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Project 2012-02

Project Title:

Anchorage of High-Strength Reinforcing Bars with 90° and 180° Standard Hooks

Institution:

University of Kansas
Lawrence, Kansas

Researchers:

Dr. David Darwin, PE
Dr. JoAnn Browning PE

Award Period:

2011-2012 school year

Status:

Ongoing

Description:

The current provisions in the ACI 318 Building Code and AASHTO Bridge Specifications for the development length of bars with standard hooks are based on tests reported in 1977 using reinforcing steel with yield strengths of 64 and 68 ksi, and concrete compressive strengths between 3,750 and 5,100 psi. The low number of tests (22 actually) has also resulted in a limited ability to evaluate the true contribution of transverse reinforcement to hook strength. Since the time of those tests, the use of reinforcing steel with yield strengths of 75 and most recently 80 ksi has become common, along with concrete with compressive strengths of 20,000 psi and above, with values of 10,000 to 15,000 psi having wide-spread application in highrise construction. Bars with yield strengths up to 120 ksi are now available, although yield strength is currently limited by code to 80 ksi, except for steel used as spiral reinforcement in columns, where the limit is 100 ksi.

Although the design expressions developed as the result of the early study are permitted to be used for higher strength steels, neither the safety nor the accuracy of those expressions have been validated for the higher-strength steels. For most applications, the ACI Code does not limit the compressive strength of concrete, but it does limit the compressive strength to 10,000 psi in design expressions for anchoring straight and hooked bars. Recently, tests were performed on hooked ASTM 1035 reinforcing bars (fy = 100 ksi). Unfortunately, the test specimens only used single hooks anchored in members with a minimum cover of 5½ in. on either side of the hook, so a bond failure was never obtained. A concrete cover of 5½ in. greatly exceeds the minimum permitted for building and bridge design; thus these test results cannot be used to determine if current design expressions are applicable at the higher bar stresses. In contrast with the lack of data for hooked bars, ample data is available on the development and splice strength of straight reinforcing bars with yield strengths up to 120 ksi, and concrete strengths ranging from 2,000 to 16,000 psi.

Thus, one of the principal impediments to achieving confidence in using higher-strength reinforcing steel for all applications is a lack of data on hooked bars. Going forward, designers will continue to pursue the use of higher strength materials in reinforced concrete construction to minimize congestion, reduce member sizes, and maximize usable floor area. There is, however, no basis for modifying the current hook provisions. Key questions involve both (1) the safety of simply substituting higher yield strength values into the current expression for development length, and (2) the ability of the current expression(s) to efficiently take advantage of higher strength concretes and higher quantities of confining transverse reinforcement. Either way, 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.

Proposed Research - The proposed study will involve testing 90°, 135°, and 180° standard hooks in bar sizes ranging from No. 4 through No. 11 to determine the required development length for reinforcing steels with yield strengths ranging from 75 to 100 ksi. While the latter strength is currently not authorized for application for building or bridge construction, running tests up through this value is needed for Grade 75 and Grade 80 bars, which regularly exhibit yield strengths above 90 ksi. Concrete strengths will range from 5,000 to 15,000 psi, and hooks will be tested in groups of 2, 3, and 4 with and without confining transverse reinforcement. The basic test specimen will represent a beam framing into a column. Additional variables will be side and tail concrete cover, and spacing of the hooked bars. The goal of the experimental study is to gain a firm understanding of the variation in hook strength with bar size, concrete strength, member geometry, and transverse reinforcement as a function of the number of hooks being anchored. The results will be used to establish a reliability-based design expression for development length. Both a simplified and detailed form of the design expression will be formulated, in a fashion that is 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 given. With the development length formulas and transverse reinforcement recommendations, many design usage barriers will be overcome.

Co-sponsorship of a research project at the University of Kansas with partners the Electric Power Research Institute (EPRI), the Charles Pankow Foundation and KU Transportation Research Institute.