A study of two-span prestressed concrete suspension bridges

Abstract

Prestressed concrete suspension bridges can be regarded as conventional prestressed concrete structures whose main girders are prestressed by cables which have large eccentricities. Alternatively, they are self-anchored suspension bridges with concrete superstructures which are prestressed by the horizontal component of the cable tension ap- plied at their ends. Due to the possible large eccentricities. prestressed concrete suspension bridges can be used for long span crossings. The aim of this study is to investigate the use of this type of bridge for medium length crossings in the form of a two-span prestressed concrete suspension bridge. Amongst the special problems studied in this thesis is the contribution of changing cable tension, due to live loading, to the stress state of the structure. As this effect can be significant, a new approach is developed which is based upon modifying earlier research carried out by Timoshenko to suit the requirements of prestressed concrete theory. Subsequently, the method is compared with the commonly used Steinman approach which was slightly altered to suit prestressed concrete suspension bridges. This leads to the formulation of a new analysis to describe the various characteris- tic features of structural behaviour. Another problem is the influence of grouting the main cable in the regions where it passes within the suspended beam. This is also studied in the thesis as it causes modifications to the standard analysis. An important aspect of the behaviour of prestressed concrete suspension bridges is their mode of failure under excessive live- load. A theory is proposed to analyse this condition. In addition, a new approach in prestressing the suspended beams by shortening their suspenders is suggested. In order to verify the validity of these analyses, two tests have been carried out on model bridges in which measurements were made within both elastic and failure stages of the structure. In one case ungrouted main cable was used and in the other the cable was grouted for a proportion of the span length. The results of these experiments confirmed the applicability of the theories which had been developed and enabled the stresses and deflections to be predicted with accepta- ble accuracy. The theoretical analysis and the experimental results indicated the advantages which arise from the use of grouted main cables. Although there was no opportunity to carry out prolonged tests, a theoretical study has been given for the time effects on the prestressing force which might be expected with this form of structure based upon well known concepts in the behaviour of standard prestressed concrete construction. The thesis also comments upon the practical problems of the construction of such bridges and draws attention to the similarities with the commonly used self-anchored suspension bridge system. Various systems of developing the prestressing force are described.

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