Chapter 4: The Construction Process of Segmental Bridges
Winter 1986). For a continuous beam on several supports, most tension will occur in the lower fibers of the cross-section around midspan and in the upper fibers above intermediate supports. It is therefore most useful to place tendons in the locations where tensile stresses will occur in the structure under service. This thought naturally leads to the idea of implementing longitudinal tendons in the beam that are not simply straight but follow a curve from the top above supports to the bottom at midspan and back to the next support. In Balanced Cantilever Construction the top cables in reaching out from the cantilever base to support the cantilever dead load are called cantilever beam cables; the bottom cables in the middle of the span are called integration cables (Mathivat 1983).
Prestressed concrete, compared with normal reinforced concrete has a higher degree of sophistication and causes higher cost for labor and for the prestressing tendons; on the other hand it saves cost through more economical use of material. Only prestressing makes long and slender concrete spans possible at all.
4.1.1 Degree of Prestressing
Menn (1990) mentions that choice of the best prestressing profile for a certain project is not predetermined but is a task for the bridge designer. He further gives an overview of the degree of prestressing. Full prestressing is supposed to withstand all tensile stresses under service conditions. When “calculated tensile stresses in the concrete must not exceed a specified permissible value” (Menn 1990, p127), so-called limited prestressing is performed. The last and most common method is partial prestressed, which does not specifically limit the concrete tensile stresses. Still, calculation of “behavior at ultimate limit state and under service conditions” (Menn 1990, p127) must be calculated, also taking into account the normal reinforcement. The purpose of the normal mild reinforcement is the control and distribution of cracking. Because of the high prestressing force, less conventional reinforcement is needed in the concrete, and members can be thinner and lighter, leading to more economical structures. The reduced susceptibility to cracking gives prestressed concrete higher durability.