Force Transfer Mechanism of Headed Anchorage Bar in Exterior Beam Column Joint with Finite Element Method and Strut and Tie Model

Abstract

In structural concrete, the provisions for anchorage of straight bars and hooks sometimes present detailing problems due to the long development lengths and large bend diameters that are required. Occasionally, the requirements for straight bar anchorage and lap splices cannot be provided within the available dimensions of elements. Hooked bars can be used to shorten anchorage length, but in many cases, the bend of the hook will not fit within the dimensions of a member or the hooks create congestion and make an element difficult to construct. This congestion may lead to high fabrication effort needed and poor concrete placement, resulting in decrease of concrete quality at the joints. An alternative is the use of headed anchorage bar, which allows for extremely small development lengths, that can reduce congestion without compromising the integrity of the structure. As a result, designing and detailing the structure are made easier and more efficient. Headed bars are formed by the attachment of a plate or the forging of an upset bearing surface at the end of a straight reinforcing bar. Such bars are anchored by a combination of bond along the straight bar length and direct bearing at the head. This papers presents strut and tie models explaining force transfer mechanism of headed anchorage bar in exterior beam-column joint under monotonic loads. The proposed model is derived from beam-column joint specimen which have been tested experimentally. Stress and strain generated by modeling the beam column joint with finite element-based program, ATENA 3D. The result of the analysis explaining the behavior of headed anchorage bar is CCT nodes (compression-compression-tension). The model is consists of a strut with the nodal zone at the head for head bearing and a fan-shaped stress field for bond stresses along the development length.