Deflection controlled precast prestressed concrete beams: new opportunities as concrete codes evolve

The new standards association of Australia concrete structures standard encourages the use of partially prestressed concrete, as will the new NAASRA bridge design specification. This focusses attention on design for serviceability criteria, especially deflections and cracking. This paper first discusses what deflection and cracking criteria are appropriate for bridges and buildings and considers the requirements set out in the new codes. The paper then considers several design examples to examine new possibilities for better deflection control of the widely used precast concrete bridge deck planks, and the parallel possibility of increasing their load capacity. The beams were designed initially for strength, and then the levels of stress in the prestressing steel were adjusted to control the deflected shape of the beam, particularly under permanent load. The potential of this approach for floor slabs in concrete buildings is also considered. The problem of fatigue loadings is then discussed. There are a number of reasons why trends in the new codes will require a more careful consideration of fatigue and the application of partial prestress given in this paper is one example. Others are suggested, including cases of reinforced concrete. Limitations on serviceability stresses are considered, and it is shown that new code requirements may be unnecessarily tight for prestressed concrete but insufficient for normal reinforcement (a).