The Line Shape Control Of Ateel Box Arch Rib Of External Slanted Arch Brigdge

Large-span steel arch bridge are developing rapidly in recent years due to their aesthetic feeling and spanning capacity. The cable-stay cantilever method and cable crane are widely used for the construction of large-span steel arch bridges. For such construction method, precise calculation of cable forces is required in order to satisfy the requirements of stress and geometry lines in the completed stage.The Nanning bridge consists of a horizontally curved steel box girder with a 300m span hanged by two unsymmetrical inclined steel arch ribs without lateral bracing between the arch ribs. The steel arch ribs are stiffened with transverse diaphragm, longitudinal stiffener and transverse stiffener. According to design documents, the cable-stay cantilever method and cable crane are used for the construction of the arch ribs of the bridge, and each arch rib is divided into 15 segments. The cable crane and stay cables are supported on the same pylons of steel truss structure. During the construction of the steel deck, the steel box girders are supported by the completed arches. The displacement control problems of arch ribs during the construction of such steel arch bridges were studied with the background of Nanning Bridge under construction now.An optimization approach based on the FEM was introduced to determine the cable forces in the process of arch rib construction. The first order optimization method adopted by the Ansys programming is used to determine cable forces . In the analysis cable force is considered as design variables, the displacement and stress of the arch and tower are considered as state variables , the square sum of the vertical displacement of arch is considered as objective function.The cable forces in each construction stage are determined by the backward analysis method . The results indicate that the cable forces obtained by proper method can not only control the displacement of the arch rib , but also can satisfy the stress requirement of arch and member of truss pylons.The paper later discusses the parameters influencing the completed geometry of arch ribs. The results indicate that geometric nonlinear and the tower's displacement have largely effect on the arch rib's displacement. Furthermore, cable sag accounts for eighty percent of the geometric nonlinear effect; The arch rib's elevation will change 2～3mm as the temperature change 1°; assemble errors have larger effect on the vertical and transverse displacements of cantilever arch ribs than on longitudinal displacements ; angle displacement errors account for the main errors of segment errors, while segmental displacement errors only account for 10～30% of the whole errors.