Camber, prestress loss, and determination of distribution factors in pretensioned concrete girder bridges

Abstract

This research investigates prestress losses and camber of Prestressed Concrete bridge beams (PC Beams), and the performance and load testing of the SH 4 Bridge over the North Canadian River in Canadian County, OK. The bridge consists of 15 spans; each span is nominally 100 ft. in length. Each span was also designed and built with unique reinforcement details. An instrument-based structural monitoring program was implemented to measure strains and temperatures within hardened concrete. Materials testing was also performed. Camber measurements were taken at several intervals during the fabrication of the PC Beams and during the construction of the SH 4 Bridge and continuing through service. Several prestress loss analysis methods were performed and compared to gathered strain data. The purpose was to compare how different strand layouts and the inclusion of mild steel reinforcement in the bottom flanges of the girders affected long-term cambers and prestress losses. Static load testing was also performed on this bridge to determine the live load distribution factors of the girders. The research investigated whether distribution factor methods (DF’s) are accurate, and whether overestimating DF’s for external girders leads to unnecessary levels of prestressing reinforcement. Excess reinforcement can result in increasing camber and increasing prestress losses, and adversely affect cracking in end regions and the constructability of the bridge. Results from the research were compared to analytical methods prescribed in AASHTO codes. A finite element model was also made to recreate the load test and determine the effectiveness of the parapets and diaphragms in distributing live load.