Strength of Continuous Beams Prestressed with Bonded and Unbonded Strands

Bonded and unbonded strands can be used together for prestressing beams when longer spans and/or external strands are needed. However, there is very limited data on the structural behavior of these members, particularly for continuous beams that have draped strands. This paper investigates the flexural and shear strengths of continuous concrete beams prestressed with a combination of bonded and unbonded strands and evaluates strength predictions of the AASHTO LRFD Bridge Design Specifications (BDS). Fiber-based beam element analyses (flexure) and modified compression field theory-based analyses (shear) were performed and validated with mid-scale test data. Flexure analyses showed that the capacity linearly increased with increasing bonded reinforcement. Shear analyses showed that the location of strands across the cross-section (web vs. flange) had little influence on shear capacity. Both flexure and shear capacity predictions of the AASHTO LRFD BDS were conservative compared to analysis results. Shear predictions of the AASHTO LRFD BDS fluctuated significantly depending on the section of interest and the area of web strands.