Realistic Evaluation of Transfer Lengths in Pretensioned, Prestressed Concrete Members

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Title: Realistic Evaluation of Transfer Lengths in Pretensioned, Prestressed Concrete Members

Author(s): Byung Hwan Oh and Eui Sung Kim

Publication: Structural Journal

Volume: 97

Issue: 6

Appears on pages(s): 821-830

Keywords: cover; prestress; prestressed concrete; pretensioning; slip; strand; stress; transfer length

Date: 11/1/2000

Abstract:
The extensive use of pretensioned, prestressed concrete in the modern construction industry, together with wider application of pretensioned components for structural purposes, requires some important consideration on the adequate transfer of prestressing force into the concrete, especially around the end zones of the pretensioned member. The current ACI design code considers only the prestress intensity and the diameter of prestressing steels to calculate the transfer length of steels. Other important parameters, however, including concrete strength and concrete cover, may affect the transfer length significantly. The main objective of this paper is to study the effects of various important parameters on the transfer length of pretensioned, prestressed concrete members. To this end, a comprehensive experimental program has been set up. The principal test variables considered were strand diameter, concrete strength, concrete cover size, and strand spacing. The results of the present study indicate that the current code equation for transfer length overestimates the actual measured transfer length. This overestimation is particularly significant for high-strength concrete with larger concrete cover, which is not covered by the ACI code. The present study shows that the transfer length decreases with an increase of concrete strength, and decreases with an increase of concrete cover. These important parameters must be, therefore, included reasonably in the current design codes. The strand spacing also affects the transfer length, and the increase of strand spacing results in a decrease of transfer length. The present study also shows that the end slip values can be efficiently used to determine the transfer length of pretensioned concrete members. The theoretical determination of transfer length based on slip agrees very well with test data. The present study provides valuable test data for the realistic and accurate determination of transfer length, which can be efficiently used for improving the design equation of transfer length in pretensioned, prestressed concrete members.