Anchor Coupler / Headed Bar Manual
Professor Dr. Engr. Md. Jahangir Alam
Former Professor, Department of Civil Engineering, BUET
Lead Consultant, Freehold Construction & Development
Table of Contents
What is Anchor Coupler or Headed Bar?
In concrete columns (pillars) and beams, as well as at beam-column joints, bars need to be bent at a 90-degree angle. The length of the bend must be 12 times the diameter of the bar. For example, a bar with a 25-millimeter (1-inch) diameter requires a 12-inch bend. Instead of bending the bar, a specially manufactured thick nut is placed at the end of the bar, called an anchor coupler. Another name for this is headed bar.
Advantages of Using Anchor Couplers
- Earthquake Resistance: To build earthquake-resistant structures, the bending of all bars at the beam-column joint must be done correctly. Columns must be provided at beam-column joints. However, in practice, workers often struggle with bending bars correctly at beam-column joints and most of the time they do not achieve proper bends. This poses a risk of building collapse initiation at beam-column joint during an earthquake. Using anchor couplers eliminates the need for difficult bending, making the building more earthquake-resistant.
- Ease of Use: Installing anchor couplers is very simple and easy. The rebar coupler supplier company uses special machines on-site to cut threads at the end of steel rebars and provides anchor couplers that resemble nuts. On-site, workers can easily attach the anchor couplers using a wrench and hand. On the other hand, bending thick bars is a challenging and time-consuming task. With anchor couplers, there are no such issues.
- Avoiding Bar Congestion: Bending bars creates congestion at the beam-column joints, which often leaves insufficient space for concrete to flow in. The use of anchor couplers eliminates the possibility of such rebar congestion at beam-column joints.
- Reducing Column Size: Compared to the bending system, the anchor coupler system requires a shorter development length, which allows for a reduction in column size.
- Reduced Wastage: Using anchor couplers reduces bar wastage and saves costs.
Anchor Coupler / Headed Bar Development Length Formula:
Anchor Coupler Diameter:
According to ACI 328-19 code, the effective cross-sectional area of a headed bar or anchor coupler must be four times the cross-sectional area of the bar. Calculations show that if the diameter of the anchor coupler is 2.25 times the diameter of the rebar, this condition is met.
Specifications for Anchor Couplers as per American Code (ACI 318-19):
- Bar Strength: Prior to 2019, anchor couplers or headed bars could only be used with Grade 60 bars. This restriction has now been lifted by ACI 328-19. Therefore, anchor couplers can be used with any grade of bar.
- Bar Size: Anchor couplers can be used with bars up to 36 millimeters in diameter.
- Coupler Size: The effective cross-sectional area of the anchor coupler must be four times the bar, and the diameter of the anchor coupler must be 2.25 times the diameter of the bar.
- Clear Cover: The clear cover of concrete must be twice the diameter of the bar.
- Bar Spacing: The center-to-center distance between bars must be three times the diameter of the bar.
- Concrete: The use of anchor couplers is prohibited in concrete made from brick aggregate. Stone aggregates must be used if anchor couplers are used.
To use anchor couplers or headed bars in compliance with these conditions, bars must be arranged as shown in the illustrations below:
Quality Test of Anchor Couplers
The quality of anchor couplers is tested using a tensile strength test. If the bar breaks inside the anchor coupler during the test, it indicates poor quality and that it is not usable. For anchor couplers, the bar should break outside the coupler, and the strength should be equal to that of the bar. If the required strength is achieved and the bar breaks outside the anchor coupler under maximum load, then the quality of the anchor coupler is considered satisfactory.
