The use of 0.6 in. (15 mm) diameter prestressing strand was prohibited by the FHWA in October 1988 due to a lack of data supporting the behavior of pretensioned concrete beams with this new size strand. (Note that the FHWA restriction on 0.6 in. strand was removed in May 1996.) In its memorandum, the FHWA imposed a 1.6 multiplier for the then-current transfer and development length equations and prohibited the use of 0.6 in. (15 mm) diameter prestressing strand. The proposed multipliers were based on the results of tests on small rectangular beams and are not fully applicable to full-scale members. As a part of a larger research project with The University of Texas at Austin, research was conducted at Texas Tech University to provide additional test data and to formulate a new development length equation. In this study, the transfer and development length results from various studies were collected in addition to the results obtained from the tests performed in regional laboratories. Proposed equations for transfer and development length are compared with current equations in ACI, AASHTO, and other codes. Comparisons show that the proposed equations give better estimates than other equations for 0.5 and 0.6 in. (13 and 15 mm) diameter prestressing strand.
