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110 CHAPTER 4 Design Recommendations 4.1 Introduction Article Partially Debonded Strands Article 5.11.5 Splices of Bar Reinforcement Article of the AASHTO LRFD Bridge Design Specifi- Article Lap Splices in Tension cations (2004) limits the applicability of the specifications for concrete compressive strengths of 10 ksi or less unless physi- cal tests are made to establish the relationships between 4.2 Design Recommendations concrete strength and other properties. A comprehensive For prestressing strands, there are five essential recom- article-by-article review of Section 5 of the AASHTO LRFD mendations stemming from the research: Bridge Design Specifications (2004), pertaining to transfer and development of prestressing strand and splice length and 1. Adoption of the Standard Test Method for the Bond of anchorage of free ends by means of standard hooks for mild Prestressing Strands. Heretofore, this test has been known reinforcement was performed under NCHRP Project 12-60 to as the NASP Bond Test. identify all provisions that would have to be revised directly or 2. Adoption of a transfer length expression incorporating a indirectly to extend their use to high-strength, normal-weight factor to account for improved bond with increasing con- concrete up to 15 ksi. These articles are the following: crete strength. The recommended expression reflects the Article 5.4.4 Prestressing Steel decrease in transfer lengths as concrete release strengths Article Resistance Factors increase. For release strengths of 4 ksi, the recommended Article 5.11 Development and Splices of Reinforcement expression would provide a transfer length of 60 strand di- Article 5.11.2 Development of Reinforcement ameters, which is the same value found in prior editions Article Deformed Bars and Deformed Wire of the AASHTO LRFD Bridge Design Specifications. For Article Tension Development Length release strengths of 6 ksi, the design transfer length would Article Modification Factors that Increase ld be approximately 50 db. Transfer lengths would be limited Article Modification Factors which Decrease ld to a minimum of 40 strand diameters. Article Deformed Bars in Compression 3. Adoption of a development length expression that incor- Article Compressive Development Length porates factors to account for improved strand bond as Article Modification Factors concrete strengths increase. The recommended code Article Bundled Bars expression is founded on the same principles as prior edi- Article Standard Hooks in Tension tions of the AASHTO LRFD Bridge Design Specifications, Article Basic Hook Development Length i.e., the development length is the sum of a transfer length Article Modification Factors expression plus a flexural bond expression. At "normal" Article Welded Wire Fabric concrete strengths, the development length expression Article Deformed Wire Fabric requires 60 strand diameters for transfer length and ap- Article Plain Wire Fabric proximately 90 strand diameters for the flexural bond Article Shear Reinforcement length, for a total of 150 strand diameters. For a concrete Article 5.11.4 Development of Prestressing Strand release strength of 6 ksi and design concrete strength of Article General 10 ksi, the development length expression provides a Article Bonded Strand development length of 120 strand diameters. For higher