Use of 0.7-in. - Diameter strands in pretensioned bridge girders

Since the Federal Highway Administration (FHWA) allowed the use of 0.6-in.-diameter (15 mm) strands at 2 in. (50 mm) minimum spacing in 1996, they have been increasingly used in the production of preten-sioned concrete bridge girders. For several years, 0.7-in.-diameter (18 mm) strands have been successfully used in cable bridges and for mining applications. Using these large-diameter strands in pretensioned concrete girders at 2 in. spacing will result in an increase of approximately 35% in the prestressing force compared with the same number of 0.6-in.-diameter strands, which will, consequently, allow for longer spans, shallower structural depth, and wider girder spacing. For the same prestressing force, using 0.7-in.-diameter strands results in fewer strands to jack and release, fewer chucks, and greater flexural capacity due to the lower center of gravity of the strands. In this paper, the design and production challenges of using 0.7-in.-diameter (18 mm) strands in pretensioned concrete bridge girders are discussed. Two full-scale NU900 I-girders pretensioned using twenty-four and thirty 0.7-in.-diameter strands were fabricated and tested to address production concerns and evaluate transfer length, development length, and end-zone cracking associated with such large-diameter strands. Positive production experience and predictable test results indicated that 0.7-in.-diameter strands can be used with no major changes to the current production practices or design criteria according to the current American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications. Also, the design and construction of the Pacific Street Bridge over Interstate 680 in Omaha, Neb., the first pretensioned concrete bridge to use 0.7-in.-diameter (18 mm) strands, is summarized.