George Morcous, Kromel Hanna, and Maher K. Tadros, University of Nebraska-Lincoln
For several years, 0.7-in. (17.8-mm) diameter strands have been used in cable-stayed bridges and mining applications in the United States, and for post-tensioning tendons in Europe and Japan. To the author’s knowledge, the Pacific Street Bridge over I-680 in Omaha, NE, is the first bridge in the world to use 0.7-in. (17.8-mm) diameter prestressing strands in pretensioned concrete girders.(1) This strand has a cross-sectional area of 0.294 in.2 (190 mm2) and a weight of 1 lb/ft (1.5 kg/m). Tensioning one 0.7-in. (17.8-mm) diameter strand up to 75% of its ultimate strength requires a prestressing force of 59.5 kips (265 kN), which is 35% greater than that of 0.6-in. (15.2-mm) diameter strand and 92% greater than that of 0.5-in. (12.7-mm) diameter strand. Using 0.7-in. (17.8-mm) diameter strands results in less strands to jack and release, requiring fewer chucks, and producing a higher flexural capacity due to the lower center of gravity of the strands.
A detailed study on optimized sections for high strength concrete bridge girders was carried out in 1996 by Russell et al.(2) Despite the unavailability of 0.7-in. (17.8-mm) diameter strand in the U.S. market at the time of the study, its cost-effectiveness compared to other strand sizes was evaluated. This study indicated that using 0.7-in. (17.8-mm) diameter strands at 2 in. (50 mm) centers in a 10,000 psi (69 MPa) BT-72 bulb-tee girder resulted in the longest girder span and most cost-effective superstructure compared to 0.5-in. (12.7-mm) and 0.6-in. (15.2-mm) diameter strands.
The Fifth Edition of the AASHTO LRFD Bridge Design Specifications(3) has transfer length and development length equations, as well as strand spacing requirements based on strands up to 0.6 in. (15.2 mm) in diameter. Transfer length is the length of the strand measured from the end of the prestressed concrete member over which the effective prestress is transferred to the concrete. Transfer length is important for shear design and concrete stresses at the girder ends following strand release. The development length of prestressing strands is defined as the minimum strand embedment in concrete required to achieve the ultimate capacity of the section without strand slippage. The development length is necessary for identifying the critical sections in flexure and shear and calculating their ultimate capacities.
Test Program
For 0.7-in. (17.8-mm) diameter strands to be used in prestressed concrete bridge girders at 2×2-in. (50×50-mm) spacing, an extensive experimental investigation was carried out at the University of Nebraska-Lincoln. The objective of this investigation was to determine whether the provisions of the AASHTO LRFD Bridge Design Specifications for transfer and development length are valid for 0.7-in. (17.8-mm) diameter strands tensioned to 75% of the ultimate strength and placed at the same 2-in. (50-mm) spacing as 0.6-in. (15.2-mm) diameter strands to avoid the costly retooling of existing prestressing beds. This investigation consisted of designing, fabricating, and testing eight 24-in. (610-mm) deep tee-girders and three NU1100 girders for transfer length and development length. The tee girders were 28 ft (8.5 m) long, prestressed with six 0.7-in. (17.8-mm) diameter strands, and made using 8000 to 14,000 psi (55 to 97 MPa) compressive strength concrete. Measured strengths at transfer ranged from 6500 to 8000 psi (45 to 55 MPa). Different spacings and lengths of confinement of the confinement reinforcement were used at the girder ends. The three NU1100 girders were 40 ft (12.2 m) long, prestressed using thirty-four 0.7-in. (17.8-mm) diameter strands, and made of high strength concrete. Different spacings and lengths of confinement of bottom flange confinement reinforcement were used.
Transfer Length
Transfer length was measured using a detachable mechanical (DEMEC) gage. Points were attached to the concrete surface at the girder ends at the elevation of the centroid of the prestressing strands before release. The change in the measured distance between the DEMEC points before and after release was used to calculate the strain in the concrete due to prestressing at different ages using the 95% average maximum strain method. According to the measured strains, the transfer length from all tests of 0.7-in. (17.8-mm) diameter strands after 28 days ranged from 24 to 31 in. (610 to 788 mm), which is well below the 42 in. (1.07 m) predicted using Article 5.11.4.1 of the Fifth Edition of the AASHTO LRFD Bridge Design Specifications.
Development Length
Development length was calculated using Equation 5.11.4.2-1 of the Fifth Edition of the AASHTO LRFD Bridge Design Specifications to be approximately 14 ft (4.3 m) for 0.7-in. (17.8-mm) diameter strands. Therefore, all the girders were tested using a point load located at 14 ft (4.3 m) from the girder end, while monitoring girder deflection at the loading point and strand slip at the girder end using linear potentiometers. The load-deflection and load-slip diagrams indicated that, even with the lowest concrete strength of 8000 psi (55 MPa) and minimum confinement reinforcement of No. 3 bars @ 6 in. (152 mm) centers for a distance of 1.5d from the end of the beam, the nominal flexural capacity was achieved without slip of any strands exceeding 0.01 in. (0.25 mm). Therefore, it can be concluded that the development length calculated using the Fifth Edition of the AASHTO LRFD Bridge Design Specification for 0.7-in. (17.8-mm) diameter strands tensioned to 75% of the ultimate strength and located at 2 in. (50 mm) centers is satisfactory.
References
1. Schuler, G., “Producer’s Experience with 10,000 psi Concrete and 0.7-in. Diameter Strands,” HPC Bridge Views, Issue No. 54, March/April 2009.
2. Russell, H. G., Volz, J. S., and Bruce, R. N., “Optimized Sections for High-Strength Concrete Bridge Girders,” FHWA, U.S. Department of Transportation, Report No. FHWA-RD-95-180, 1997, 156 pp.
3. AASHTO LRFD Bridge Design Specifications, Fifth Edition, American Association of State Highway and Transportation Officials, Washington, DC, 2010.