A gantry to be used in the construction process of bridges, viaducts and other structures, which is equipped with a system that automatically adjusts the prestressing of the gantry's structure in accordance to the external actions being applied on it when loadings occur. The adjustment of the prestressing is achieved through the use of at least one sensor (2) that monitors the structure and which sends the measurements taken to a controller (6), . This controller is then capable of activating at least one actuator which alters the tension of the structure's prestressing cable or cables. Amongst the many advantages of the present invention, one is the possibility of applying a large amount of prestressing without this implying undesirable deformations in the main structure (1) when exterior loads are not applied.