why central prestressing is required during construction for incremental launching method for bridge construction
During the construction of incremental launching bridge, Superstructure will continuously be subjected to positive and negative bending moment, each section will cross-section moves from a positive bending moment to negative moment and vice verca, this means some part of the superstructure tension will be at top, and some parts the bottom of section will be subjected to tensile stresses. Figure 1 showing the bending moment pattern during the construction of a bridge using incremental launching method, central prestressing used during construction to reduce the tensile stress on superstructure to a permissible value.
In central prestressing cables are arranged in a way that the resultant compressive prestressing force at all points are equal so it doesn't matters if the tensile stress during construction at the bottom or top section of superstructure. the central prestressing in conjugation with reinforcement is providing a factor of safety against structure rupture during construction.