The engineering thinking behind the development of creative designs for two bridges is described. The main stages are included, from the initial and alternative designs to the selection of the best solution for the construction. An important expressway in Buenos Aires had to be widened. The traffic flow is intense (greater than 250,000 vehicles/day) and because there are no alternative routes it was impossible to replace the existing structures. The three central supports of 2, six-span continuous prestressed concrete overpasses had to be removed. At least four lanes had to be kept open to traffic flow in each direction at all stages of the construction work. This was a requirement imposed on the designers. To deal with these heavy constraints steel over-bridges were designed from which the existing structures were suspended. Some details of complex structural problems that derived from the interaction between the old and the new superstructures are described. Jorge Fontán Balestra, Structural Engineer – Partner: Del Carril – Fontán Balestra y Asoc., Buenos Aires, Argentina Tomás del Carril, Structural Engineer – Partner: Del Carril – Fontán Balestra y Asoc., Buenos Aires, Argentina Member of the National Academy of Engineering, Buenos Aires, Argentina
This paper examines the installation of cable stays which are made up of individual parallel strands. For cables that are erected using a strand-by-strand or isotension method, a simple procedure is outlined for computing the installation forces, taking into account the flexibilities of the cable and the structure, and allowing for changes in loading and thermal effects. The procedure is also extended to compute the top-up forces in a two-stage stressing procedure in which the results of the first stage are utilised to predict an improved final force without the need for extensive and laborious geometric monitoring of the anchorages’ displacements. Comparison of the theorectical results is finally made with measured data from the recently completed Flintshire Bridge in North Wales where the technique has been successfully employed by the author. The formulations presented in this paper are equally applicable to cable stays which are made of parallel wires or bars. H. Al-Qarra, BSc (Hons), PhD, CEng, MICE, Chief Structural Engineer, DIWI Consult International GmbH, Essen, Germany (formerly Gifford and Partners, Southampton, UK)