Economical Concrete Grade for Design of Variable Span Cast-in-situ Prestressed Box Girder Railway Bridge

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Addis Ababa University


Elevated rail transportation has the advantage that with limited construction time a more open and lighter space below the structure can be realized, which is crucial to keep cities accessible. This study is interested in one of the elevated railway superstructure type, cast- in-situ posttensioned box girder, which gives flexibility in tendon arrangement and span length. Moreover, it makes statical verification of preliminary design requirement and on the basis of which the main design quantities estimated, then comparative study undertaken to know the benefit or loss behind the concrete grade used for considered span lengths of 30m, 40m, 50m, and 60m investigated. Concrete grade used for girder governs the prestressing layout, the amount of prestress in bridge girder, stress limit flexibility; and Generally, any specific comparisons between different concrete grades, used for each span length box girder possible depth given. For variable span lengths considered in this study, with the same possible box girder depth provision for each span and for considered, 30MPa, 40MPa and 50MPa, cylindrical compressive strength concrete grade, by making the girder concrete strength higher, saving in prestress tendon used amount and a bit more noticeable benefit from non prestress reinforcement reduction can be enhanced for each span length particularly in context design to Euro code. Also, for variable span lengths and concrete grade considered, as the span get longer, using higher strength concrete for each span:  More reduced depth can be provided (10cm, 15cm, 15cm and 20cm, total box girder depth decrement can be enhanced for span length of 30m, 40m, 50m and 60m, respectively by varying the concreter grade of the girder from 30MPa to 50MPa cylindrical compressive strength);  More saving in prestress tendon amount and non-prestress reinforcement quantity can be made by keeping the possible box girder depth the same for each span. Finally, based on cost assumptions made, as the span get longer, using higher strength concrete more reduced depth can be provided with comparative cost and with the same possible box girder depth provision for each span length, varying the girder concrete from 30MPa to 50MPa makes the longer span box girders, more cost effective or saving in cost/span ratio become significant.



Railway Bridge, Concrete Grade, Variable Span, Prestressed Box Girder