Evaluation of Torsional Strength Design Expressions Using Experimentally Validated Numerical Analysis of Post-tensioned Girders

Post-tensioning is a popular method of reinforcing concrete in modern bridge construction, providing economical and efficient design solutions. For hollow members such as girders with box-shaped cross sections, post-tensioning ducts that pass through thin flanges or webs can interrupt shear flow along the perimeter of the cross section. The members can also be prone to torsion as a result of tensile stresses created by eccentric post-tensioning. Although the current AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (BDS) addresses torsional strength, very few studies have evaluated the torsional design procedures for post-tensioned girders. This study aims to identify the influence of ducts and post-tensioning force on the torsional behavior of box-shaped girders post-tensioned with bonded or unbonded strands. Nonlinear finite-element models of girders under torsion are developed and validated using test data. Torsional strengths obtained from the analyses are compared with the ones from AASHTO LRFD BDS. The comparison indicates that the AASHTO predictions can be very conservative and have a significant scatter. The predictions were still conservative when the factor of AASHTO LRFD BDS that accounts for ducts for bonded strands was eliminated.