The NCHRP Report 849 provides proposed revisions to the current debonding provisions found within the AASHTO LRFD Bridge Design Specifications with detailed examples of the application of the proposed revisions. The proposed revisions are based on comprehensive analytical and testing programs for investigating the effects of end anchorages, beam sections, end-diaphragm details, concrete strengths up to 15 ksi, and strand sizes. The material in this report will be of immediate interest to highway design engineers.
Strand debonding is an alternative for reducing stresses in the end regions of pretensioned concrete beams. The AASHTO LRFD Bridge Design Specifications currently limit the amount of partial debonding to twenty-five percent of the total strand area within a pretensioned girder. The limit was imposed in recognition of the detrimental effects that excessive debonding can have on shear performance. Nevertheless, several states allow significantly higher percentages of debonding (up to seventy-five percent) to be used routinely in design. These higher percentages are based on successful past practices that have not been challenged. It is clear that unless the experimental evidence provides sufficient clarity, a consensus agreement among bridge owners will continue to be difficult to achieve.
A comprehensive study of partial debonding effects on the performance of pretensioned girders was therefore needed. Review of the various debonding practices used throughout the United States and existing test data allowed focused experimental research on the critical parameters. The research was designed to produce definitive recommendations regarding the number and configuration of debonded strands within commonly used cross-sectional shapes (i.e. I-, U-, and box beams). Final statements regarding the integral role of strand anchorage in the service and strength performance of pretensioned beams should be well-substantiated and ultimately highlight the importance of a unified approach to strand debonding.
Research was performed under NCHRP Project 12-91 by the University of Cincinnati to develop a proposed revision to the current debonding provisions found within the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications. The proposed revisions consider service and strength limit states for strand debonding within pretensioned flexural superstructure members (i.e., I-, U-, and box beams).
A number of deliverables, provided as appendices, are not published but are available on the TRB project website. These appendices are titled as follows: