In France most of prestressed concrete box-girder bridges are the object of reinforcements these last years per addition of prestressing tendons in most cases following classical complementary investigations carried out within the framework of project of repair. An innovating investigation was realized on a motorway viaduct close to Lyon in May 2013: strain gauges will be positioned on four steel tendons of prestressed right to significant cracks of the box-girder, cracks also instrumented by displacement sensors. Indeed, risks of overstretching in the tendons are expected under road traffic. A first relation will be established between the overstretchings evaluated from the measurements of the gauges and the openings of cracks from the dis-placement sensors. Moreover, measurements of temperature gradients were also carried out to distinguish solicitations from traffic loads and temperature effects. All these measurements are realized from a new Wi-Fi platform called PEGASE developed by IFSTTAR and the society A3IP. Finally, a fatigue analysis has been carried out in order to establish the residual lifetime of the bridge. An analysis in robustness will be undertaken. Three approaches are described in this paper. The most realistic approach is the experimented one. At the conclusion of this analysis, a decision will be made by the owner based on the opinion of experts to possibly launch the project of heavy repair of the bridge per external additional prestressing.