Anchorage of High-Strength Reinforcing Bars
University of Kansas Center for Research, Inc.
December 2011 - January 2015
Professor David Darwin
.Neal Anderson, Vice President, CRSI; Cary Kopczynski, President, Cary Kopczynski & Co. Inc.; Javeed Munshi, Principal Engineer and Concrete Design Specialist, Bechtel Power; Conrad Paulson, Principal, Wiss, Janney, Elstner Associates, Inc.
Concrete Reinforcing Steel Institute (CRSI)
A principal impediment to the use of higher-strength reinforcing steel for all reinforced concrete applications are the limitations in the ACI Code on the parameters for performance of anchorage hooks. The profession is currently operating without an understanding of the performance of hooks using reinforcing steel with yield strengths above 68 ksi and concrete with compressive strengths above 5,100 psi. The use of reinforcing steel with yield strengths of 75 to 80 ksi is now common; concrete with compressive strengths between 10,000 to 15,000 psi is now used for many applications, and bars with yield strengths up to 120 ksi are now available. Yet yield strength for bars is currently limited to 80 ksi by code.
The initial test matrix consists of 150 hook tests with varying bar sizes and hook angles and lengths. The basic test specimen will represent a beam anchored by hooked bars framing into an exterior column. The goal of the experimental study is to gain a firm understanding of the variation in hook strength as a function of bar size, concrete strength, member geometry, and transverse reinforcement.
The test results will be used to establish a reliability-based design expression for development length. Both simplified and detailed forms of the design expression will be formulated in a fashion parallel with the current approach for straight reinforcing steel. Recommendations on the use and detailing of transverse reinforcing steel in the hook region will be provided. The research team will work closely with members of ACI Committees 318, 349, and 359 to implement the proposed design criteria in the shortest possible time.
Co-Funding: Total of $320,000: Electric Power Research Institute (EPRI): $150,000; Concrete Reinforcing Steel Institute (CRSI): $90,000; KU Transportation Research Institute (KUTRI): $80,000