The Development Length and Anchorage Behavior of Headed Reinforcing Bars

Research has been conducted by several investigators on a new, innovative type of reinforcement, referred to as the headed bar. The development of the headed bar resulted out of the need to reduce the development length of the bar and anchor the reinforcement in a shorter length. Construction in earthquake prone regions and designing for blast or impact conditions requires dense reinforcement configurations.. In these areas, designs call for connection details of major structural members that become congested with reinforcement. So much congestion occurs that standard 90° or 180° hooks, as prescribed by codes such as AC~ become unmanageable and are not feasible in complex reinforcement configurations. As the main focus of this present study, the anchorage behavior of a headed bar embedded in concrete in terms of development length and bond strength is investigated. In addition, further research will be proposed to evaluate the use of headed bars in lap splice applications. This research includes a testing program consisting of concrete beam-end tests used to investigate the anchorage behavior of the headed bar. In order to form a basis for comparison, other beam-end specimens are tested using straight reinforcing bar, and still others using standard 180° hooked bars in addition to tests of the headed bar. Variables evaluated include the clear cover, bonded length, and transverse reinforcement. The results of this program show the headed bar to provide almost immediate development of the bar provided that an adequate amount of confinement in terms of cover or transverse reinforcement is used. The results show the headed bar to be an adequate, if not an improved substitute, for the standard hooked bar as set forth by ACI. Based on these results and comparisons to previously developed expressions from past research, a design equation is proposed to describe the development length of headed reinforcement. From this equation, a set of design guidelines also is developed and presented to ACI for inclusion in a future version of the ACI Building Code. Currently, there are no ACI code provisions that cover the use of the headed bar in structural design. Through this research, as well as studies being done at other institutions, a sufficient and accurate basis can be provided for the adoption of such standards into future editions of the ACI Building Code.