Anchorage Behavior Of 90-degree Hooked BeamBars In Reinforced Concrete Knee Joints

Abstract

Beam bars of reinforced concrete structures are usually anchored with 90-degree hooks in exterior beam-column joints. However, there have been far fewer studies on the anchorage behavior of 90-degree hooked-bar than on straight-bar anchorage. In our previous paper, we divided the anchorage failure of 90-degree hooked bars in exterior beam-column joints in a middle storey in a building into three modes: side split failure; local compression failure; and raking-out failure, in which a concrete block, approximating the inside dimensions of the hooked bar in size, is raked out toward the beam side of the column due to the presence of many beam bars and/or to short development length within the joint. The purpose of this paper was to clarify the anchorage performances on the raking-out failure mode of beam top-bars with 90-degree hooks arranged in an exterior beam-column joint in the roof story of a building. Fifteen specimens of knee joints with various arrangements of L-shaped beam bar anchorage and various material properties, were subjected to pull-out loading on the beam top-bars. From the experimental results, we were able to conclude that: (1) the anchorage mechanism depends on stress transmission from the tail portion of the beam bar hooks to the adjacent column bars in a joint and that the anchorage mechanism in a knee joint was quite different from that in a exterior beam-column joint in a middle story; (2) the main factors in influencing anchorage strength are tail length, horizontal distance between tail and column bars and between tail bars and the beam end, lateral reinforcement ratio within the joint and concrete strength; and (3) accurate estimation of anchorage strength can be obtained by taking into account the influence of the above mentioned factors.