Transverse Post-Tensioning in Long-Span Concrete Box-Girder Bridges: Refined Modeling and Alternative System

Abstract

Transverse post-tensioning is commonly introduced in top slabs of concrete box girders to control transverse tensile stresses induced by dead and live wheel loads. However, longitudinal cracks with a definite pattern have occurred time and again on the bottom of top slabs. This unexpected cracking distress urges a reevaluation of the structural effect of transverse prestressing. First, a step-by-step three-dimensional (3D) modeling was employed to simulate the balanced cantilever construction of box-girder bridges. It was found that the stressing sequence of transverse tendons has a significant influence on the slab prestress distribution along the bridge. The commonly adopted immediate tensioning method (referring to stressing transverse tendons in segment i immediately after the casting and hardening of segment i) yields a ±40% fluctuation of slab prestress within each segment, resulting in much lower prestress near the segment’s rear joint, where cracks are easily occurred. In contrast, the delayed tensioning method (referring to stressing transverse tendons in segment i after the casting and hardening of segment i+1) will greatly alleviate the nonuniform distribution. Second, an explicit equation based on an analytical model is proposed for calculating the slab transverse prestress. To overcome the problem of large friction losses in the current flat anchorage (FA) system, an alternative single-large-strand (SLS) system was designed and its performance verified by full-scale comparative tests. The test results show that the friction loss of the SLS system is only one-third of the FA system, and the overall structural efficiency can be increased by 20%.