Design of composite partially prestressed concrete bridge planks for deflection control

Precast concrete planks have been used extensively in Australia for the construction of small-span highway bridges. While this form of construction has proven to be both economic and structurally effective, some difficulties have been experienced because of lack of control of deflections. In particular, excessive camber has been a common problem. The simplest and most cost-effective way of eliminating excessive camber and to achieve good deflection control is to abandon the allowable stress criteria and adopt a performance-oriented approach to design. This almost inevitably leads to the use of partially prestressed concrete, i.e. to planks which undergo flexural cracking under full design live load. This paper describes a simple performance-based procedure for the preliminary design of composite, prestressed concrete planks. Deflection control is achieved by load balancing, which is extremely simple to apply in practice. Nevertheless, a step-by-step analysis is required in the detailed design calculations to determine the effect on deflection of creep and shrinkage in the plank and in the topping. Trial and error calculations are needed in order to choose appropriate values for the section depth and magnitude of prestress. To assist in this process, results of preliminary plank designs are presented for spans up to 20 m (A).