Anchorage Strength of Reinforcing Bars With Standard Hooks

Abstract

Hooked bars are often used to anchor reinforcing steel where concrete dimensions are not sufficient to provide the required development length for straight reinforcement, such as in external beam-column joints. The purpose of this study is to expand the understanding of the behavior of hooked bars in high-strength concrete and to develop design guidelines allowing for the use of high-strength reinforcing steel and high-strength concrete. In this study, 122 simulated beamcolumn joints were tested as a continuation of previous work at the University of Kansas. The test parameters included bar size (No. 5, No. 8 and No. 11), hook bend angle (90° or 180°), embedment length (5.5 to 23.5 in.), amount of confining reinforcement within the joint (no confining reinforcement to nine No. 3 hoops), location of the hooked bar with respect to member depth, hooked bar stresses (22,800 to 138,800 psi), concrete compressive strength (4,490 to 14,050 psi), center-to-center spacing between hooked bars (2 to 11.8db), number of hooked bars (2, 3, 4, or 6), arrangement of hooked bars (one or two layers), and ratios of beam effective depth to embedment length (0.6 to 2.13). Some specimens contained strain gauges mounted along the straight portion of the hooked bars and on the confining reinforcement within the joint rejoin. Test results from this study, along with test results from earlier work covering specimens without and with confining reinforcement, concrete compressive strengths between 2,570 and 16,510 psi, and bars stresses at anchorage failure ranging from 22,800 and 144,100 psi, were used to develop descriptive equations for the anchorage strength of hooked bars. The results of this study show that the current Code provisions overestimate the contribution of the concrete compressive strength and the bar size on the anchorage strength of hooked bars. The incorporation of the modification factors for cover and confining reinforcement in the provisions in the ACI Building Code (ACI 318-14) produces an unconservative estimation of anchorage strength of hooked bars, particularly with large hooked bars and closely-spaced hooked bars (hooked bars with center-to-center spacing less than 6db). Closely-spaced hooked bars exhibit less anchorage strength than widely-spaced hooked bars. The reduction in anchorage strength of closely spaced hooked bars is a function of both the spacing between hooked bars and the amount of confining reinforcement. Both the hooks and the straight portion of hooked bars contribute to anchorage strength. The anchorage strength of staggered hooked bars can be represented by considering the minimum spacing between the bars. Hooked bars anchored in beam-column joints with ratio of beam effective depth to embedment length greater than 1.5 exhibit low anchorage strengths compared to hooked bars with a ration below 1.5. These observations are used to develop proposed Code provisions for the development length of reinforcing bars anchored with standard hooks. The proposed provisions provide a higher level of reliability than current provisions and can be used for reinforcing steels with yield strengths up to 120,000 psi and concretes with compressive strengths up to 16,000 psi.