Anchorage of High-Strength Reinforcing Bars in Concrete

Abstract

Hooked and headed reinforcing bars are commonly used as a means of shortening development length of reinforcing bars, but a limited amount of previous research has resulted in restrictions on their use in practice. This study included two phases: In the first phase, 31 tests of simulated column-foundation joints were conducted to investigate the anchorage strength and behavior of large and high-strength headed bars as functions of the distance between the anchored headed bar and the compression reaction, number of headed bars tested simultaneously (1 or 2), size of the headed bars (No. 11 or No. 14), center-to-center spacing between headed bars loaded simultaneously (3.2 or 8.2db), amount of confining reinforcement within the joint region (zero to six No. 4 closed stirrups), and concrete compressive strength (5,060 to 14,470 psi). The embedment length of the headed bars ranged from 125 /8 to 14 in., and the stresses in the headed bars at failure ranged from 41,800 to 144,400 psi. The test results are compared with anchorage strengths based on the descriptive equations for headed bars developed at the University of Kansas, ACI 318-19 Code provisions, and proposed Code provisions. Recommended changes to Chapters 17 and 25 of ACI 318-19 are presented. In the second phase of the study, descriptive equations for beam-column joints tested under monotonic loading are investigated their applicability to predict the anchorage strength of hooked bars anchored in members subjected to reversed cyclic loading. Comparisons are made with test results from 24 studies of 146 exterior beam-column joint specimens subjected to reversed cyclic loading in which the beam bars are anchored by hooks. Key variables include embedment lengths of the hooked bars (6 to 21 in.), concrete compressive strength (3,140 to 13,700 psi), center-to-center spacing between the hooked bars (1.75 to 6.5 in.), bar size (No. 3 to No. 9), and confining reinforcement within the joint region parallel to the straight portion of the hooked bars (none to nine hoops spaced at 1.25 to 6.0 in.). The yield strength of the hooked bars ranged from 42,900 to 103,000 psi. Proposed changes to Chapters 18 of ACI 318-19 are presented. The results of the experimental study show that the anchorage strength of headed bars anchored in column-foundation joints is improved by parallel tie reinforcement located on all sides of the headed bars, a contribution that is not included in the provisions of ACI 318-19. Similar to observations for beam-column joints, the anchorage strength of headed bars anchored in simulated column-foundation joints decreases as the center-to-center spacing decreases below 8db. The descriptive equations developed based on tests of beam-column joints are suitable for predicting the anchorage strength of headed bars anchored in column-foundation joints. Chapter 17 of ACI 318-19 does not accurately predict the anchorage strength of headed bars tested when parallel tie/anchor reinforcement is used and should be modified to combine the contributions of concrete strength and parallel tie reinforcement. The descriptive equations developed for beam-column joints apply to column-foundation joints and could serve as a basis for the anchorage provisions in Chapter 17 of ACI 318. The provisions in Chapter 25 of ACI 318-19 should be updated to include the effect of parallel tie reinforcement in connections other than beam-column joints. The descriptive equations for the anchorage strength of hooked bars in beam-column joints tested under monotonic loading are suitable for predicting the anchorage strength of hooked bars anchored in members subjected to reversed cyclic loading. The ACI Code provisions for the development length of hooked bars in tension in beam-column joints in special moment frames (Section 18.8.5.1 of ACI 318-19), derived from the development length provisions for non-seismic loading in earlier Codes, permit development lengths that are shorter needed for gravity load by Chapter 25. Changes in Chapter 18 of ACI 318-19 are proposed that require the use of the provisions in Chapter 25 to establish the minimum development length for hooked bars anchored in joints for frames subjected to seismic loading