Modeling and Detailing Pretensioned Concrete Bridge Girder End Regions Using the Strut-and-Tie Approach

Many research initiatives over the last two decades have had the objective of increasing the available spans of prestressed concrete bridge girders and have resulted in optimized cross-sectional shapes. To achieve these long spans, greater levels of prestress force are required. In addition, sections have been redesigned to increase the eccentricity of the prestress, which involves flattening and widening of the bottom bulb. The greater pretension forces and more slender bulbs have a number of effects at the girder end that affect both the serviceability and the ultimate behavior of the girder. This article identifies some behaviors that are potentially exacerbated when larger prestress forces are introduced and proposes a strut-and-tie modeling approach to better understand and mitigate these effects through improved girder end-region detailing. The utility of the proposed strut-and-tie model is demonstrated in two instances: (1) an investigation of the effects of strand debonding and (2) an investigation of the potential impacts of the larger prestress forces resulting from increasing girder span [specifically by adopting larger-diameter strands of 17.8 mm (0.7 in.)]. In the first instance, the strut-and-tie approach was used to establish guidance for preferred strand and strand debonding patterns. In the second instance, the impact of providing greater prestress forces through the use of larger strands was shown to have little effect on the development of transverse stresses at girder ends. The strut-and-tie approach was also demonstrated to be able to be rapidly applied over a wide range of parameters, allowing trends related to girder geometry, for instance, to be established. Thus, the utility of the strut-and-tie method in general, and the proposed model in particular, is demonstrated to provide a powerful tool for the rational analysis of the complex stress state occurring at the end regions of prestressed concrete bridge girders.