws and the formulation is sufficiently general to permit the analysis of different construction sequences. The solution is performed by means of the Ritz method and the numerical problem is solved by using the Newton-Raphson procedure. The numerical applications developed by the authors show some effects of the most important design parameters, like section geometry and prestressing force, on deformations, ultimate load capacities, cable force increments and collapse modalities.