The Superstructure launched using the incremental method will be subjected to high tensile stresses, this stress arises from the nature of the launching process, the superstructure will be pushed from abutment to piers without any supporting from the bottom and this will cause high hogging and sagging moment, some measures can be taken to reduce the effects of hogging and sagging moment:
- during the launching of the superstructure, the lead of superstructure will be subjected to high hogging moment, a steel nose of length equal to 0.60-.65 of the span length is provided at the edge of superstructure lead to reduce the cantilever moment. the nose usually consist of two main girders (lattice or solid web girders) of steel, the weight range between 1 to 2 t/m or more if the length is more than 30 m, sometimes guying from mast are used instead of nose to serve the same purpose as shown in figure 2, guying from mast will required continuous adjustment of the forces during the movement of the superstructure whereas nose don’t require any attention.
- the launched superstructure will be subjected to hogging and sagging moment alternatively, this moment will cause tensile stress at the top and bottom of the superstructure, the superstructure will get cracked and damaged because the concrete has low tensile strength, therefore central prestressing used to reduce tensile stress to acceptable value, in central prestressing cables are arranged to produce a similar resultant compressive stress at any point of structure, as a result, it doesn’t matter whether tensile stress at top or at bottom.
- for bridges with span length more than 50 m, auxiliary piers can be used to reduce stress on superstructure, also using of these piers will reduce the requirement for central prestressing, spans with high lengths required high stressing force, using of auxiliary piers will reduce prestressing and the material used, it is recommended to used concrete piers and avoid using steel auxiliary piers, because steel is elastic and sensitive to temperature. figure 3 showing auxiliary piers.