Design of Post-Tensioned Girder Anchorage Zones

Several large thin-webbed box girders, with post-tensioned anchorage zones designed in accordance with AASHTO and ACI requirements, have experienced large cracks along the tendon path in the anchorage zones at the design stressing load. Cracking of this nature provides a path for penetration of moisture and salts and thus presents a potential corrosion and frost damage threat. In addition, such cracking negates a major reason for the use of prestressed concrete, the minimization of service load cracking.

This report summarizes the major design-related observations and conclusions from an extensive analytical and experimental program which studied anchorage zone behavior of post-tensioned box girders. The experimental program investigated the primary variables affecting the formation of the tendon path crack: tendon inclination and eccentricity, section height and width, tensile splitting strength of the concrete, anchor width and geometry, and the effect of supplementary anchorage zone reinforcement, both active and passive. An extensive series of three-dimensional linear elastic finite element computer analyses was used to generalize these results and develop a failure theory to explain tendon path crack initiation based upon specific peak spalling strains at the edge of the anhorage. The theory agreed well with the experimental data over a wide sectrum of variables.