To respond to an FHWA memorandum restricting the use of certain sizes of seven-wire strand in prestressed concrete girders, the PCI sponsored a research program at the University of Tennessee at Knoxville. Twenty full-scale AASHTO Type I beams with various strand diameters were statistically tested to failures. Transfer and development lengths for 1/2 in. (13 mm), 1/2 in. special (13.3 mm), 9/16 in. (14 mm) and 0.6 in. (15 mm) diameter strands were determined. Also, minimum strand spacing was investigated for 1/2 in. (13 mm) diameter strand. Factors that affect both transfer and diameter length are evaluated and discussed. Based on the test data, equations for transfer and development length of strand are proposed. The computed and measured moment capacities of the girder sections are compared and reasons for variation are explained. It is concluded that the use of 0.6 in (15 mm) diameter strand should be accepted as standard practice and a center-to-center spacing of 1.75 in. (44.5 mm) should be allowed for 1/2 in. (13 mm) diameter strand.
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Deatherage et al. (1994) studied this question.