Headed reinforcing bars serve as a viable alternative to hooked bars for anchorage in concrete because they provide a more efficient anchorage mechanism and limit congestion of the reinforcement. Due to a lack of information, design provisions for the development length of headed bars in ACI 318-14 and ACI 349-13, however, impose more stringent restrictions on the application of headed bars than on hooked bars. For headed bars, the yield strength is limited to 60,000 psi and the concrete compressive strength is limited to 6,000 psi when calculating the required development length, compared, respectively, to 80,000 and 10,000 psi for hooked bars. The contribution of confining reinforcement to anchorage strength is also not addressed. In addition, headed bars are required to have a clear spacing between bars no less than four times the bar diameter. These limits prevent from taking the full advantage of using headed bars.
A comprehensive study on headed bars was conducted to establish the primary factors that affect the development length of headed bars and to develop new design guidelines that allow higher strength steel and concrete to be utilized in members subjected to monotonic as well as seismic loading. A total of 233 specimens were tested: 202 beam-column joints, 15 column-foundation joints, 10 compression-compression-tension (CCT) nodes, and 6 headed splices. The major test parameters included embedment length, concrete compressive strength, bar diameter/spacing, head size, and confining reinforcement. In addition, test results of 84 exterior, seven roof-level interior, and seven knee beam-column joints subjected to reversed cyclic loading were analyzed. Concrete compressive strengths ranged from 3,480 to 21,520 psi and bar stresses at failure ranged from 49,500 psi to 149, 900 psi. No. 4 to No. 11 headed bars with net bearing area ranging from 1.7 to 15 times bar area were evaluated. Some of these heads had obstructions larger than allowed under current Code requirements.
The results show that headed bars with obstructions exceeding the dimensional limits in ASTM A970-16 provide adequate anchorage strength. The findings are incorporated in ASTM A970-17. Headed bars did not provide sufficient anchorage in knee beam-column joints subjected to reversed cyclic loading. Reinforcement perpendicular to headed bars in column-foundation joints does not improve the anchorage strength. The descriptive equations and proposed design provisions, in which the anchorage strength of the headed bar is a function of key factors affecting the anchorage strength (embedment length, concrete compressive strength, bar spacing, bar diameter, and confining reinforcement), are applicable to a wide range of reinforced concrete members subjected to monotonic as well as reversed cyclic loading. The proposed design provisions allow the minimum clear spacing of 3db between headed bars permitted in joints in special moment frames (Section 18.104.22.168 of ACI 318-14) to be reduced to 1db, allowing for the use of more closely spaced headed bars. The anchorage provisions of Chapter 17 of ACI 318-14 with a strength reduction factor of 1.0 provide a very conservative and highly variable estimate of anchorage strength for headed bars compared to the proposed design provisions.