| dc.description.abstract | In reinforced concrete members, headed bars are often used to anchor reinforcing steel when member geometry is insufficient for the full development of straight bars; the shorter development lengths and compact size also reduce congestion. Currently, very limited research has been performed on the behavior of headed bars in beam-column joints, particularly with regard to their use with high-strength steel or high-strength concrete. As a result, current ACI design provisions (ACI 318-14) limit the yield strength of headed bars to 60,000 psi and concrete compressive strength to 6,000 psi when calculating the development length of headed bars. Clear spacing between headed bars must be no less than four bar diameters (in non-seismic design). These limits hinder the full use of headed bars and prevent the advantages of higher-strength steel and concrete from being fully realized. Results are presented for 202 simulated exterior beam-column joint specimens. The main variables included concrete compressive strength (ranging from 3,960 to 16,030 psi), embedment length (ranging from 4 to 19.25 in.), bar size (No. 5, No. 8, and No. 11), head size (net bearing areas ranging from 3.8 to 14.9Ab, where Ab is bar area), number of headed bars in a specimen (two, three, and four), spacing between the bars (center-to-center bar spacing ranging from 3 to 11.8db, where db is bar diameter), and the amount of confining reinforcement within the joint region (ranging from none to six No. 3 hoops). Bar stresses at failure ranged from 26,100 to 153,200 psi. Some of the heads investigated in this study had obstructions larger than allowed in ASTM A970 for HA heads. These non-HA heads had a net bearing area (unobstructed area adjacent to the head) of at least 4.5Ab. The test results were used to evaluate the applicability of the current ACI provisions on the development of headed bars with high-strength steel and concrete. The comparisons show that current ACI 318-14 code provisions do not accurately account for the effects of concrete compressive strength, bar size, bar spacing, or confining reinforcement within the joint region. The test results also show that the headed bars with non-HA heads in this study perform similarly to those with HA heads. The test results of beam-column joint specimens were used to develop descriptive equations to characterize the anchorage strength of headed bars. The equations were compared with test results from previous studies to establish new design criteria for development length that safely allow for the use of headed bars with yield strengths up to 120,000 psi and concrete with compressive strengths up to 16,000 psi. | |
