Design of bridge with external prestressing: design example

There has been increasing interest in recent years in the design and construction of post-tensioned concrete bridges with external unbonded tendons. To assist in the design of these structures, the Highways Agency issued a Standard and Advice Note (BD 58 and BA 58) in 1994. These documents were based on a review of existing knowledge that included recommendations on where further research was required. To provide further information for designers an outline design for a continuous bridge with an overall length of 80m is presented in this report. The objective is to illustrate the concept and to present sample calculations. The bridge considered is a box girder structure, representing a highway bridge with two spans, 32m and 48m, carrying a two-lane carriageway over a motorway. The report contains a description of the main features of the bridge and describes how the concept design was devised. It describes the principles involved in the analysis appropriate for this type of construction and how these differ from the analysis of conventionally post-tensioned bridges. Detailed calculations for shear and flexural calculations are included in the report. An outline design for an equivalent internally prestressed bridge is also given and costs are presented for both structures. The internally prestressed bridge is found to be marginally cheaper when initial costs only are considered. However, other factors could be considered to outweigh this. These include the costs of maintenance, inspection and potential strengthening, as well as traffic delay costs associated with these activities. (A)