Shear Resistance of Cast-in-Place Post-Tensioned Bridge Girders

The Caltrans permits departures from AASHTO provisions in the design of post-tensioned (PT) box-girder bridges with fully grouted ducts, specifically with respect to (1) the effective web width used in shear capacity calculations, and (2) the permissible bundling of ducts. To assess the structural implications of these deviations, load tests to failure were conducted on near-full-scale girder cross sections representative of typical Caltrans practice. Transverse and longitudinal reinforcement details were proportioned to inhibit flexural yielding and promote shear-governed behavior. For the tested specimens, the concrete contribution to shear resistance ( V c ) ranged from approximately 15% to 25% of the nominal shear capacity. Adoption of the Caltrans-modified effective web width resulted in less than a 5% variation in predicted nominal shear strength, indicating that the amendment has minimal influence on shear design for PT girders with fully grouted ducts. At load levels approaching shear failure, Specimen 1 exhibited no visible distress in the vicinity of the corrugated metal duct, suggesting adequate bond performance and composite action between the duct and surrounding concrete. Specimen 2 developed minor cracking along the duct alignment and ultimately failed in a sudden, brittle shear mode. Overall, the experimental results indicate that the Caltrans provisions for effective web width and duct bundling ensure sufficient shear capacity and do not adversely affect duct integrity under the conditions investigated.