State-of-the-industry of anchorage zone design for pretensioned bulb-tee bridge girders

Abstract

The purpose of this project is to develop an improved prestressed concrete anchorage zone detail that mitigates end-zone cracking and reduces the congestion of reinforcement in the anchorage zone for VDOT. The current detail provides an adequate level of crack control, but it generally requires a very large amount of steel. This project is considering solutions such as added horizontal reinforcement in the form of WWR, splayed draping, and debonding. The purpose of this report is to provide background information on the current state of the industry and recent research to inform the analysis of the possible solutions. The project team performed a literature review, surveyed departments of transportation nationwide, and visited five precast plants in Virginia and surrounding states. The results of these investigations will be used in the development of a parametric finite-element study to determine the optimal configuration of possible solutions. The literature review includes current design methods used in practice, published design methods from research, and investigation of the influence of different factors on design performance. The AASHTO and VDOT design approaches for mild crack control reinforcement were presented. Four design methods for mild steel in the anchorage zone found in literature were discussed including empirical design, cracked equilibrium, strut-and-tie model, and shear transfer design approaches. Six factors affecting the performance of anchorage zones have been studied in prior research including debonding. Additional research into modified prestressing arrangements and the addition of horizontal steel, particularly WWR, should be considered. A survey was sent out in May 2023 to determine the detailing and repair practices of different DOTs for prestressed concrete bulb-tee girders. From the responses of 34 DOTs, it was found that most states allow the use of prestressed concrete bulb-tee girders. Many states modify AASHTO provisions for anchorage zones based on their own research and experience. Many also have their own typical details that vary from AASHTO’s prescriptions. In terms of repair practices, most states did not have consistent crack widths to trigger different repairs. Five precast plants in Virginia and surrounding areas were toured and stock beams were examined for end zone cracking. The observed beams had a wide variety of end zone cracking severities, but some states’ details were more prone to cracking than others. Pennsylvania details tended to perform very well relative to other states, and Maryland design details performed poorly. Most other state details performance fell between Pennsylvania and Maryland. A cursory analysis of the data collected from beams at the precast plants was performed to identify key factor interactions. One significant relationship identified was between the prestress force, the depth of the beam, the transfer length and net crack width. This trend showed a positive correlation in net cracking with level of prestress and depth of the section. Many other combinations of factors were compared, but none yielded a clear correlation. Further analysis through statistics and strut-and-tie modeling is recommended.