An extradosed prestressing concept, is a new type of structural system in which the tendons are installed outside and above the main girder and deviated by short towers located at supports.
Extradosed PC bridges have several positive characteristics. The girder height may be lower than that of ordinary girder bridges, thus reducing self-weight of structures. The ratio of the girder height to the span length (H/L) in extradosed bridges ranges between 1/15 and 1/35, while it is approximately 1/15~1/17 for box-girder bridges. Comparing to cable-stayed bridges, the height of the main tower in extradosed bridges is lower; hence, a reduction in labor costs of construction can be achieved.
Because of a lower main tower in extradosed bridges, vertical loads are partly resisted by main girders and stress variations in stay cables produced by live loads are smaller than those in cable-stayed bridges. This is quite similar to the behavior of box-girder bridges, where the main girder itself has a decisive influence on the structure rigidity and live loads produce only limited stress variations in tendons.