Anchorage Zone Design for Pretensioned Precast Bulb-T Bridge Girders in Virginia

About the report:

Precast/prestressed concrete girders are commonly used in bridge construction in the United States.  The application and diffusion of the prestress force in a pretensioned girder cause a vertical tension force to develop near the end of the beam.  Field surveys of the beam ends of pretensioned bridge girders indicate that many of the precast bulb-T (PCBT) beams used in Virginia develop cracks within the anchorage zone region.  The lengths and widths of these cracks range from acceptable to poor and in need of repair.  Field observations also indicate deeper cross sections, very heavily prestressed sections, and girders with lightweight concrete tend to be most susceptible to crack formation. 

This research examined a new strut-and-tie based design approach to the anchorage zone design of the PCBT bridge girders used in Virginia.  Case study girders surveyed during site visits were used to illustrate the nature of the problem and support the calibration of the strut-and-tie-based model.  A parametric study was conducted using this proposed design model, and the results of this study were consolidated into anchorage zone design tables.  The results of the parametric study were compared to the results obtained using existing anchorage zone design models, international bridge codes, and standard anchorage zone details used by other states.  A set of new standard details was developed for the PCBT girders that incorporates elements of the new design approach and is compatible with the anchorage zone design aids.

A 65-ft PCBT-53 girder was fabricated offsite and tested at the Virginia Tech Structures Lab to verify the new strut-and-tie-based design model.  This girder contained anchorage zone details designed with the new model.  The new anchorage zone details were successful at controlling the development of anchorage zone cracks.  The new design approach is recommended for implementation by the Virginia Department of Transportation.

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