Stability considerations for the construction of steel I-girder bridges using the incremental launching method

The incremental launching method (ILM) is commonly used for bridges where erection procedures cannot be conducted from below the superstructure. In this method, the steel components are assembled on one side of the obstacle and moved longitudinally to the other supports. Deep valleys with steep slopes and environmentally sensitive areas are examples where the use of the ILM is justified. During the launching operation, the bridge I-girders cantilever out from their initial position and may be prone to stability related problems such as global lateral-torsional buckling of the system, as well as high strength demands at the cantilever support. This paper presents studies of the behavior of a three-span steel I-girder bridge consisting of four parallel girders erected with the ILM. Critical construction stages are investigated to determine the structural responses according to the AASHTO LRFD Bridge Design Specifications. These results are compared to the responses obtained from refined three-dimensional finite element models (FEM) to verify the applicability of current design methodologies for bridges erected with the ILM. In particular, analyses are conducted to assess the validity of the construction checks in the AASHTO Specifications of girders in cantilever and of systems potentially subjected to global lateral- torsional buckling. Based on the results of the studies, recommendations are provided to conduct a stability assessment of bridges erected by applying this method.