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Project 2009-02

Project Title:

Evaluation of the Orientation of 90-Degree and 180-Degree Reinforcing Bar Hooks in Wide Members

Institution:

Missouri University of Science & Technology
Rolla, Missouri

Researchers:

Dr. Lesley H. Sneed, P.E., Assistant Professor of Civil Engineering
Nichole Podhorsky, Graduate Research Assistant

Award Period:

2010-2011 school year

Status:

Completed - Click here to download the Research Note

Description:

Background – The overall goal of the proposed project is to study the influence of tilt angle on the development of ACI standard reinforcing bar hooks. Specific project objectives include:

  • Evaluate 90° and 180° ACI standard reinforcing bar hooks to determine the influence of hook tilt angle (from vertical) on the development of the reinforcing bar;
  • Study the influence of confinement and transverse reinforcement on the development of tilted, hooked reinforcing; and
  • Develop design recommendations for limits of tilt of hooked reinforcing bars.

The topic of this study was identified by CRSI as high priority for the concrete reinforcing steel industry. Longitudinal reinforcing steel in flexural elements is often developed at the end of a concrete member by a 90° or 180° standard hook, usually oriented such that the hook is in the vertical direction. In some instances, such as the case of a shallow member that is heavily-reinforced with reinforcing bars of large diameter, the standard hook height in accordance with ACI 318-08 plus concrete cover above and below the bar may exceed the thickness of the concrete member.

To address this issue, the CRSI Design Handbook notes that the hook may be tilted from vertical to maintain the required clear cover. The limits of this tilt, however, are not defined or known, as shown in Figure 1.

To address this issue, research is needed to determine the limits of reinforcing steel hook tilt, if any, so ultimate performance of the member is not compromised. In addition, research is also needed to determine the effects of supplemental transverse reinforcement used to hold the hook angle constant on mitigating horizontal splitting. Design recommendations developed as part of this study will provide clarification to engineers and building code officials regarding limits of tilt of hooked reinforcing bars so that the original intent of hooked bar development is met.