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The effectiveness of headed bars terminating in exterior beam-column joints was assessed. Nine inter-story and five roof-level joint specimens were tested under reversed cyclic loading, where the primary test parameters were the anchorage type, the size and arrangement of the beam bars and the heads, and the detailing provided for roof joints. The test results indicate that hysteretic behavior of the exterior joints constructed with headed bars was similar or superior to joints constructed and tested with hooked bars commonly used in Korean practice. Head size with a net area of three to four times the bar area was sufficient to anchor the beam reinforcement effectively within the exterior beam-column joint. The test results also indicate that, in addition to providing vertical U-bars at roof joints, heads on column bars should extend beyond the beam top bars to provide improved behavior.
Keywords: anchorage; beam-column joint; reinforcement.
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INTRODUCTION
Proper anchorage of reinforcement is essential to reinforced concrete structures to ensure composite action between reinforcement and concrete to resist the member design forces. In general, anchorage is achieved by a combination of bond (that is, bearing on bar ribs) and bearing on hooks or anchor plates. To ensure that the capacity is greater than demand, and to minimize slip between the bars and the concrete, current ACI 3181 provisions specify the development length of straight reinforcement, as well as the hook length, bend radius, and extension. Use of anchor plates or heads, either welded or threaded to the longitudinal bar, has been identified as an alternative to the use of hooked bars for exterior joints.2 In this paper, the term headed bar will be used to refer to a reinforcing bar with an attached head to provide mechanical anchorage.
Standard 90-degree hooks are commonly used to anchor longitudinal bars within an exterior beam-column joint; however, use of hooks often results in steel congestion, difficult fabrication and construction, and greater potential for poor concrete placement. In addition, cyclic loading tends to degrade the anchorage capacity due to the slip. The use of headed bars offers a potential solution to these problems and may also ease fabrication, construction, and concrete placement.
Headed bars with rectangular, round, or elliptical anchor plates...