Modelling Construction of Footbridges with Cables

The usual moderate dimensions of footbridges allow design architects (and sometimes engineers) to provide such structures with capricious forms. Under such circumstances, the analysis of the service state is usually very thorough, devoting to the study of the construction stages much less attention. However, it is very well known that the construction procedure can determine the service stage. Moreover, dimensions of footbridges also allow a wide range of construction procedures. For example, when cables are used to help to support the deck, as in cable stayed bridges, those are, namely, cantilever method, temporary support method, movement of the bridge to its final position as well as bridge launching. When the cables are used in tied arch bridges most of the previously mentioned methods might be used, especially temporary support method, movement of the bridge to its final position as well as bridge launching techniques. Having this in mind, it is crucial for the designer and for the contractor to have practical tools for construction control of the tensioning process of the stays, as this will condition the service stresses of the bridges. This is challenging, as the prestressing of one stay affects the stresses of the already installed stays. This paper will present a procedure to calculate the stress to be given to the first strand of a stay when the strand by strand tensioning technique is used. Also, a procedure to modify the tensioning process of a cable supported bridges to minimize deviation between design and actual stay stresses in service will be presented.

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