Optimization of cable preloading on cable-stayed bridges

Generally geometric nonlinearities of cable-staved bridges depend on the behaviors of cables, pylons, the bridge deck and their interactions. Theses are geometry change, cable sag! and the interactions of axial forces. the bending moment and their deformations in the pylons and bridge deck. Therefore, a large cable-staved bridges system having a large number of cables can be analyzed under different load conditions. In investigating nonlinear behaviors of cable-stayed bridges, the nonlinear behavior of cables needs to be considered because it may cause the nonlinear behavior of whole bridge system. The nonlinear behavior of a cable gained from its sag. With an increasing axial load, the elongation of the cable is increased but the total cable sag is decreased. Cable-stayed bridge uses cables instead of the internal piers to support its bridge deck. During the construction period, the cables need to be preloading in order to support the bridge deck. Usually, cable-stayed bridge decks are straight with a little camber compared to the total length of the bridge. Keeping the bridge deck in the position where is the designer desired (straight or with little camber of the bridge deck) is not only for bridge aesthetics but also for people on the bridge in terms of psychological effect of improving confidence in structure and engineering considerations. To achieve the serviceability and engineering requirements, preloading of the cable is necessary. In this paper, one such a bridge with geometry similarly to an existing cable-stayed bridge, Quincy Bayview Bridge, located in Illinois: USA, has been considered. Quincy Bayview Bridge has 58 cables in the two planes. Four methods have been considered in this paper to make the optimum selection of cable preloading. The objective is to select appropriate method to determine cable prestrains in order to minimize the deformations and stresses due to dead load of the bridge. Thus, it is not a trivial problem since a change in the prestress of a cable influence the deformation every where in the structure. The best method would be determined by comparing. the calculated bending and vertical displacements of the bridge deck.