Performance and Design of Anchorage Zones for Post-Tensioned Box Girder Bridges
AuthorMaree, Ahmed Mohamed Farghal
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Post-tensioned box girder bridges are very common form of bridge construction. The post-tensioning anchorage zone is the location where very large prestressing forces are applied to the box girder, and then spread into the box section. The spreading of the large compressive forces creates transverse tension forces that must be considered in design. The diaphragm, web, deck and soffit adjacent to prestressing anchors are affected by force spreading and included in the post-tensioning general anchorage zone. These parts need to have adequate reinforcement and proper concrete placement. As part of this study, a database of Caltrans bridges was developed, which included 29 anchorage zones of box girder bridges. Based on this database, it was concluded that the diaphragm reinforcement selection varies substantially between bridges, and seems to be based more on “rules of thumb”. Lack of an accurate methodology of design and detailing for anchorage zone has led to highly congested anchorage zones with construction issues and cracking problems. In order to study the performance of anchorage zones, four box girders end zones were instrumented in the field in California. These four bridges cover a wide variety of anchorage zone configuration including different diaphragm width, number of girders, box girder height and openings in the end diaphragm. Strain gauges were used in order to capture strains in reinforcing bars and within the concrete elements. End diaphragm cracking was observed during post-tensioning for the investigated bridges. Through the captured strains, the flow of forces was estimated as well as the different parameters affecting force spreading in the general anchorage zone.Experimental work included two phases. The first phase included two single half-scale I-section girders with rectangular solid end diaphragms. The main parameter investigated in these specimens was the diaphragm width. The second phase contained two double girder half-scale box section with different openings in the end diaphragms as well as one solid diaphragm. Loading was applied with post-tensioning tendons to represent different design levels as well as to reach the ultimate load of the anchorage zone. Increasing of diaphragm width reduces the effect of bursting forces developed in different directions of the general anchorage zones. Effect of diaphragm openings were studied. Anchorage zone failure occurred in the double girders with openings in the end diaphragm. Finite element models were developed for the experimental specimens using the DIANA finite element package. Results of the developed models showed good correlation with the experimental results of tested specimens. These models were used to extend the investigated parameters affecting performance of general anchorage zone including: geometry of box girder end zone, edge eccentricity, number of anchors, tendon inclination and skew angle.Results obtained from field monitoring of bridges, experimental work and finite element modeling were combined using the strut-and-tie methodology to develop a set of design equations for bursting forces of anchorage zone. A simplified design table is developed based on the proposed equations. It provides percentage of bursting forces to ultimate jacking force based on effective diaphragm width to box girder height ratio as well as girder spacing to box girder height ratio. The proposed equations and simplified design table provide adequate procedures to design general anchorage zone of box girder bridges.