Proposed lump-sum formulas for long-term prestress losses

The current American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications’ approximate formula for estimating long-term prestress losses is the outcome of the research work presented in National Cooperative Highway Research Program report 496. It is produced by simplifying the detailed method and taking into account the variability of concrete properties and the interaction between the precast concrete girder and cast-in-place deck. This paper presents two detailed parametric studies based on the average con­ditions for the design and construction of commonly used bridge girders. Three spans and, consequently, three levels of prestressing for each section have been considered. The first study establishes the creep multiplier Nc, whereas the second study evaluates the shrinkage multiplier Ns. Both multipliers are used in the lump-sum formulas for estimat­ing long-term prestress losses for different bridge girders. The multipliers produced by these studies are compared with that of the current AASHTO LRFD specifications’ approximate method, and new lump-sum formulas for long-term prestress losses are proposed.